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  Home > Glossary

 
100-year Flood Plain
The flood plain that would be inundated in the event of a 100-year flood.
500 hPa
Pressure surface (geopotential height) in the troposphere equivalent to about 18,000 feet above sea level. Level of the atmosphere at which half the mass of the atmosphere lies above and half below, as measured in pressure units. This area is important for understanding surface weather, upper air storms tend to be steered in the direction of the winds at this level and are highly correlated with surface weather.
Absorption
The process in which incident radiant energy is retained by a substance by conversion to some other form of energy.
Abutment Seeping
Reservoir water that moves through seams or pores in the natural abutment material and exits as seepage.
Acid Precipitation
Precipitation, such as rain, snow or sleet, containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
ACPY
Accompany
Active Prominence
In solar-terrestrial terms, a prominence displaying material motion and changes in appearance over a few minutes of time.
Active Prominence Region (APR)
In solar-terrestrial terms, a portion of the solar limb displaying active prominences.
Active Storage Capacity
In hydrologic terms, the total amount of reservoir capacity normally available for release from a reservoir below the maximum storage level. It is total or reservoir capacity minus inactive storage capacity. More specifically, it is the volume of water between the outlet works and the spillway crest.
ADAPTATION (ADAPTABLE) PARAMETER
Generally, data related to a specific WSR-88D unit. These data may consist of meteorological or hydrological parameters or of geographic boundaries, political boundaries, system configuration, telephone numbers (auto dial), or other like data. Such data may be generated at either a centralized location or locally at the WSR-88D unit.
Adiabatic Lapse Rate
The rate of decrease of temperature experienced by a parcel of air when it is lifted in the atmosphere under the restriction that it cannot exchange heat with its environment. For parcels that remain unsaturated during lifting, the (dry adiabatic) lapse rate is 9.8°C per kilometer.
Adiabatic Process
A process which occurs with no exchange of heat between a system and its environment.
Adirondack Type Snow Sampling Set
In hydrologic terms, a snow sampler consisting of a 5-foot fiberglass tube, 3 inches in diameter, with a serrated-edge steel cutter at one end and a twisting handle at the other. This sampler has a 60-inch snow depth capacity.
ADPC
Acoustic Doppler Current Profiler
Advanced Hydrologic Prediction Service (AHPS)
A web-based suite of accurate and information-rich forecast products. They display the magnitude and uncertainty of occurrence of floods or droughts, from hours to days and months in advance. These graphical products are useful information and planning tools for many economic and emergency managers.
AGDISP
A particular atmospheric disperison model used for treating the transport and diffusion of aerially sprayed pest control agents in agricultural applications.
Air Pollutant
Harmful substance or product introduced into the atmosphere.
Air Pollution Potential
The meteorological potential for air pollution problems, considered without regard to the presence or absence of actual pollution sources.
Air Transportable Mobile Unit
A modularized transportable unit containing communications and observational equipment necessary to support a meteorologist preparing on-site forecasts at a wildfire or other incident.
Airborne Snow Survey Program
In hydrologic terms, Center (NOHRSC) program that makes airborne snow water equivalent and soil moisture measurements over large areas of the country that are subject to severe and chronic snowmelt flooding.
Alberta Clipper
A fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a "Saskatchewan Screamer".
Along-slope Wind System
A closed, thermally driven diurnal mountain wind circulation whose lower branch blows up or down the sloping sidewalls of a valley or mountain. The upper branch blows in the opposite direction, thereby closing the circulation.
Amplifier
A device used to increase the strength of an analog signal
Amplitude
The maximum magnitude of a quantity. Often used to refer to the maximum height of a wave.
Annual Exceedance Probability (AEP)
The probability that a stream reach will have a flow of a certain magnitude in any given year.
ANOMALOUS PROPAGATION (AP)
Non-standard atmospheric temperature or moisture gradients will cause all or part of the radar beam to propagate along a non-normal path. When non-standard index-of-refraction distributions prevail, "abnormal" or "anomalous" propagation occurs. When abnormal downward bending occurs, it is called "superrefraction." If the beam is refracted downward sufficiently, it will illuminate the ground and return signals to the radar from distances further than is normally associated with ground targets. The term "subrefraction" is applied when there is abnormal upward bending of the radar beam.
Antedecent Precipitation Index
(Abbrev. API) - an index of moisture stored within a drainage basin before a storm.
Anthropogenic Source
A pollutant source caused or produced by humans.
AOPA
Aircraft Owners and Pilots Association
AP
Anomalous Propagation. Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric conditions.
AP Index
In solar-terrestrial terms, an averaged planetary A Index based on data from a set of specific stations.
APD
On a buoy report, the average wave period (seconds) of all waves during the 20-minute period.
Aphelion
The point on the annual orbit of a body (about the sun) that is farthest from the sun; at present, the earth reaches this point (152 million kilometer from the sun) on about 5 July. Opposite of perihelion.
API Method
In hydrologic terms, a statistical method to estimate the amount of surface runoff which will occur from a basin from a given rainstorm based on the antecedent precipitation index, physical characteristics of the basin, time of year, storm duration, rainfall amount, and rainfall intensity.
Apogee
The farthest distance between the moon and earth or the earth and sun.
Apparent Temperature
A measure of human discomfort due to combined heat and humidity (e.g., heat index).
Apparent Wind
The speed and true direction from which the wind appears to blow with reference to a moving point. Sometimes called RELATIVE WIND.
APRCH
Approach
APRCHG
approaching
APRNT
apparent
APST
Aviation Products and Services Team
Area Wide Hydrologic Prediction System
(Abbrev. AWHPS) - A computer system which automatically ingests areal flash flood guidance values and WSR-88D products and displays this data and other hydrologic information on a map background.
Area-Capacity Curve
In hydrologic terms, a graph showing the relation between the surface area of the water in a reservoir, the corresponding volume, and elevation.
ASAP
1. AHOS SHEF Automatic Processing System

2. As soon as possible (may be used in Area Forecast Discussions)
ASAPTRAN
The software component of ASAP.
ASSOCIATED PRINCIPAL USER
A Principal User with dedicated communications to a WSR-88D unit.
Atmosphere
The air surrounding and bound to the Earth.
Atmospheric Boundary Layer
Same as Boundary Layer - in general, a layer of air adjacent to a bounding surface. Specifically, the term most often refers to the planetary boundary layer, which is the layer within which the effects of friction are significant. For the earth, this layer is considered to be roughly the lowest one or two kilometers of the atmosphere. It is within this layer that temperatures are most strongly affected by daytime insolation and nighttime radiational cooling, and winds are affected by friction with the earth's surface. The effects of friction die out gradually with height, so the "top" of this layer cannot be defined exactly.
Atmospheric Circulation Model
A mathematical model for quantitatively describing, simulating, and analyzing the structure of the circulation in the atmosphere and the underlying causes. Sometimes referred to as Atmospheric General Circulation Models or AGCMs.
Atmospheric Pressure
The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.
Atmospheric Radiation
Infrared radiation (energy in the wavelength interval of 3- 80 micrometer) emitted by or being propagated through the atmosphere. It consists of both upwelling and downwelling components. Compare with terrestrial radiation.
ATMP
On a buoy report, the air temperature (Celsius).
Augmented report
A meteorological report prepared by an automated surface weather observing system for transmission with certified observers signed on to the system to add information to the report.
Automated Event Reporting Gage
(also see Tipping Bucket Rain Gage); for river stage gages, IFLOWS pressure transducer type gages can be programmed to report if water surface rises or falls by a predetermined amount.
Automated Report
A meteorological report prepared by an automated surface weather observing system for transmission, and with no certified weather observers signed on to the system.
AVP
On a buoy report, Average Wave Period is the average period (seconds) of the highest one-third of the wave observed during a 20 minute sampling period.
AWHPS
Area Wide Hydrologic Prediction System
AWIPS
Advanced Weather Interactive Processing System. This system replaced the Automation of Field Operations and Services (AFOS). This system allows the operator to overlay meteorological data from a variety of sources.
BANDPASS FILTER
A filter whose frequencies are between given upper and lower cutoff values, while substantially attenuating all frequencies outside these values (this band).
BAPSU
Bay Area Public Service Unit. Public Service section of the San Francisco Bay Area Weather Service Forecast Office.
Barber Pole
[Slang], a thunderstorm updraft with a visual appearance including cloud striations that are curved in a manner similar to the stripes of a barber pole. The structure typically is most pronounced on the leading edge of the updraft, while drier air from the rear flank downdraft often erodes the clouds on the trailing side of the updraft.
Barograph
A barometer that records its observations continuously.
Barometric Pressure
The pressure of the atmosphere as indicated by a barometer.
Barotropic System
A weather system in which temperature and pressure surfaces are coincident, i.e., temperature is uniform (no temperature gradient) on a constant pressure surface. Barotropic systems are characterized by a lack of wind shear, and thus are generally unfavorable areas for severe thunderstorm development. See baroclinic zone.

Usually, in operational meteorology, references to barotropic systems refer to equivalent barotropic systems - systems in which temperature gradients exist, but are parallel to height gradients on a constant pressure surface. In such systems, height contours and isotherms are parallel everywhere, and winds do not change direction with height.

As a rule, a true equivalent barotropic system can never be achieved in the real atmosphere. While some systems (such as closed lows or cutoff lows) may reach a state that is close to equivalent barotropic, the term barotropic system usually is used in a relative sense to describe systems that are really only close to being equivalent barotropic, i.e., isotherms and height contours are nearly parallel everywhere and directional wind shear is weak.
Barotropy
The state of a fluid in which surfaces of constant density (or temperature) are coincident with surfaces of constant pressure; it is the state of zero baroclinity.
BASE PRODUCTS
Those products that present some representation of the base data. This representation may not necessarily be either in full resolution or depict the full area of coverage. Base products can be used to generate a graphic display or further processing.
Beginning of Freezup
In hydrologic terms, date on which ice forming a stable winter ice cover is first observed on the water surface
Beginning of the Breakup
In hydrologic terms, date of definite breaking, movement, or melting of ice cover or significant rise of water level.
Bergeron Process
The process by which ice crystals in a cloud grow at the expense of supercooled liquid water droplets.
BLDUP
Buildup
Breakup
In hydrologic terms, the time when a river whose surface has been frozen from bank to bank for a significant portion of its length begins to change to an open water flow condition. Breakup is signaled by the breaking of the ice and often associated with ice jams and flooding.
Breakup Date
In hydrologic terms, date on which a body of water is first observed to be entirely clear of ice and remains clear thereafter.
Breakup Jam
In hydrologic terms, an ice jam that occurs as a result of the accumulation of broken ice pieces.
Breakup Period
In hydrologic terms, the period of disintegration of an ice cover.
Brocken Specter
An optical phenomenon sometimes occurring at high altitudes when the image of an observer placed between the sun and a cloud is projected on the cloud as a greatly magnified shadow. The shadow's head is surrounded by rings of color, called a glory.
Cap
(also called "Lid") A layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and produce thunderstorms. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur.

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.
Cap Cloud
A stationary cloud directly above an isolated mountain peak, with cloud base below the elevation of the peak.
CAPE
Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.

However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent. CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.) See also CIN.
Capillarity
In hydrologic terms,

1.The degree to which a material or object containing minute openings or passages, when immersed in a liquid, will draw the surface of the liquid above the hydrostatic level. Unless otherwise defined, the liquid is generally assumed to be water.

2. The phenomenon by which water is held in interstices above the normal hydrostatic level, due to attraction between water molecules.
Capillary Fringe
In hydrologic terms, the soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials. The capillary fringe is also called the capillary zone.
Capillary Waves
Waves caused by the initial wind stress on the water surface causes what are known as capillary waves. These have a wavelength of less than 1.73 cm, and the force that tries to restore them to equilibrium is the cohesion of the individual molecules. Capillary waves are important in starting the process of energy transfer from the air to the water.
Capillary Zone
Used interchangably with Capillary Fringe; the soil area just above the water table where water can rise up slightly through the cohesive force of capillary action. This layer ranges in depth from a couple of inches, to a few feet, and it depends on the pore sizes of the materials.
Capping
A region of negative buoyancy below an existing level of free convection (LFC) where energy must be supplied to the parcel to maintain its ascent. This tends to inhibit the development of convection until some physical mechanism can lift a parcel to its LFC. The intensity of the cap is measured by its convective inhibition. The term capping inversion is sometimes used, but an inversion is not necessary for the conditions producing convective inhibition to exist.
Capping Inversion
Alternate term for Cap; a layer of relatively warm air aloft, usually several thousand feet above the ground, which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further and produce thunderstorms. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However, if the cap is removed or weakened, then explosive thunderstorm development can occur.

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.
CAPS
Center for Analysis and Prediction of Storms
Central Meridian Passage (CMP)
In solar-terrestrial terms, the passage of an Active Region or other feature across the longitude meridian that passes through the apparent center of the solar disk.
CFP
Cold Front Passage
Chromosphere
In solar-terrestrial terms, the layer of the solar atmosphere above the photosphere and beneath the transition region and the corona.
Chromospheric Events
In solar-terrestrial terms, flares that are just Chromospheric Events without Centimetric Bursts or Ionospheric Effects. (SID) (Class C flare)
Climate Prediction Center
This Center is one of several centers under the National Centers for Environmental Prediction (NCEP) part of the National Weather Service (NWS) in the National Oceanic and Atmospheric Administration (NOAA). The Center serves the public by assessing and forecasting the impacts of short-term climate variability, emphasizing enhanced risks of weather-related extreme events, for use in mitigating losses and maximizing economic gains.
CMPLT
Complete
CMPLX
Complex
Cold Pool
A region of relatively cold air, represented on a weather map analysis as a relative minimum in temperature surrounded by closed isotherms. Cold pools aloft represent regions of relatively low stability, while surface-based cold pools are regions of relatively stable air.
Complex Gale/Storm
In the high seas and offshore forecasts, an area for which gale/storm force winds are forecast or are occurring but for which no single center is the principal generator of these winds.
Complex Terrain
Typically used to refer to mountainous terrain. In general usage, it may also refer to coastal regions and heterogeneous landscapes.
Composite
An average that is calculated according to specific criteria. For example, one might want a composite for the rainfall at a given location for all years where the temperature was much above average.
Composite Hydrograph
A stream discharge hydrograph which includes base flow, or one which corresponds to a net rain storm of duration longer than one unit period.
Comprehensive Flare Index (CFI)
In solar-terrestrial terms, the indicative of solar flare importance.
Cone of Depression
In hydrologic terms, the depression, roughly conical in shape, produced in a water table, or other piezometric surface, by the extraction of water from a well at a given rate. The volume of the cone will vary with the rate of withdrawal of water. Also called the Cone of Influence.
Conjugate Points
Two points on the earth's surface, at opposite ends of a geomagnetic field line
Constant Pressure Chart
Alternate term for Isobaric Chart; a weather map representing conditions on a surface of equal atmospheric pressure. For example, a 500 mb chart will display conditions at the level of the atmosphere at which the atmospheric pressure is 500 mb. The height above sea level at which the pressure is that particular value may vary from one location to another at any given time, and also varies with time at any one location, so it does not represent a surface of constant altitude/height (i.e., the 500 mb level may be at a different height above sea level over Dallas than over New York at a given time, and may also be at a different height over Dallas from one day to the next).
Control Points
In hydrologic terms, small monuments securely embedded in the surface of the dam. Any movement of the monument indicates a movement in the dam itself. Movements in the dam are detected by comparing control points location to location of fixed monuments located off the dam using accurate survey techniques.
Convective Overdevelopment
Convection that covers the sky with clouds, thereby cutting off the sunshine that produces convection.
Convective Temperature
The approximate temperature that the air near the ground must warm to in order for surface-based convection to develop, based on analysis of a sounding.

Calculation of the convective temperature involves many assumptions, such that thunderstorms sometimes develop well before or well after the convective temperature is reached (or may not develop at all). However, in some cases the convective temperature is a useful parameter for forecasting the onset of convection.
Cooperative Observer
An individual (or institution) who takes precipitation and temperature observations-and in some cases other observations such as river stage, soil temperature, and evaporation-at or near their home, or place of business. Many observers transmit their reports by touch-tone telephone to an NWS computer, and nearly all observers mail monthly reports to the National Climatic Data Center to be archived and published.
Core Punch
[Slang], a penetration by a vehicle into the heavy precipitation core of a thunderstorm. Core punching is not a recommended procedure for storm spotting.
Coupled Atmosphere-Ocean Model
Same as Coupled Model; in the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other.
Coupled Model
In the context of climate modeling this usually refers to a numerical model which simulates both atmospheric and oceanic motions and temperatures and which takes into account the effects of each component on the other.
Couplet
Adjacent maxima of radial velocities of opposite signs.
CPC
Climate Prediction Center
Crepuscular Rays
The alternating bands of light and dark (rays and shadows) seen at the earth's surface when the sun shines through clouds.
Critical Depth
In hydrologic terms, The depth of water flowing in an open channel or conduit, partially filled, corresponding to one of the recognized critical velocities.
Critical Rainfall Probability
(Abbrev. CRP) - In hydrologic terms, the Probability that the actual precipitation during a rainfall event has exceeded or will exceed the flash flood guidance value.
Crop Moisture Index
In 1968, Palmer developed the index to assess short-term crop water conditions and needs across major crop-producing regions. This index is a useful tool in forecasting short-term drought conditions.
CRP
Critical Rainfall Probability - in hydrologic terms, the probability that a given rainfall will cause a river, or stream to rise above flood stage.
Cubic Feet per Second
(Abbrev. CFP) - In hydrologic terms, a unit expressing rates of discharge. One cubic foot per second is equal to the discharge through a rectangular cross section, 1 foot wide by 1 foot deep, flowing at an average velocity of 1 foot per second. It is also approximately 7.48 gallons per second.
Cumulus Buildups
Clouds which develop vertically due to unstable air. Characterized by their cauliflower-like or tower-like appearance of moderately large size
Daily Climatological Report
As the name indicates, this climatological product is issued daily by each National Weather Service office. Most of the climatological data in this report are presented in a tabular form; however, some narrative statements may also be used in the product. The report is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, sunrise and sunset times, etc.).
Daily Flood Peak
In hydrologic terms, the maximum mean daily discharge occuring in a stream during a given flood event.
DAPM
In hydrologic terms, the Data Acquisition Program Manager.
Data Point
In the context of hydrologic observations, a location on a river/stream for which observed data is input to RFC or WFO hydrologic forecast procedures, or included in public hydro­logic products. Flood forecasts and warnings are not issued for data points (see /forecast point/).
DCP
(Data Collection Platform) In hydrologic terms, an electronic device that connects to a river or rainfall gage that records data from the gage and at pre-determined times transmits that data through a satellite to a remote computer.
Deep Percolation Loss
In hydrologic terms, water that percolates downward through the soil beyond the reach of plant roots.
Deep Seepage
In hydrologic terms, infiltration which reaches the water table.
Deep well
In hydrologic terms, a well whose pumping head is too great to permit use of a suction pump.
Deepening
A decrease in the central pressure of a surface low pressure system. The storm is intensifying.
DEP
Departure
Depletion Curve
In hydrologic terms, the part of the hydrograph extending from the point of termination of the Recession Curve to the subsequent rise or alternation of inflow due to additional water becomming available for stream flow.
Depression
A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation. The term is also sometimes used as a reference to a Tropical Depression.
Depression Storage
In hydrologic terms, the volume of water contained in natural depressions in the land surface, such as puddles.
Depth of Runoff
In hydrologic terms, the total runoff from a drainage basin, divided by its area. For convenience in comparing runoff with precipitation, the term is usually expressed in inches of depth during a given period of time over the drainage area or acre-feet per square mile.
Derived Products
Processed base data on the Doppler radar.
Developing Gale/Storm
In the high seas and offshore forecasts, a headline used in the warnings section to indicate that gale/storm force winds are not now occurring but are expected before the end of the forecast period.
Dew Point
(Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).
Dew Point Depression
The difference in degrees between the air temperature and the dew point.
Dew Point Front
A narrow zone (mesoscale feature) of extremely sharp moisture gradient and little temperature gradient. It separates moist air from dry air. Severe weather can be associated with this front. It is also known as a "dryline" or "dry front".
DEWP
On a buoy report, the dewpoint temperature taken at the same height as the air temperature measurement.
Disappearing Solar Filament (DSF)
In solar-terrestrial terms, the sudden (timescale of minutes to hours) disappearance of a solar filament (prominence).
Dispersion
The process of separating radiation into various wavelengths.
Distribution (Hydro)Graph
In hydrologic terms, a unit hydrograph of direct runoff modified to show the proportions of the volume of runoff that occur during successive equal units of time.
Diurnal Temperature Range
The temperature difference between the minimum at night (low) and the maximum during the day (high).
Domestic Consumption
In hydrologic terms, the quantity, or quantity per capita, of water consumed in a municipality or district for domestic uses or purposes during a given period, generally one day. It is usually taken to include all uses included within the term Municipal Use of Water and quantity wasted, lost, or otherwise unaccounted for.
Doppler Radar
Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).
Downslope Flow
A thermally driven wind directed down a mountain slope and usually occurring at night; part of the along-slope wind system.
Downstream Slope
In hydrologic terms, the slope or face of the dam away from the reservoir water. This slope requires some kind of protection (e.g.; grass) from the erosive effects of rain and surface flow
DP
1. Deep

2. Dew Point
DPD
On a buoy report, dominant wave period (seconds) is the period with the maximum wave energy.
DPNG
Deepening
DPTH
Depth
DPTR
Departure
DPVA
Differential Positive Vorticity Advection
Drop-size Distribution
The distribution of rain drops or cloud droplets of specified sizes.
Dry Adiabatic Lapse Rate
The rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5°F per 1000 ft or 9.8°C per km.
Dry Floodproofing
In hydrologic terms, a dry floodproofed building is sealed against floodwaters. All areas below the flood protection level are made watertight. Walls are coated with waterproofing compounds or plastic sheeting. Openings like doors windows, sewer lines and vents are closed, whether permanently, with removable shields, or with sandbags. The flood protection level should be no more than 2 or 3 feet above the top of the foundation because the buildings walls and floors cannot withstand the pressure of deeper water.
Dry Punch
[Slang], a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.
DSIPT
Dissipate
Dust Plume
A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front.
DVLP
Develop
DWNSLP
Downslope
DWPT
Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).
Dynamic Wave Routing Model (DWOPER)
A computerized hydraulic routing program whose algorithms incorporate the complete one-dimensional equations of unsteady flow
E-19, Report on River Gage Station
In hydrologic terms, a report to be completed every 5 years providing a complete history of a river station and all gages that have been used for public forecasts since the establishment of the station.
E-19a, Abridged Report on River Gage Sta
In hydrologic terms, an abridged version of an E-19, an E-19a updates the E-19 as additional information, or changes occur at the station during the intervening five year period. An E-19a is to be completed anytime a significant change occurs at a forecast point. An E-19a is also used to take the place of an E-19 in documenting any gage history, or information of any non-forecast point (i.e; data point).
E-3, Flood Stage Report
In hydrologic terms, a form that a Service Hydrologist/ Hydrology Focal Point completes to document the dates in which forecast points are above flood stage, as well as the crest dates and stages. Discussion of the flood event must also be included in the E-5, Monthly Report of River and Flood conditions. An E-3 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).
E-5, Monthly Report of River and Flood c
In hydrologic terms, a monthly narrative report covering flooding which occurred over the past month. Flood stage, flood crest and dates in which flooding occurred is covered within this report for each data point which was in flood. If the flooding involved a forecast point, an E-3 must be filled out as well. If no flooding has occurred within the past month, a climatic summary of the past month can be included as well as other interesting non-flood events, such as water supply, ice jams and the occurrence of drought. An E-5 report is sent to Regional Headquarters, the appropriate RFC, as well as the Office of Hydrology (OH).
E-7, Flood Damage Report
In hydrologic terms, a report to be completed anytime there is reported flood damage or loss of life as a direct result of flooding. An E-7 report is sent to Regional Headquarters, as well as the Office of Hydrology (OH).
Echo Tops
The height above ground of the center of the radar beam using the tilt, or scan, that contains the highest elevation where reflectivities greater than 18 dBZ can be detected.
Effective Porosity
In hydrologic terms, the ratio, usually expressed as a percentage, of the volume of water or other liquid which a given saturated volume of rock or soil will yield under any specified hydraulic condition, to the given volume of soil or rock.
Effective Precipitation
1) That part of the precipitation that produces runoff.

2) A weighted average of current and antecedent precipitation that is "effective" in correlating with runoff.

3) That part of the precipitation falling on an irrigated area that is effective in meeting the consumptive use requirements.
Effective Topography
The topography as seen by an approaching flow, which may include not only the actual terrain but also cold air masses trapped within or adjacent to the actual topography.
Effluent Seepage
In hydrologic terms, diffuse discharge of ground water to the ground surface
Emergency Action Plan
In hydrologic terms, a predetermined plan of action to be taken to reduce the potential for property damage and loss of life in an area affected by a dam break or excessive spillway.
Energy Dissipator
In hydrologic terms, a structure which slows fast-moving spillway flows in order to prevent erosion of the stream channel.
Entropy
The amount of energy that is not available for work during a certain process.
Environmental Lapse Rate
The rate of decrease of air temperature with height, usually measured with a radiosonde.
Environmental Temperature Sounding
An instantaneous or near-instantaneous sounding of temperature as a function of height. This sounding or vertical profile is usually obtained by a balloon-borne instrument, but can also be measured using remote sensing equipment.
EPA
Environmental Protection Agency
EPCTG
Expecting
EPV
Equivalent Potential Vorticity
Equi-Potential Line
In hydrologic terms, a line, in a field of flow, such that the total head is the same for all points on the line, and therefore the direction of flow is perpendicular to the line at all points.
Equivalent Potential Temperature
The equivalent potential temperature is the temperature a parcel at a specific pressure level and temperature would have if it were raised to 0 mb, condensing all moisture from the parcel, and then lowered to 1000 mb.
Eruptive
In solar-terrestrial terms, solar activity levels with at least one radio event (10 cm) and several chromospheric events per day (Class C Flares).
Eruptive Prominence on Limb (EPL)
In solar-terrestrial terms, a solar prominence that becomes activa- ted and is seen to ascend from the sun.
ESP
Extended Streamflow Prediction
Evaporation
The process of a liquid changing into a vapor or gas, usually water in meteorology.
Evaporation Pan
In hydrologic terms, a pan used to hold water during observations for the determination of the quantity of evaporation at a given location. Such pans are of varying sizes and shapes, the most commonly used being circular or square.
Evaporation Rate
In hydrologic terms, the quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is usually expressed in inches depth, per day, month, or year.
Evaporation-mixing Fog
Fog that forms when the evaporation of water raises the dew point of the adjacent air.
Evaporative Demand Drought Index (EDDI)
An experimental drought monitoring and early warning guidance tool. It examines how anomalous the atmospheric evaporative demand is for a given location and across a time period of interest.
Evaporimeter
In hydrologic terms, an instrument which measures the evaporation rate of water into the atmosphere.
Evapotranspiration
Combination of evaporation from free water surfaces and transpiration of water from plant surfaces to the atmosphere.
Exclusive Flood Control Storage Capacity
In hydrologic terms, the space in a reservoir reserved for the sole purpose of regulating flood inflows to abate flood damage
Exosphere
The upper most layer of the earth's atmosphere; the only layer where atmospheric gases can escape into outer space.
Experimental Product
An experimental product is in the final stages of testing and evaluation. If the product proves accurate and valuable to users then the next step is to make it an operational product.
Explosive Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 mb/hr for at least 12 hours or 5 mb/hr for at least six hours.
Extratropical
A term used in advisories and tropical summaries to indicate that a cyclone has lost its "tropical" characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone's primary energy source from the release of latent heat of condensation to baroclinic (the temperature contrast between warm and cold air masses) processes. It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force.
Extratropical Cyclone
A cyclone in the middle and high latitudes often being 2000 kilometers in diameter and usually containing a cold front that extends toward the equator for hundreds of kilometers.
Extratropical Low
A low pressure center which refers to a migratory frontal cyclone of middle and higher latitudes. Tropical cyclones occasionally evolve into extratropical lows losing tropical characteristics and become associated with frontal discontinuity.
Federal Snow Sampler
In hydrologic terms, a snow sampler consisting of five or more sections of sampling tubes, one which has a steel cutter on the end. The combined snowpack measuring depth is 150 inches. This instrument was formerly the Mount Rose Type Snow Sampling Set.
Field (Moisture) Capacity
The amount of water held in soil against the pull of gravity
Flash Multiplicity
The number of return strokes in a lightning flash.
Float Recording Precipitation gage
In hydrologic terms, a rain gage where the rise of a float within the instrument with increasing rainfall is recorded. Some of these gages must be emptied manually, while others employ a self-starting siphon to empty old rainfall amounts.
Flood Inundation Mapping (FIM)
In the context of the National Water Model (NWM) FIM, the expected maximum inundation extent over a particular period of time derived using the Height Above Nearest Drainage (HAND) method. Stage heights are interpolated from the NWM discharge values using synthetic rating curves, and are rounded to the nearest foot.
Flood Plain
In hydrologic terms, the portion of a river valley that has been inundated by the river during historic floods.
Flood Potential Outlook
(ESF on AFOS) (FPO for Acronym): In hydrologic terms, An NWS outlook that is issued to alert the public of potentially heavy rainfall that could send area rivers and streams into flood or aggravate an existing flood.
Flood Prevention
In hydrologic terms, measures that are taken in order to keep flood problems from getting worse. Planning, land acquisition, river channel maintenance, wetlands protection, and other regulations all help modify development on floodplains and watersheds to reduce their susceptibility to flood damage. Preventive measures are usually administered by the building, zoning, planning and/ or code enforcement offices of the local government.
Flood Problems
In hydrologic terms, problems and damages that occur during a flood as a result of human development and actions. Flood problems are a result from:
1) Inappropriate development in the floodplain (e.g., building too low, too close to the channel, or blocking flood flows);
2) Development in the watershed that increases flood flows and creates a larger floodplain, or;
3) A combination of the previous two.
Flood Profile
In hydrologic terms, a graph of elevation of the water surface of a river in flood, plotted as ordinate, against distance, measured in the downstream direction, plotted as abscissa. A flood profile may be drawn to show elevation at a given time, crests during a particular flood, or to show stages of concordant flows
Floodproofing
In hydrologic terms, the process of protecting a building from flood damage on site. Floodproofing can be divided into wet and dry floodproofing. In areas subject to slow-moving, shallow flooding, buildings can be elevated, or barriers can be constructed to block the water’s approach to the building. These techniques have the advantage of being less disruptive to the neighborhood. It must be noted that during a flood, a floodproofed building may be isolated and without utilities and therefore unusable, even though it has not been damaged.
Flow Separation
The process by which a separation eddy forms on the windward or leeward sides of bluff objects or steeply rising hillsides.
Flow Splitting
The splitting of a stable airflow around a mountain barrier, with branches going around the left and right edges of the barrier, often at accelerated speeds.
Foehn Pause
A temporary cessation of the foehn at the ground due to the formation or intrusion of a cold air layer which lifts the foehn off the ground.
Forecast Periods
Official definitions for NWS products:

Today...............................Sunrise to sunset
This afternoon..................noon till 6 p.m.
This evening.....................6 p.m. till sunset
Tonight.............................sunset till sunrise
Tomorrow.........................sunrise to sunset of the following day
Forecast Point
A location along a river or stream for which hydrologic forecast and warning services are provided by a WFO. The observed/forecast stage or discharge for a given forecast point can be assumed to represent conditions in a given reach (see /reach/).
FPO
Flood Potential Outlook
FPS
Fujita-Pearson Scale
Free Atmosphere
The part of the atmosphere that lies above the frictional influence of the earth's surface.
Freezeup date
In hydrologic terms, the date on which the water body was first observed to be completely frozen over
Freezing Spray
An accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Freezing Spray Advisory
An advisory for an accumulation of freezing water droplets on a vessel at a rate of less than 2 centimeters (cm) per hour caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Freezup jam
In hydrologic terms, ice jam formed as frazil ice accumulates and thickens
FROPA
Frontal Passage
Frost Point
Dew point below freezing.
FTPMAIL
An Internet server application which provides access to Internet FTP server files via e-mail. The National Weather Service operates an FTPMAIL server which provides e -mail access to any product available on the tgftp.nws.noaa.gov FTP server including marine text and graphic forecasts. For further information see: http://weather.noaa.gov/pub/fax/ftpmail.txt, or send an e-mail to ftpmail@ftpmail.nws.noaa.gov with the word "help" in the body.
Full-Physics Numerical Model
A computer model used to calculate air pollution concentrations. A full-physics numerical model uses a full set of equations describing the thermodynamic and dynamic state of the atmosphere and can be used to simulate atmospheric phenomena.
Gap Winds
Strong winds channeled through gaps in the Pacific coastal ranges, blowing out into the Pacific Ocean or into the waterways of the Inside Passage. The winds blow through low passes where major river valleys issue onto the seaways when strong east-west pressure gradients exist between the coast and the inland areas, with low pressure over the ocean.
Gaussian Plume Model
A computer model used to calculate air pollution concentrations. The model assumes that a pollutant plume is carried downwind from its emission source by a mean wind and that concentrations in the plume can be approximated by assuming that the highest concentrations occur on the horizontal and vertical midlines of the plume, with the distribution about these mid-lines characterized by Gaussian- or bell-shaped concentration profiles.
Gaussian Puff Model
A model used to calculate air pollution concentrations. The model assumes that a continuously emitted plume or instantaneous cloud of pollutants can be simulated by the release of a series of puffs that will be carried in a time- and space-varying wind field. The puffs are assumed to have Gaussian or bell-shaped concentration profiles in their vertical and horizontal planes.
GEMPAK
General Environmental Meteorological Package (programming language)
Geophysical Events
In solar-terrestrial terms, flares (Importance two or larger) with Centimetric Outbursts (maximum of the flux higher than the Quiet Sun flux, duration longer 10 minutes) and/or strong SID. Sometimes these flares are followed by Geomagnetic Storms or small PCA. (Class M Flares)
Geophysics
In hydrologic terms, the study of the physical characteristics and properties of the earth; including geodesy, seismology, meteorology, oceanography, atmospheric electricity, terrestrial magnetism, and tidal phenomena.
Geopotential Height
The height above sea level of a pressure level. For example, if a station reports that the 500 mb height at its location is 5600 m, it means that the level of the atmosphere over that station at which the atmospheric pressure is 500 mb is 5600 meters above sea level. This is an estimated height based on temperature and pressure data.
Geostrophic Wind
A wind that is affected by coriolis force, blows parallel to isobars and whose strength is related to the pressure gradient (i.e., spacing of the isobars).
Global Temperature Change
The net result of four primary factors including the greenhouse effect, changes in incoming solar radiation, altered patterns of ocean circulations, and changes in continental position, topography and/or vegetation. Three feedback mechanisms which affect global temperature change include cloud height and amount, snow and ice distribution, and atmospheric water vapor levels.
GPS
An acronym for Global Positioning System. A network of satellites which provide extremely accurate position and time information. Useful in remote locations or for moving platforms.
Graupel
Same as snow pellets or small hail.
Gravity Recovery and Climate Experiment (GRACE)
NASA satellites that detect small changes in the Earth’s gravitational field caused by the redistribution of water on and beneath the land surface.
Great Lakes Freeze-Up/Break-Up Outlook
(FBO) - A National Weather Service product to keep mariners informed of the projected freeze-up date or break-up date of ice on the Great Lakes.
Group Velocity
The speed at which a particular wave front or swell train advances.
GSP
On a buoy report, maximum 5-second peak gust during the measurement hour, reported at the last hourly 10-minute segment.
H-Alpha
In solar-terrestrial terms, this absorption line of neutral hydrogen falls in the red part of the visible spectrum and is convenient for solar observations. The H-alpha line is universally used for patrol observations of solar flares.
H-component of the Geomagnetic Field
(Geomagnetic Elements) In solar-terrestrial terms, the components of the geomagnetic field at the surface of the earth. In SESC use, the northward and eastward components are often called the H and D components, where the D component is expressed in gammas and is derived from D (the declination angle) using the small angle approximation.
Hail Spike
An area of reflectivity extending away from the radar immediately behind a thunderstorm with extremely large hail. In an area of large hail, radiation from the radar can bounce from hailstone to hailstone before being reflected back to the radar. The time delay between the backscattered radiation from the storm and the bounced and scattered radiation from the large hail causes the reflectivity from the hail to appear to come from a farther range than the actual storm.
Heavy Freezing Spray
An accumulation of freezing water droplets on a vessel at a rate of two cm per hour or greater (0.7 cm per hour or greater in Alaska Region) caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Heavy Freezing Spray Warning
A warning for an accumulation of freezing water droplets on a vessel at a rate of two cm per hour or greater (0.7 cm per hour or greater in Alaska Region) caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.
Heavy Freezing Spray Watch
A watch for an increased risk of a heavy freezing spray event to meet Heavy Freezing Spray Warning criteria but its occurrence, location, and/or timing is still uncertain.
Hectopascal
A unit of pressure equal to a millibar (1 hPa = 1 mb). Abbreviated hPa.
High Resolution Rapid Refresh (HRRR)
A real-time 3-km resolution, hourly updated, cloud-resolving, convection-allowing atmospheric model, initialized by 3km grids with 3km radar assimilation.
High Water Probability
The probability (%) that a stream will reach its high water flow within the forecast period.
High-Speed Stream
In solar-terrestrial terms, a feature of the solar wind having velocities that are about double average solar wind values.
Hodograph
A polar coordinate graph which shows the vertical wind profile of the lowest 7000 meters of the atmosphere. These plots are used to determine the advection patterns aloft, whether a thunderstorm will rotate, and the type of thunderstorms that you will likely see that day.
Hourly Precipitation Data (HPD)
It contains data on nearly 3,000 hourly precipitation stations (National Weather Service, Federal Aviation Administration, and cooperative observer stations) in inches to tenths or inches to hundredths at local standard time. HPD includes maximum precipitation for nine (9) time periods from 15 minutes to 24 hours, for selected stations.
HP Storm
or HP Supercell - High-Precipitation storm (or High-Precipitation supercell). A supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone.

Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large damaging hail events.
hPa
Hectopascal- A unit of pressure equal to a millibar (1 hPa = 1 mb).
HPC
Hydrometeorological Prediction Center
Hydraulic Jump
A steady disturbance in the lee of a mountain, where the airflow passing over the mountain suddenly changes from a region of low depth and high velocity to a region of high depth and low velocity.
Hydraulic Permeability
In hydrologic terms, the flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
Hydrograph
In hydrologic terms, a graph showing the water level (stage), discharge, or other property of a river volume with respect to time.
Hydrograph Separation
In hydrologic terms, the process where the storm hydrograph is separated into baseflow components and surface runoff components.
Hydrographic Survey
In hydrologic terms, an instrumental survey to measure and determine characteristics of streams and other bodies of water within an area, including such things as location, areal extent, and depth of water in lakes or the ocean; the width, depth, and course of streams; position and elevation of high water marks; location and depth of wells, etc.
Hyetograph
A graphical representation of rainfall intensity with respect to time.
Hygroscopic
Absorbing or attracting moisture from the air.
Hypothermia
A rapid, progressive mental and physical collapse that accompanies the lowering of body temperature.
Ice Pellets
(abbrev. IP) Same as Sleet; defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. A Winter Storm Warning is issued for sleet or a combination of sleet and snow based on total accumulation which is locally defined by area.
Ice Push
In hydrologic terms, compression of an ice cover particularly at the front of a moving section of ice cover.
Impermeable
Material that does not permit fluids to pass through it.
Impervious
In hydrologic terms, the ability to repel water, or not let water infiltrate
IMPL
Impulse- Alternate term for Upper Level System and Shortwave; a general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.
Import
In hydrologic terms, water piped or channeled into an area
IMPT
Important
Impulse
(abbrev. IMPL) Alternate term for Upper Level System and Shortwave; a general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.
Inactive Storage Capacity
In hydrologic terms, the portion of capacity below which the reservoir is not normally drawn, and which is provided for sedimentation, recreation, fish and wildlife, aesthetic reasons, or for the creation of a minimum controlled operational or power head in compliance with operating agreements or restrictions.
Industrial Consumption
The quantity of water consumed in a municipality or district for mechanical, trade, and manufacturing purposes, in a given period, generally one day. The per capita use is generally based on the total population of the locality, municipality, or district.
Infiltration Capacity
In hydrologic terms, the maximum rate at which water can enter the soil at a particular point under a given set of conditions.
Influent Seepage
In hydrologic terms, movement of gravity water in the zone of aeration from the ground surface toward the water table.
Interception
In hydrologic terms, the process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation, and lost by evaporation, never reaching the surface of the ground. Interception equals the precipitation on the vegetation minus streamflow and through fall.
Interception Storage Requirements
In hydrologic terms, water caught by plants at the onset of a rainstorm. This must be met before rainfall reaches the ground.
Intermediate Synoptic Times
The times of 0300, 0900, 1500, and 2100 UTC.
Interplanetary Magnetic Field
(abbrev. IMF) In solar-terrestrial terms, the magnetic field carried with the solar wind.
Interpolate
To estimate a value within an interval between two known values. This technique is sometimes used with computer models for locations in between the model's "gridpoints."
Intertropical Convergence Zone
(ITCZ) The region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.
Ionosphere
A complex atmospheric zone of ionized gases that extends between 50 and 400 miles (80 to 640 kilometers) above the earth's surface. It is located between the mesosphere and the exosphere and is included as part of the thermosphere.
Ionospheric Storm
A disturbance in the F region of the ionosphere, which occurs in connection with geomagnetic activity
IP
Ice pellets (sleet). See "PL"
IPV
Improve
Isentropic Analysis
A way in the forecaster can look at the atmosphere in 3-dimensions instead of looking at constant pressure surfaces (such as the 850 mb, 700 mb, 500 mb, etc.) which are in 2-dimensions. In this analysis method, the forecaster looks at constant potential temperature (the temperature that it would take if we compressed or expanded it adiabatically to the pressure of 1000 mb) surfaces. Air parcels move up and down these surfaces; therefore, the forecaster can see where the moisture is located and how much moisture is available.
Isentropic Lift
Lifting of air that is traveling along an upward-sloping isentropic surface.

Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.
Isentropic Surface
A two-dimensional surface containing points of equal potential temperature.
Isobaric Process
Any thermodynamic change of state of a system that takes a place at constant pressure.
Isodop
A contour of constant Doppler velocity values.
Isopleth
A broad term for any line on a weather map connecting points with equal values of a particular atmospheric variable (temperature, dew point, etc.). Isotherms, isotachs, etc. are all examples of isopleths.
Isotropic
Having the same characteristics in all directions, as with isotropic antennas. Directional or focused antennas are not isotropic.
Jet Wind Speed Profile
A vertical wind speed profile characterized by a relatively narrow current of high winds with slower moving air above and below. A large wind (speed) shear occurs above and below the jet axis.
Jokulhlaup
In hydrologic terms, an Icelandic term meaning glacier dammed lake outburst flood
Kelvin Temperature Scale
An absolute temperature scale in which a change of 1 Kelvin equals a change of 1 degree Celsius; 0ºK is the lowest temperature on the Kelvin scale. The freezing point of water is +273ºK (Kelvin) and the boiling point of +373ºK. It is used primarily for scientific purposes. It is also known as the Absolute Temperature Scale.
Kilopascal
The internationally recognized unit used by the Atmospheric Environment Service for measuring atmospheric pressure. Abbreviated kPa.
Kp Index
A 3-hourly planetary geomagnetic index of activity generated in Gottingen, Germany, based on the K Index from 12 or 13 stations distributed around the world
Landspout
[Slang], a tornado that does not arise from organized storm-scale rotation and therefore is not associated with a wall cloud (visually) or a mesocyclone (on radar). Landspouts typically are observed beneath Cbs or towering cumulus clouds (often as no more than a dust whirl), and essentially are the land-based equivalents of waterspouts.
Lapse Rate
The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.
Last Update
The time and date in which the forecast was issued or updated. The forecast may be updated at any time as weather conditions warrant.
Layer Composite Reflectivity Average
This WSR-88D radar product displays the average reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.
Layer Composite Reflectivity Maximum
This WSR-88D radar product displays the maximum reflectivities for a layer. Data is taken from all elevation angles contained in a given layer for each grid box. It is available for 3 layers (low, mid, high). Currently, the low layer extends from the surface to 24,000 feet, the mid layer extends from 24,000 feet to 33,000 feet, and high layer extends above 33,000 feet. It is used to aid in determining storm intensity trends by comparing mid level layer composite products with a low level elevation angle base reflectivity product and aid in routing air traffic.
Leader Spot
In solar-terrestrial terms, in a magnetically bipolar or multipolar sunspot group, the western part precedes and the main spot in that part is called the leader.
LEWP
Line Echo Wave Pattern. A bulge in a thunderstorm line producing a wave-shaped "kink" in the line. The potential for strong outflow and damaging straight-line winds increases near the bulge, which often resembles a bow echo. Severe weather potential also is increased with storms near the crest of a LEWP.
Line Echo Wave Pattern
(abbrev. LEWP) A radar echo pattern formed when a segment of a line of thunderstorms surges forward at an accelerated rate.
Lithosphere
In hydrologic terms, that part of the earth which is composed predominantly of rocks (either coherent or incoherent, and including the disintegrated rock materials known as soils and subsoils), together with everything in this rocky crust.
Live Capacity
In hydrologic terms, the total amount of storage capacity available in a reservoir for all purposes, from the dead storage level to the normal water or normal pool level surface level. Does not include surcharge, or dead storage, but does include inactive storage, active conservation storage and exclusive flood control storage.
Loop Prominence System
(abbrev. LPS) In solar-terrestrial terms, a system of loop prominences associated with major flares.
Looping
A pattern of plume dispersion in an unstable atmosphere, in which the plume undergoes marked vertical oscillations as it is alternately affected by rising convective plumes and the subsiding motions between the plumes.
LOPRES
low pressure
Low Pressure System
An area of a relative pressure minimum that has converging winds and rotates in the same direction as the earth. This is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Also known as an cyclone, it is the opposite of an area of high pressure, or a anticyclone.
LP Storm
Low-Precipitation storm (or Low-Precipitation supercell). A supercell thunderstorm characterized by a relative lack of visible precipitation. Visually similar to a classic supercell, except without the heavy precipitation core. LP storms often exhibit a striking visual appearance; the main tower often is bell-shaped, with a corkscrew appearance suggesting rotation. They are capable of producing tornadoes and very large hail. Radar identification often is difficult relative to other types of supercells, so visual reports are very important. LP storms almost always occur on or near the dry line, and thus are sometimes referred to as dry line storms.
LPS
Loop Prominence System- In solar-terrestrial terms, a system of loop prominences associated with major flares.
Magnetopause
In solar-terrestrial terms, the boundary layer between the solar wind and the magnetosphere.
Magnetosphere
In solar-terrestrial terms, the magnetic cavity surrounding the earth, carved out of the passing solar wind by virtue of the geomagnetic field, which prevents, or at least impedes, the direct entry of the solar wind plasma into the cavity
Main Synoptic Times
The times of 0000, 0600, 1200, and 1800 UTC. Also known as the standard synoptic times.
MAP
Mean Areal Precipitation- The average rainfall over a given area, generally expressed as an average depth over the area.
MAREP
(MArine REPort) A voluntary marine observation program of the National Weather Service whose goal is to solicit meteorological and oceanographic observations in plain language from recreational and small commercial mariners who are not part of Voluntary Observing Ship program.
Marine Push
A replacement of the current air mass with air from off the ocean. Temperatures are much cooler and relative humidities much higher. The air mass is generally much more stable in this situation.
Maritime Polar Air Mass
An air mass characterized by cold, moist air. Abbreviated mP.
Maritime Tropical Air Mass
An air mass characterized by warm, moist air. Abbreviated mT.
Max Parcel Level (MPL)
This signifies the highest attainable level that a convective updraft can reach; therefore, it is a good indication of how tall a thunderstorm may reach.
Maximum Spillway Discharge
In hydrologic terms, spillway discharge (cfs) when reservoir is at maximum designed water surface elevation.
Maximum Temperature
This is the highest temperature recorded during a specified period of time. Common time periods include 6, 12 and 24 hours. The most common reference is to the daily maximum temperature, or "high."
Mean Annual Temperature
The average temperature for the entire year at any given location.
Mean Areal Precipitation
(abbrev. MAP)- The average rainfall over a given area, generally expressed as an average depth over the area.
Mean Daily Temperature
The average of the highest and lowest temperatures during a 24-hour period.
Mean Depth
In hydrologic terms, the average depth of water in a stream channel or conduit. It is equal to the cross-sectional area divided by the surface width.
Mean Doppler Velocity
Reflectivity-weighted average velocity of targets in a given pulse resolution volume. Usually determined from a large number of successive radar pulses. Also called mean radial (towards or away from the antenna) velocity. Doppler velocity refers to spectral density first moment, radial velocity to base data.
Melting Point
The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.
Mesopause
The top of the mesosphere, corresponding to the level of minimum temperature in the atmosphere found at 70 to 80 km.
Mesoscale Convective Complex
(abbrev. MCC)- MCC - Mesoscale Convective Complex. A large Mesoscale Convective System (MCS), generally round or oval-shaped, which normally reaches peak intensity at night. The formal definition includes specific minimum criteria for size, duration, and eccentricity (i.e., "roundness"), based on the cloud shield as seen on infrared satellite photographs: * Size: Area of cloud top -32 degrees C or less: 100,000 square kilometers or more (slightly smaller than the state of Ohio), and area of cloud top -52 degrees C or less: 50,000 square kilometers or more. * Duration: Size criteria must be met for at least 6 hours. * Eccentricity: Minor/major axis at least 0.7. MCCs typically form during the afternoon and evening in the form of several isolated thunderstorms, during which time the potential for severe weather is greatest. During peak intensity, the primary threat shifts toward heavy rain and flooding.
Mesosphere
The atmospheric shell between about 20 km and about 70 to 80 km, extending from the top of the stratosphere (the stratopause) to the upper temperature minimum that defines the mesopause (the base of the thermosphere).
Microbarograph
A instrument designed to continuously record a barometer's reading of very small changes in atmospheric pressure.
Minimum Temperature
This is the lowest temperature recorded during a specified period of time. The time period can be 6, 12 or 24 hours. The most common reference is to the daily minimum temperature, or "low."
Mixed Precipitation
Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.
Mixing Depth
Vertical distance between the ground and the altitude to which pollutants are mixed by turbulence caused by convective currents or vertical shear in the horizontal wind.
MLCAPE
Mean Layer CAPE - CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level. See Convective Available Potential Energy (CAPE).
Model Output Statistics
(abbrev. MOS) - the Hydrometeorological Center (HPC) produces a short range (6 to 60 hours) MOS (Model Output Statistics) guidance package generated from the NGM, GFS, and ETA models for over 300 individual stations in the continental United States. These alphanumeric messages are made available at approximately 0400 and 1600 UTC for the 0000 and 1200 UTC forecast cycles, respectively. Model Output Statistics are a set of statistical equations that use model output to forecast the probability of precipitation, high and low temperature, cloud cover, and precipitation amount for many cities across the USA. The statistical equations were specifically tailored for each location, taking into account factors such as each location's climate.
Moist Adiabatic Lapse Rate
(abbrev. MALR)- The rate at which the temperature of a parcel of saturated air decreases as the parcel is lifted in the atmosphere. The moist adiabatic lapse rate (abbreviated MALR) is not a constant like the dry adiabatic lapse rate but is dependent on parcel temperature and pressure.
Monthly Climatological Report
This climatological product is issued once a month by each National Weather Service office. It is a mix of tabular and narrative information. It is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, heating/cooling degree information, etc.).
Mountain-Plain Wind System
A closed, large-scale, thermally driven circulation between the mountains and the surrounding plain. The mountain-to-plain flow making up the lower branch of the closed circulation usually occurs during nighttime, while the plain-to-mountain flow occurs during daytime.
MSLP
Mean sea level pressure
Multiple Doppler Analysis
The use of more than one radar (and hence more than one look angle) to reconstruct spatial distributions of the 2D or 3D wind field, which cannot be measured from a single radar alone. Includes dual Doppler, triple Doppler, and overdetermined multiple Doppler analysis.
Multiple Vortex Tornado
A tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other. Multiple-vortex tornadoes can be especially damaging.
Multipurpose Reservoir
In hydrologic terms, a reservoir constructed and equipped to provide storage and release of water for two or more purposes such as flood control, power development, navigation, irrigation, recreation, pollution abatement, domestic water supply, etc.
Municipal Use of Water
In hydrologic terms, the various uses to which water is put to use developed urban areas, including domestic use, industrial use, street sprinkling, fire protection, etc.
National Hurricane Operations Plan
(NHOP) - The NHOP is issued annually by the Federal Coordinator for Meteorological Services and Supporting Research. It documents interdepartmental agreements relating to tropical cyclone observing, warning, and forecasting services. National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and the JTWC serve as the principal offices in coordinating the day-to-day activities of the NWS in support of the Plan in their region of responsibility.
National Weather and Crop Summary
A product of the National Agricultural Statistics Service, Agricultural Statistics Board, and U.S. Department of Agriculture. It contains weekly national agricultural weather summaries, including the weather's effect on crops; summaries and farm progress for 44 states and New England area.
NCEP
National Centers for Environmental Prediction. A part of the National Weather Service which provides nationwide computerized and manual guidance to Warning and Forecast Offices concerning the forecast of basic weather elements.
Neap Tide
A minimum tide occurring at the first and third quarters of the moon.
NHOP
(National Hurricane Operations Plan) - The NHOP is issued annually by the Federal Coordinator for Meteorological Services and Supporting Research. It documents interdepartmental agreements relating to tropical cyclone observing, warning, and forecasting services. National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and the JTWC serve as the principal offices in coordinating the day-to-day activities of the NWS in support of the Plan in their region of responsibility.
Nieve Penitente
A spike or pillar of compacted snow, firn or glacier ice, caused by differential melting and evaporation. The pillars form most frequently on low-latitude mountains where air temperatures are near freezing, dew points are much below freezing and insolation is strong. Penitents are oriented individually toward the noon-day sun, and usually occur in east-west lines.
NOGAPS
Navy Operational Global Atmospheric Prediction System; a 144-hour numerical model of the atmosphere run by the U.S. Navy twice daily.
North Pacific High
A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate. Comparable systems are the Azores High and the Bermuda High.
Numerical Weather Prediction
Same as Numerical Forecasting; a computer forecast or prediction based on equations governing the motions and the forces affecting motion of fluids. The equations are based, or initialized, on specified weather or climate conditions at a certain place and time.
NWP
Numerical Weather Prediction
Obscuring Phenomena
Any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).
Oceanography
The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's physical boundaries, the chemistry and physics of sea water, and marine biology.
Office of Global Programs
The Office of Global Programs (OGP) sponsors focused scientific research, within approximately eleven research elements, aimed at understanding climate variability and its predictability. Through studies in these areas, researchers coordinate activities that jointly contribute to improved predictions and assessments of climate variability over a continuum of timescales from season to season, year to year, and over the course of a decade and beyond.
Opaque
A condition where a material, such as a cloud, blocks the passage of radiant energy, especially light. Opaque sky cover refers to the amount of sky cover that completely hides all that might be above it.
OPC
Ocean Prediction Center (Formally the Marine Prediction Center. An NCEP center which produces marine forecasts north of 30oN.
Open Lakes Forecast
(GLF) - A National Weather Service marine forecast product for the U.S. waters within a Great Lake not including the waters covered by an existing Nearshore Waters Forecast (NSH). When the seasonal Nearshore forecast is not issued, the Open Lake forecast includes a forecast of nearshore waters.
Operational Products
A product that has been fully tested and evaluated and is produced on a regular and ongoing basis.
Orographic
Related to, or caused by, physical geography (such as mountains or sloping terrain).
Orographic Lifting
Same as Upslope Flow; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.
Orographic Precipitation
Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.
Orographic Uplift
Same as Orographic Lifting; occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.
Orographic Waves
A wavelike airflow produced over and in the lee of a mountain barrier.
Orphan Anvil
Slang for an anvil from a dissipated thunderstorm, below which no other clouds remain.
Overshooting Top
(or Penetrating Top) - A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell.

A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm.
PAC
Pacific
Pacific Decadal Oscillation
(Abbrev. PDO) - a recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands.
Palmer Drought Severity Index
(Abbrev. PDSI) - an index used to gage the severity of drought conditions by using a water balance equation to track water supply and demand. This index is calculated weekly by the National Weather Service.
Pan Pan
A headline within National Weather Service high seas forecasts transmitted via the GMDSS to indicate that a hurricane or hurricane force winds are forecast.
Pancake Ice
In hydrologic terms, circular flat pieces of ice with a raised rim; the shape and rim are due to repeated collisions
Panhandle Hook
Low pressure systems that originate in the panhandle region of Texas and Oklahoma which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter, these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track.
Parameter
A subset of the group of evaluations that constitute each element of an observation.
Parapet Wall
In hydrologic terms, a solid wall built along the top of the dam for ornament, safety, or to prevent overtopping
Parcel
A volume of air small enough to contain uniform distribution of its meteorological properties and large enough to remain relatively self-contained and respond to all meteorological processes.
Parhelion
The scientific name for sun dogs. Either of two colored luminous spots that appear at roughly 22 degrees on both sides of the sun at the same elevation. They are caused by the refraction of sunlight passing through ice crystals. They are most commonly seen during winter in the middle latitudes and are exclusively associated with cirriform clouds. They are also known as mock suns.
Partial Beam Filling
A limitation of the rainfall estimation techniques used by NEXRAD. At far ranges from the radar, a storm may occupy only a portion of the radar beam (which may be several miles across). However, the radiation received by the radar antenna consists of the average reflectivity across the entire beam, so the reflectivity and associated rainfall rates are underestimated.
Partial-Duration Flood Series
In hydrologic terms, a list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year.
Particle Trajectory Model
A computer sub-model that tracks the trajectories of multiple particles that are released into an atmospheric flow model.
Partly Cloudy
Between 3/8 and 5/8 of the sky is covered by clouds.
Partly Sunny
Between 3/8 and 5/8 of the sky is covered by clouds. The term "Partly Sunny" is used only during daylight hours.
Pascal
The unit of pressure produced when one newton acts on one square meter (1 N/m2). It is abbreviated Pa.
PAT
Pattern
PBL
Probable
PC-GRIDDS
PC-Gridded Interactive Display and Diagnostic System - Allows the forecaster to view fields of gridded model output in contour or vector format. By doing this, the forecaster can extract relevant information from the numerical model grid-point data.
PCPN
Precipitation
PCT
precent
PD
Period
PDI
Palmer Drought Index
PDMT
Predominant
PDO
Pacific Decadal Oscillation - a recently described pattern of climate variation similar to ENSO though on a timescale of decades and not seasons. It is characterized by SST anomalies of one sign in the north-central Pacific and SST anomalies of another sign to the north and east near the Aleutians and the Gulf of Alaska. It primarily affects weather patterns and sea surface temperatures in the Pacific Northwest, Alaska, and northern Pacific Islands. Two main characteristics distinguish PDO from El Niño/Southern Oscillation (ENSO): first, 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months; second, the climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics- the opposite is true for ENSO. Several independent studies found evidence of just two full PDO cycles in the past century: cool" PDO regimes prevailed from 1890-1924 and again from 1947-1976, while "warm" PDO regimes dominated from 1925-1946 and from 1977 through (at least) the mid-1990's. Causes for the PDO are not currently known. Likewise, the potential predictability for this climate oscillation are not known.
PDS
Particularly Dangerous Situation (PDS) wording is used in rare situations when long-lived, strong and violent tornadoes are possible. This enhanced wording may also accompany severe thunderstorm watches for intense convective wind storms.
PDS Watch
The Particularly Dangerous Situation (PDS) wording is used in rare situations when long-lived, strong and violent tornadoes are possible. This enhanced wording may also accompany severe thunderstorm watches for intense convective wind storms.
PDSI
Palmer Drought Severity Index - an index used to gage the severity of drought conditions by using a water balance equation to track water supply and demand. This index is calculated weekly by the National Weather Service.
PDT
Pacific Daylight Time
Peak Discharge
In hydrologic terms, the rate of discharge of a volume of water passing a given location
Peak Flow Arrival Time (PFAT)
The time at which a stream reach is forecast to achieve its maximum flow within the forecast period.
Peak Gust
The highest instantaneous wind speed observed or recorded.
Peak Pulse
The amount of power transmitted by a radar during a given pulse. Note that because these pulses are widely spaced, the average power will be much smaller.
Peak Wind Speed
The maximum instantaneous wind speed since the last observation that exceeded 25 knots.
Pendant Echo
Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.
Penetrating Top
Same as Overshooting Top; a dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top (anvil dome) often is present on a supercell. A short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm.
Penumbra
In solar-terrestrial terms, the sunspot area that may surround the darker umbra or umbrae. It consists of linear bright and dark elements radial from the sunspot umbra.
Perched Groundwater
In hydrologic terms, local saturated zones above the water table which exist above an impervious layer of limited extent.
Percolation
In hydrologic terms, the movement of water, under hydrostatic pressure, through the interstices of a rock or soil, except the movement through large openings such as caves
Percolation Path
In hydrologic terms, the course followed by water moving or percolating through any other permeable material, or under a dam which rests upon a permeable foundation.
Perennial Stream
In hydrologic terms, a stream that flows all year round.
Perigee
The closest distance between moon and earth or the earth and sun.
Perihelion
The point on the annual orbit of a body (about the sun) that is closest to the sun; at present, the earth reaches this point on about 5 January. Opposite of aphelion.
Permafrost
A layer of soil at varying depths below the surface in which the temperature has remained below freezing continuously from a few to several thousands of years.
Permeability
In hydrologic terms, the ability of a material to transmit fluid through its pores when subjected to a difference in head.
Permeability Coefficient
In hydrologic terms, the rate of flow of a fluid through a cross section of a porous mass under a unit hydraulic gradient, at a temperature of 60 degrees Fahrenheit.
Permeameter
In hydrologic terms, a laboratory instrument for determining permeability by measuring the discharge through a sample of the material when a known hydraulic head is applied.
Persistence
Continuation of existing conditions. When a physical parameter varies slowly, the best prediction is often persistence
Persistence Forecast
A forecast that the current weather condition will persist and that future weather will be the same as the present (e.g., if it is raining today, a forecast predicting rain tonight).
Perturbation Model
A computer model used to calculate air pollution concentrations. A perturbation model produces a wind field from solutions to a simplified set of equations that describe atmospheric motions.
Pervious Zone
In hydrologic terms, a part of the cross section of an embankment dam comprising material of high permeability
PFD
QPF Discussion (issued by the HPC)
Phenomenological Model
A computer model used to calculate air pollution concentrations. A phenomenological model focuses on an individual phenomenon, such as plume impingement or fumigation.
PHFO
Honolulu National Weather Service Forecast Office
Phiezometer
In hydrologic terms, an instrument used to measure pressure head in a conduit, tank, soil, etc. They are used in dams to measure the level of saturation.
Photochemical Smog
Air pollution containing ozone and other reactive chemical compounds formed by the reaction of nitrogen oxides and hydrocarbons in the presence of sunlight.
Photosphere
The intensely bright portion of the sun visible to the unaided eye; the "surface" of the sun. Reaching temperatures estimated at about 11,000 degrees Fahrenheit, it is the portion of the sun's atmosphere which emits continuous electromagnetic radiation.
Phreatic water
In hydrologic terms, water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table. Also termed Groundwater.
PIBAL
Pilot balloon. A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Pilot Balloon
(Abbrev. PIBAL)- A small helium-filled meteorological balloon that is tracked as it rises through the atmosphere to determine how wind speed and direction change with altitude.
Pilot Report
(Abbrev. PIREP)- A report of inflight weather by an aircraft pilot or crew member. A complete coded report includes the following information in this order: location and/or extent of reported weather phenomenon: type of aircraft (only with reports turbulence or icing).
Pingo
A large frost mound of more than one-year duration.
PIREP
Pilot Report. A report of inflight weather by an aircraft pilot or crew member. A complete coded report includes the following information in this order: location and/or extent of reported weather phenomenon: type of aircraft (only with reports turbulence or icing).
Pitot Tube
In hydrologic terms, a device for measuring the velocity of flowing water using the velocity head of the stream as an index of velocity. It consists essentially of an orifice held to a point upstream in the water, connected with a tube in which the rise of water due to velocity head may be observed and measured. It also may be constructed with an upstream and downstream orifice, with two water columns, in which case the difference in height of water column in the tubes is the index of velocity.
PIX
picture
PK
Peak
PL
Sleet (Ice Pellets)- defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of ½" or more.
PL
Ice pellets (sleet)
Plage
In solar-terrestrial terms, an extended emission feature of an active region that exists from the emergence of the first magnetic flux until the widely scattered remnant magnetic fields merge with the background.
Plage Corridor
In solar-terrestrial terms, a space in chromospheric plage lacking plage intensity, coinciding with polarity inversion line.
Plagenil
In solar-terrestrial terms, spotless disc free of calcium plage.
Plan Position Indicator
An acronym for Plan Position Indicator. A PPI displays radar data horizontally using a map projection. In PPI mode, the radar makes a 360-degree sweep with the antenna at a specific elevation angle. A PPI display is the familiar radar display shown on the television weather programs.
Planetary Boundary Layer
The layer within the atmosphere between 1 km and the earth's surface where friction affects wind speed and wind direction.
Plasma
Any ionized gas; that is, any gas containing ions and electrons.
Platform
A generic radar term, often used to encompass the pedestal and antenna assembly; sometimes including the radar control, display and analysis hardware and software as well.
Plow Wind
A term used in the midwestern United States to describe strong, straight-line winds associated with the downdrafts spreading out in advance of squall lines and thunderstorms. Resulting damage is usually confined to narrow zones like that caused by tornadoes; however, the winds are all in one direction (straight-line winds).
Plume Blight
Visibility impairment caused by air pollution plumes aggregated from individual sources.
Plume Impingement
The collision of a plume with topography that rises above the plume altitude; often a temporary condition that occurs as the plume sweeps by the face of a hill as the wind shifts.
Plume-dominated Fire
A fire whose behavior is governed primarily by the local wind circulation produced in response to the strong convection above the fire rather than by the general wind.
Pluvial
In hydrology, anything that is brought about directly by precipitation.
PMD
Prognostic Discussion
PMO
Port Meteorological Officer.
PNA
Pacific North American teleconnection
PNHDL
Panhandle
PNS
Public Information Statement - a narrative statement issued by a National Weather Service Forecast Office that can be used for:

1) A current or expected nonhazardous event of general interest to the public that can usually be covered with a single message (e.g., unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell).

2) Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc.

3) Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data. For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.
POH
Probability of Hail - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm.
Point Precipitation
Precipitation at a particular site, in contrast to the mean precipitation over an area.
Point Source
A pollutant source that can be treated in a dispersion model as though pollutants were emitted from a single point that is fixed in space. Example: the mouth of a smokestack. Compare area source and line source.
Polar Cap Absorption (PCA)
In solar-terrestrial terms, an anomalous condition of the polar ionosphere whereby HF and VHF (3 - 300 MHz) radiowaves are absorbed, and LF and VLF (3 - 300 kHz) radiowaves are reflected at lower altitudes than normal. In practice, the absorption is inferred from the proton flux at energies greater than 10 MeV, so that PCAs and proton events are simultaneous. Transpolar radio paths may still be disturbed for days, up to weeks, following the end of a proton event.
Polar Front
A semipermanent, semicontinuous front that separates tropical air masses from polar air masses.
Polar Jet
Marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.
Polar Jet Stream
Used interchangably with Polar Jet; a jet stream marked by a concentration of isotherms and strong vertical shear, this jet is the boundary between the polar air and the subtropical air. It often divides into two branches, the north and the south, and marks the high speed core of the prevailing westerlies. It is associated with the location and motion of the high and low pressure areas of the middle latitudes, and therefore, is variable in position, elevation, and wind speed. Its position tends to migrate south in the Northern Hemispheric winter and north in the summer, and its core winds increase during the winter and become less strong in the summer.
Polar Orbiting Satellite
A weather satellite which travels over both poles each time it orbits the Earth. It orbits about 530 miles (850 km) above the Earth's surface. A satellite with an orbit nearly parallel to the earth's meridian lines which crosses the polar regions on each orbit.
Polarization Radar
A radar which takes advantage of ways in which the transmitted waves' polarization affect the backscattering. Such radars may alternately transmit horizontal and vertically polarized beams, and measure differential reflectivity.
Pollutant
Particles, gases, or liquid aerosols in the atmosphere which have an undesirable effect on humans or their surroundings. Something unfavorable to health and life that has been added to the environment.
Pondage
In hydrologic terms,
(1) The holding back of water for later release for power development above the dam of a hydroelectric plant to
(a) equalize daily or weekly fluctuations of streamflow or
(b) to permit irregular hourly use of water by the wheels to care for fluctuations in the load demand.
(2) In general the holding back of water for later releases.
(3) The storage capacity available for the use of such water.
Ponding
In hydrologic terms, in flat areas, runoff collects, or ponds in depression and cannot drain out. Flood waters must infiltrate slowly into the soil, evaporate, or be pumped out.
Pool
The elevation of the surface of a body of water such as a lake. Specifically, the pool at a lock and dam or a reservoir is the elevation of the water surface immediately upstream from the dam.
Pool Height
In hydrologic terms, the height of the water behind a dam. (Various datums may be used and various pool height may be used, e.g., conservation pool, flood control pool, etc.)
POP
Probability of Precipitation
Popcorn Convection
Slang for showers and thunderstorms that form on a scattered basis with little or no apparent organization, usually during the afternoon in response to diurnal heating. Individual thunderstorms typically are of the type sometimes referred to as air-mass thunderstorms: they are small, short-lived, very rarely severe, and they almost always dissipate near or just after sunset.
POPS
Probability of Precipitation
Porosity
In hydrologic terms,
(1) The ratio of pore volume to total volume of the formation. Sandy soils have large pores and a higher porosity than clays and other fine-grained soils.
(2) An index of the void characteristics of a soil or stream as pertaining to percolation; degree of previousness.
POS
Positive
Positive Area
The area on a sounding representing the layer in which a lifted parcel would be warmer than the environment; thus, the area between the environmental temperature profile and the path of the lifted parcel. Positive area is a measure of the energy available for convection; see CAPE.
Positive Cloud to Ground Lightning
A CG flash that delivers positive charge to the ground, as opposed to the more common negative charge. Positive CGs have been found to occur more frequently in some severe thunderstorms. Their occurrence is detectable by most lightning detection networks, but visually it is not considered possible to distinguish between a positive CG and a negative CG. (Some claim to have observed a relationship between staccato lightning and positive CGs, but this relationship is as yet unproven.)
Positive Vorticity Advection
(Abbrev. PVA) - Advection of higher values of vorticity into an area, which often is associated with upward motion (lifting) of the air. PVA typically is found in advance of disturbances aloft (i.e., shortwaves), and is a property which often enhances the potential for thunderstorm development.
Positive-tilt Trough
An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.
Post-Flare Loops
In solar-terrestrial terms, a loop prominence system often seen after a major two-ribbon flare, which bridges the ribbons.
Post-storm Report
A report issued by a local National Weather Service office summarizing the impact of a tropical cyclone on it's forecast area. These reports include information on observed winds, pressures, storm surges, rainfall, tornadoes, damage and casualties.
Potential Temperature
The temperature a parcel of dry air would have if brought adiabatically (i.e., without transfer of heat or mass) to a standard pressure level of 1000 mb.
Potential Vorticity
This plays an important role in the generation of vorticity in cyclogenesis, especially along the polar front. It is also very useful in tracing intrusions of stratospheric air deep into the troposphere in the vicinity of jet streaks.
Powder Snow
Dry, loose, unconsolidated snow.
Power
A radar equation to describe the amount of power that a radar emits.

P = I * V (or)
P = V2 / R (or)
P = I2 / R

where I is current (amps), V is voltage (volts), R is resistance (ohms), P is power (watts).
PPI
Plan Position Indicator
PPINE
Plan Position Indicates No Echoes, referring to the fact that a radar detects no precipitation within its range. An intensity-modulated display on which echo signals are shown in plain view with range and azimuth angle displayed in polar coordinates, forming a map-like display. Each PPI is taken at a single, fixed elevation angle, and thus forms a cone of coverage in space. PPIs may be run in sequence, creating a "volume scan".
PQPF
Probabilistic QPF; a form of QPF (see below) that includes an assigned probability of occurrence for each numerical value in the forecast product.
PRBLTY
Probability
PRBLY
probably
PRCP
Precipitation
Pre-Frontal Squall Line
A line of thunderstorms that precedes an advancing cold front.
Pre-Frontal Trough
An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.
Pre-Hurricane Squall Line
It is often the first serious indication that a hurricane is approaching. It is a generally a straight line and resembles a squall-line that occurs with a mid-latitude cold front. It is as much as 50 miles or even more before the first ragged rain echoes of the hurricane's bands and is usually about 100 to 200 miles ahead of the eye, but it has been observed to be as much as 500 miles ahead of the eye in the largest hurricanes.
PRECD
Precede
Precipitable Water
Measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
Precipitation
The process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.
Precipitation Attenuation
The loss of energy that radar beam experiences as it passes through an area of precipitation.
Precipitation Mode
The standard, or default, operational mode of the WSR-88D. The radar automatically switches into precipitation mode from clear-air mode if the measured reflectivity exceeds a specific threshold value. The precipitation mode of NEXRAD is more sensitive than previous weather radars. The minimum detectable reflectivity in NEXRAD's precipitation mode is 5 dBZ, compared to 28 dBZ with the old WSR-57.
Precipitation Processing System
The WSR-88D system that generates 1-hour running, 3-hourly, and running storm total precipitation accumulations. Five functional steps are performed to calculate the best estimate of precipitation: 1) development of a sectorized hybrid scan, 2) conversion to precipitation rate, 3) precipitation accumulation, 4) adjustment using rain gages, 5) product update.
Precision
The accuracy with which a number can be represented, i.e., the number of digits used to represent a number.
Predominant Wind
The wind that prevails and generates the local component of the significant sea conditions across the forecast area. This is the wind included in all marine forecast products and is defined as a 10-meter wind, except over the nearshore marine zones where it is defined to be the wind at a 3-meter height.
Preliminary Report
Now known as the "Tropical Cyclone Report". A report summarizing the life history and effects of an Atlantic or eastern Pacific tropical cyclone. It contains a summary of the cyclone life cycle and pertinent meteorological data, including the post-analysis best track (six-hourly positions and intensities) and other meteorological statistics. It also contains a description of damage and casualties the system produced, as well as information on forecasts and warnings associated with the cyclone. NHC writes a report on every tropical cyclone in its area of responsibility.
PRES
Pressure
Prescribed Fire
A management ignited or natural wildland fire that burns under specified conditions where the fire is confined to a predetermined area and produces the fire behavior and fire characteristics required to attain planned fire treatment and resource management objectives.
Present Movement
The best estimate of the movement of the center of a tropical cyclone at a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.
Present Weather
The type of weather observed at the reporting time. These conditions may include types and intensity of precipitation such as light rain or heavy snow, as well as the condition of the air environment such as foggy, hazy or blowing dust.
Pressure
The exertion of force upon a surface by a fluid (e.g., the atmosphere) in contact with it.
Pressure Altimeter
An aneroid barometer calibrated to indicate altitude in feet instead of units of pressure. It is read accurately only in a standard atmosphere and when the correct altimeter setting is used.
Pressure Altitude
The altitude in standard atmosphere at which a given pressure will be observed. It is the indicated altitude of a pressure altimeter at an altitude setting of 29.92 inches of mercury, and is therefore the indicated altitude above the 29.92 constant pressure surface.
Pressure Change
The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.
Pressure Characteristic
The pattern of the pressure change during the specified period of time, usually the three hour period preceding an observation. This is recorded in three categories: falling, rising, or steady.
Pressure Couplet
It is an area where you have a high pressure area located adjacent to a low pressure area.
Pressure Falling Rapidly
A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.
Pressure Gage
A device for registering the pressure of solids, liquids, or gases. It may be graduated to register pressure in any units desired.
Pressure Gradient
The amount of pressure change occurring over a given distance.
Pressure Gradient Force
A three-dimensional force vector operating in the atmosphere that accelerates air parcels away from regions of high pressure and toward regions of low pressure in response to an air pressure gradient. Usually resolved into vertical and horizontal components.
Pressure Head
Energy contained by fluid because of its pressure, usually expressed in feet of fluid (foot pounds per pound).
Pressure Ice
Floating sea, river, or lake ice that has been deformed, altered, or forced upward in pressure ridges by the lateral stresses of any combination of wind, water currents, tides, waves, and surf.
Pressure Induced Wave
A rare type of wave that does not develop from wind or seismic activity. Instead, these waves develop as a pressure perturbation moves over the water surface. The water surface adjusts to account for the atmospheric pressure change. As atmospheric pressure decreases, the force exerted upward by the water increases, creating a pressure induced wave.
Pressure Jump
A sudden, sharp increase in atmospheric pressure, typically occurring along an active front and preceding a storm.
Pressure Rising Rapidly
An increase in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.
Pressure Tendency
The character and amount of atmospheric pressure change during a specified period of time, usually 3-hour period preceding an observation.
Pressure Unsteady
A pressure that fluctuates by 0.03 inch of mercury or more from the mean pressure during the period of measurement.
Pressure-driven Channeling
Channeling of wind in a valley by synoptic-scale pressure gradients superimposed along the valley's axis. Compare forced channeling.
Prevailing Visibility
The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous.
Prevailing Westerlies
The westerly winds that dominant in middle latitudes.
Prevailing Winds
A wind that consistently blows from one direction more than from any other.
Prevention of Significant Deterioration
A program, specified in the Clean Air Act, whose goal is to prevent air quality from deteriorating significantly in areas of the country that are presently meeting the ambient air quality standards.
PRIM
Primary
Primary Ambient Air Quality Standards
Air quality standards designed to protect human health.
Primary Control Tide Station
A tide station where continuous observations have been made for a minimum of 19 years. Its purpose is to provide data for computing accepted values essential to tide predictions and for determining tidal datums for coastal and marine boundaries. The data series from primary control tide stations serves as a primary control for the reduction of tidal datum for subordinate tide stations with a shorter period of record. The 19 year period is the official tidal epoch for calculating tidal datums.
Primary Pollutant
Substances that are pollutants immediately on entering the atmosphere. Compare secondary pollutant.
Primary Swell Direction
Prevailing direction of swell propagation.
PRIN
Principal
Probability
A chance, or likelihood, that a certain event might happen.
Probability Forecast
A forecast of the probability that one or more of a mutually exclusive set of weather conditions will occur.
Probability of Hail
(Abbrev. POH) - a product from the NEXRAD hail detection algorithm that estimates the likelihood that hail is present in a storm.
Probability of Precipitation
(Abbrev. PoP)- The probability that precipitation will be reported at a certain location during a specified period of time.
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.
Probability of Tropical Cyclone Conditio
The probability, in percent, that the cyclone center will pass within 50 miles to the right or 75 miles to the left of the listed location within the indicated time period when looking at the coast in the direction of the cyclone's movement.
Product Resolution
The smallest spatial increment or data element that is distinguishable in a given Doppler radar product.
Profiler
An instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.
PROG
Forecast (prognostication)
PROGGED
Forecasted
Prognostic Discussion
This Hydrometeorological Prediction Center (HPC) discussion may include analysis of numerical and statistical models, meteorological circulation patterns and trends, and confidence factors. Reference is usually made to the manually produced 6- to 10-day Northern Hemisphere prognoses for mean 500 millibar heights and mean 500 millibar height anomalies. Discussions may also refer to the method of operational ensemble predictions.
Progressive Derecho
Derecho characterized by a short curved squall line oriented nearly perpendicular to the mean wind direction with a bulge in the general direction of the mean flow. Downburst activity occurs along the bulging portion of the line. This type of derecho typically occurs in the warm season (May through August) and is most frequent in a zone extending from eastern South Dakota to the upper Ohio Valley. The severe wind storms typically begin during the afternoon and continue into the evening hours. Several hours typically pass between initial convection and the first wind damage report.
Prominence
A term identifying cloud-like features in the solar atmosphere. The features appear as bright structures in the CORONA above the solar LIMB and as dark FILAMENTs when seen projected against the solar DISK
PROPA
Propagation (movement)
Propagation
1. The movement of an atmospheric phenomenon. This term is frequently applied to the motion of thunderstorms into regions favorable for their continued development (into a maritime tropical airmass).

2. The transmission of electromagnetic energy as waves through or along a medium.
Property Protection
Measures that are undertaken usually by property owners in order to prevent, or reduce flood damage. Property protection measures are often inexpensive for the community because they are implemented by or cost-shared with property owners. In many cases the buildings' appearance or use is unaffected, so these measurements are particularity appropriate for historical sites and landmarks. These measures include relocation and acquisition, flood proofing, and buying flood insurance.
Proton
Solar activity levels with at least one high energy event (Class X Flares).
Proton Event
In solar-terrestrial terms, the measurement of at least 10 protons/cm2/sec/steradian at energies greater than 10 MeV.
Proton Flare
In solar-terrestrial terms, any flare producing significant fluxes of greater-than-10 MeV protons in the vicinity of the earth.
PRST
Persist
PRVD
Provide
PSBL
Possible
PSBLY
possibly
Pseudo-Cold Front
A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the mesocyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded mesocyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front.
Pseudo-Warm Front
A boundary between a supercell's inflow region and the forward-flank downdraft (or FFD). It extends outward from at or near the mesocyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the mesocyclone.
PSG
Passage
PST
Pacific Standard Time
Psychrometer
An instrument used to measure the water vapor content of the air; a hygrometer consisting essentially of two similar thermometers with the bulb of one being kept wet so that the cooling that results from evaporation makes it register a lower temperature than the dry one and with the difference between the readings constituting a measure of the dryness of the atmosphere
PTCHY
Patchy
PTCLDY
Partly Cloudy
PTDY
On a buoy report, Pressure Tendency is the sign (plus or minus) and the amount of pressure change (hPa) for a three hour period ending at the time of observation.
PTLY
Partly
PTN
Portion
PTTN
pattern
PTWC
(Pacific Tsunami Warning Center) - The Pacific Tsunami Warning Center in Ewa Beach (pronounced Eva Beach), HI has an international warning responsibility for the entire Pacific and a regional warning responsibility for the State of Hawaii. See also WC/ATWC.
PTYPE
precipitation type
Public Information Statement
A narrative statement issued by a National Weather Service Forecast Office that can be used for:

1) A current or expected nonhazardous event of general interest to the public that can usually be covered with a single message (e.g., unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora borealis, lenticular clouds, and stories about a long-term dry/cold/wet/warm spell).

2) Public educational information and activities, such as storm safety rules, awareness activities, storm drills, etc.

3) Information regarding service changes, service limitations, interruptions due to reduced or lost power or equipment outages, or special information clarifying interpretation of NWS data. For example, this product may be used to inform users of radar equipment outages or special information clarifying interpretation of radar data originating from an unusual source which may be mistaken for precipitation (such as chaff drops, smoke plumes, etc., that produces echoes on the radar display.
Public Severe Weather Outlook
These are issued when the Storm Prediction Center (SPC) in Norman, Oklahoma anticipates an especially significant and/or widespread outbreak of severe weather. This outlook will stress the seriousness of the situation, defines the threat area, and provides information on the timing of the outbreak. The lead time on this outlook is normally less than 36 hours prior to the severe weather event.
Puddle
In hydrologic terms,
(1) The act of compacting earth, soil clay, etc., by mixing them with water and rolling or tamping the mixture.
(2) A compact mass of earth, soil, clay, or a mixture of material, which has been compacted through the addition of water, rolling and tamping. This makes the material less permeable.
(3) A small pool of water, usually a few inches in depth and from several inches to several feet in it greatest dimension.
Puget Sound Convergence Zone
A situation where wind forced around the Olympic Mountains converges over the Puget Sound. Causes extreme variability in weather conditions around Seattle, Washington with some areas of sunshine and others in clouds and rain.
Pulse
A short burst of electromagnetic energy that a radar sends out in a straight line to detect a precipitation target. The straight line that this pulse travels along is called a radar beam.
Pulse Duration
The time over which a radar pulse lasts. The pulse duration can be multiplied by the speed of light to determine the pulse length or pulse width.
Pulse Length
The linear distance in range occupied by an individual pulse from a radar. h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space. Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously. This is also called a pulse width.
Pulse Radar
A type of radar, designed to facilitate range (distance) measurements, in which are transmitted energy emitted in periodic, brief transmission.
Pulse Repetition Frequency (PRF)
The amount of time between successive pulses, or bursts, of electromagnetic energy that is transmitted by a radar. The PRF determines the maximum range at which echoes can be detected and also the maximum radial velocity that can be detected by a Doppler radar.
Pulse Repetition Time (PRT)
The time elapsed between pulses by the radar. This is also called the pulse interval.
Pulse Resolution Volume
A discrete radar sampling volume, of dimensions (horizontal beamwidth * vertical beamwidth * 1 range gate).
Pulse Severe Thunderstorms
Single cell thunderstorms which produce brief periods of severe weather (3/4 inch hail, wind gusts in the excess of 58 miles an hour, or a tornado).
Pulse Storm
A thunderstorm within which a brief period (pulse) of strong updraft occurs, during and immediately after which the storm produces a short episode of severe weather. These storms generally are not tornado producers, but often produce large hail and/or damaging winds. See also overshooting top.
Pulse Width
Same as Pulse Length; the linear distance in range occupied by an individual pulse from a radar. h = c * t , where t is the duration of the transmitted pulse, c is the speed of light, h is the length of the pulse in space. Note, in the radar equation, the length h/2 is actually used for calculating pulse volume because we are only interested in signals that arrive back at the radar simultaneously.
Pulse-Pair Processing
Nickname for the technique of mean velocity estimation by calculation of the signal complex covariance argument. The calculation requires two consecutive pulses, hence "pulse-pair".
PVA
Positive Vorticity Advection - the advection of higher values of vorticity into an area.
PVL
Prevail
PW
Precipitable Water - measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
PWAT
Precipitable Water - measure of the depth of liquid water at the surface that would result after precipitating all of the water vapor in a vertical column over a given location, usually extending from the surface to 300 mb.
PWO
Public Severe Weather Outlook
QPF
Quantitative Precipitation Forecast. A spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.
QPF Discussion
This HPC forecast discussion is directed completely to explaining manual forecasts of areas in the contiguous 48 states expected to receive 1/4 inch or more precipitation during a 24-hour period. The manual forecasts are explained in terms of initial conditions and differences and/or similarities in the numerical model forecasts. General confidence in the manual forecast is expressed where it is appropriate and possible alternatives may be offered. This product is issued 3 times a day.
QPFERD
NCEP Excessive Rainfall Discussion
QPFHSD
NCEP Heavy Snow Discussion
QPFPFD
NCEP Precipitation Forecast Discussion
Quantitative Precipitation Estimate (QPE)
A spatial and temporal analysis estimating the amount of precipitation that has occurred using a variety of techniques including observational and remote sensing data.
Quantitative Precipitation Forecast
A spatial and temporal precipitation forecast that will predict the potential amount of future precipitation for a specified region, or area.
Quiescent Prominence (Filament)
Long, sheet-like prominences nearly vertical to the solar surface
RADAP II
RAdar DAta Processor II, attached to some WSR-57 and WSR-74 radar units. It automatically controls the tilt sequence and computes several radar-derived quantities at regular intervals, including VIL, storm tops, accumulated rainfall, etc.
Radar Product Generator (RPG)
The RPG is the computer in the NEXRAD system that receives polar-coordinate base radar data from the RDA and processes these data into end-user products. Algorithms are utilized for pattern-recognition, rainfall estimation, computation of VIL and other products. The RPG communicates these products to end-users. A specific subset of available products is always generated for the NIDS vendors for distribution outside of the NWS, DoD, and FAA. Other products are generated by the RPG upon request from a PUP.
Radioisotope Snow Gage
A snow water equivalent gage based on the absorption of gamma radiation by snow; this gage can measure up to 55 inches water equivalent with a 2 to 5 percent error.
Rapid Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 mb/hr or 42 mb for 24 hours
Rapid Onset Flooding (ROF)
In the context of the National Water Model (NWM), rapid onset flooding refers to stream reaches that are forecast to at least double their flow within an hour, and meet or exceed their high water flow within six hours of this flow increase.
Rapidly Intensifying
Any maritime cyclone whose central pressure is dropping, or is expected to drop, at a rate of 1 MB per hour for 24 hours.
Record Event Report
This non-routine narrative product is issued by the National Weather Service to report meteorological and hydrological events that equal or exceed existing records.
Recreation Report
This National Weather Service product is used to relay reports on conditions for resorts and recreational areas and/or events. This report may also contain forecast information. Reports for recreational areas and resorts are often routine products, typically for a season, but possibly year-round.
REP
Represent/Representative
Replace and Route (R&R)
A methodology that ingests the official streamflow forecasts issued by the NWS RFCs at AHPS gauge locations, and utilizes the National Water Model (NWM) to route these forecasts downstream. This method is used to delineate the River Forecast Center Flood Inundation Map (RFC FIM).
Report
A weather report is a statement of the actual weather conditions observed at a specific time at a specific site.
Response Time
In hydrologic terms, the amount of time in which it will take a watershed to react to a given rainfall event
Rip Current
A relatively small-scale surf-zone current moving away from the beach. Rip currents form as waves disperse along the beach causing water to become trapped between the beach and a sandbar or other underwater feature. The water converges into a narrow, river-like channel moving away from the shore at high speed.
Rip Tide
See RIP CURRENTS
Riparian Zone
In hydrologic terms, a stream and all the vegetation on its banks.
Rope
(Also "Rope Funnel") - a narrow, often contorted condensation funnel usually associated with the decaying stage of a tornado. See rope stage.
Rope Cloud
In satellite meteorology, a narrow, rope-like band of clouds sometimes seen on satellite images along a front or other boundary. The term sometimes is used synonymously with rope or rope funnel.
Rope Stage
The dissipating stage of a tornado, characterized by thinning and shrinking of the condensation funnel into a rope (or rope funnel). Damage still is possible during this stage.
RPD
Rapid
RPLC
Replace
RPRT
Report
Saffir-Simpson Hurricane Wind Scale
The Saffir-Simpson Hurricane Wind Scale is a 1 to 5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the United States associated with winds of the indicated intensity. In general, damage rises by about a factor of four for every category increase. The maximum sustained surface wind speed (peak 1-minute wind at the standard meteorological observation height of 10 m [33 ft] over unobstructed exposure) associated with the cyclone is the determining factor in the scale. The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes.
Sampling Frequency
The rate at which sensor data is read or sampled.
Satellite Hydrology Program
A NOHRSC program that uses satellite data to generate areal extent of snow cover data over large areas of the western United States.
Saturation Vapor Pressure
The vapor pressure of a system, at a given temperature, wherein the vapor of a substance is in equilibrium with a plane surface of that substance's pure liquid or solid phase.
SBCAPE
Surface Based CAPE; CAPE calculated using a Surface based parcel.
Sea Level Pressure
The sea level pressure is the atmospheric pressure at sea level at a given location. When observed at a reporting station that is not at sea level (nearly all stations), it is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.
Sea Surface Temperatures
The term refers to the mean temperature of the ocean in the upper few meters.
Secondary Pollutant
Pollutants generated by chemical reactions occurring within the atmosphere. Compare primary pollutant.
Sediment Storage Capacity
In hydrologic terms, the volume of a reservoir planned for the deposition of sediment.
Seepage
In hydrologic terms, the interstitial movement of water that may take place through a dam, its foundation, or abutments.
Separation Eddy
An eddy that forms near the ground on the windward or leeward side of a bluff object or steeply rising hillside; streamlines above this eddy go over the object.
Servo Loop
In radar meteorology, a generic description of hardware needed to remotely control the motion of the antenna dish.
Set-up
The process whereby strong winds blowing down the length of a lake cause water to "pile up" at the downwind end, raising water levels there and lowering them at the upwind end of the lake.
Severe Weather Potential Statement
This statement is designed to alert the public and state/local agencies to the potential for severe weather up to 24 hours in advance. It is issued by the local National Weather Service office.
Severe Weather Probability
This WSR-88D radar product algorithm displays numerical values proportional to the probability that a storm will produce severe weather within 30 minutes. Values determined using a statistical regression equation which analyzes output from the VIL algorithm. It is used to quickly identify the most significant thunderstorms.
Short-Term Prediction Research and Transition (SPoRT)
A NASA- and NOAA-funded activity to transition experimental/quasi-operational satellite observations and research capabilities to the operational weather community to improve short-term weather forecasts on a regional and local scale.
Short-Term Prediction Research and Transition Center - Land Information System (SPoRT-LIS)
Provides high-resolution (~3 km) gridded soil moisture products in real-time to support regional and local modeling and improve situational awareness.
Sling Psychrometer
An instrument used to measure the water vapor content of the atmosphere in which wet and dry bulb thermometers are mounted on a frame connected to a handle at one end by means of a bearing or a length of chain. The psychrometer is whirled by hand to provide the necessary ventilation to evaporate water from the wet bulb.
SLP
Sea Level Pressure
Smoke Dispersal
Describes the ability of the atmosphere to ventilate smoke. Depends on the stability and winds in the lower layers of the atmosphere, i.e., a combination of mixing heights and transport winds.
Smoothed Sunspot Number
An average of 13 monthly RI numbers, centered on the month of concern.
Snow Depth
The combined total depth of both the old and new snow on the ground.
Snow Pack
Same as Snowcover; the combined layers of snow and ice on the ground at any one time.
Snow Pellets
Precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Same as graupel or small hail.
Snow Pillow
1) A window of snow deposited in the immediate lee of a snow fence or ridge.
or
2) In hydrologic terms, an instrument used to measure snow water equivalents. Snow pillows typically have flat stainless steel surface areas. The pillow below this flat surface is filled with antifreeze solution and the pressure in the pillow is related to the water-equivalent depth of the snow on the platform. One great advantage of snow pillows over a snow survey is the frequency of observations, which can be as high as twice per day.
Snowpack
The total snow and ice on the ground, including both the new snow and the previous snow and ice which has not melted.
Space Environment Center
(SEC) - This center provides real-time monitoring and forecasting of solar and geophysical events, conducts research in solar-terrestrial physics, and develops techniques for forecasting solar and geophysical disturbances. SEC's parent organization is the National Oceanic and Atmospheric Administration (NOAA). SEC is one of NOAA's 12 Environmental Research Laboratories (ERL) and one of NOAA's 9 National Centers for Environmental Prediction (NCEP). SEC's Space Weather Operations is jointly operated by NOAA and the U.S. Air Force and is the national and world warning center for disturbances that can affect people and equipment working in the space environment.
SPC
Storm Prediction Center
SPCLY
Especially
SPD
1) Speed
2) On a buoy report, ten-minute average wind speed values in m/s.
Spearhead Echo
A radar echo associated with a downburst with a pointed appendage extending toward the direction of the echo motion. The appendage moves much faster than the parent echo, which is drawn into the appendage. During it's mature stage, the appendage turns into a major echo and the parent echo loses its identity.
Special Avalanche Warning
Issued by the National Weather Service when avalanches are imminent or occurring in the mountains. It is usually issued for a 24 hour period.
Special Fire Weather
Meteorological services uniquely required by user agencies which cannot be provided at an NWS office during normal working hours. Examples are on-site support, weather observer training, and participation in user agency training activities.
Special Marine Warning
(SMW) A warning product issued for potentially hazardous weather conditions usually of short duration (up to 2 hours) producing sustained marine thunderstorm winds or associated gusts of 34 knots or greater; and/or hail 3/4 inch or more in diameter; and/or waterspouts affecting areas included in a Coastal Waters Forecast, a Nearshore Marine Forecast, or an Great Lakes Open Lakes Forecast that is not adequately covered by existing marine warnings. Also used for short duration mesoscale events such as a strong cold front, gravity wave, squall line, etc., lasting less than 2 hours and producing winds or gusts of 34 knots or greater.
Special Tropical Disturbance Statement
This statement issued by the National Hurricane Center furnishes information on strong and formative non-depression systems. This statement focuses on the major threat(s) of the disturbance, such as the potential for torrential rainfall on an island or inland area. The statement is coordinated with the appropriate forecast office(s).
Specific Gravity
The ratio of the density of any substance to the density of water.
Specific Humidity
In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.
Specific Yield
In hydrologic terms, the ratio of the water which will drain freely from the material to the total volume of the aquifer formation. This value will always be less than the porosity.
Spectral Density
A radar term for the distribution of power by frequency.
Spectral Wave Density
On a buoy report, energy in (meter*meter)/Hz, for each frequency bin (typically from 0.03 Hz to 0.40 Hz).
Spectral Wave Direction
On a buoy report, mean wave direction, in degrees from true North, for each frequency bin.
Spectrum Width
This WSR-88D radar product depicts a full 360 degree sweep of spectrum width data indicating a measure of velocity dispersion within the radar sample volume. It is available for every elevation angle sampled, it provides a measure of the variability of the mean radial velocity estimates due to wind shear, turbulence, and/or the quality of the velocity samples. It is used to estimate turbulence associated with boundaries, thunderstorms, and mesocyclones; check the reliability of the velocity estimates; and locate boundaries (cold front, outflow, lake breeze, etc.).
Spectrum Width Cross Section
This WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet on the vertical axis and distance up to 124 nm on the horizontal axis. Two end points to create cross section are radar operator selected along a radial or from one AZRAN to another AZRAN within 124 nm of the radar that are less than 124 nm apart.
It is used to:
1) Verify features on the Reflectivity Cross Section (RCS) and Velocity Cross Section (VCS) and to evaluate the quality of the velocity data
2) Estimate vertical extent of turbulence (aviation use).
Speed Shear
The component of wind shear which is due to a change in wind speed with height, e.g., southwesterly winds of 20 mph at 10,000 feet increasing to 50 mph at 20,000 feet. Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.
SPENES
NESDIS Satellite Precipitation Estimates
Sphere Calibration
Reflectivity calibration of a radar by pointing the dish at a metal sphere of (theoretically) known reflectivity. The sphere is often tethered to a balloon.
Spillway
In hydrologic terms, a structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.
Spillway Crest
In hydrologic terms, the elevation of the highest point of a spillway.
Spin-Up
Slang for a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.
SPKL
Sprinkle
Split Flow
A flow pattern high in the atmosphere characterized by diverging winds. Storms moving along in this type of flow pattern usually weaken.
Splitting Storm
A thunderstorm which splits into two storms which follow diverging paths (a left mover and a right mover). The left mover typically moves faster than the original storm, the right mover, slower. Of the two, the left mover is most likely to weaken and dissipate (but on rare occasions can become a very severe anticyclonic-rotating storm), while the right mover is the one most likely to reach supercell status.
SPLNS
Southern Plains
Sporadic E
In solar-terrestrial terms, a phenomenon occurring in the E region of the ionosphere, which significantly affects HF radiowave propagation. Sporadic E can occur during daytime or nighttime and it varies markedly with latitude.
SPOTNIL
In solar-terrestrial terms, a spotless disk.
Spotting
Outbreak of secondary fires as firebrands or other burning materials are carried ahead of the main fire line by winds.
Spray
An ensemble of water droplets torn by the wind from an extensive body of water, generally from the crests of waves, and carried up into the air in such quantities that it reduces the horizontal visibility.
SPRD
Spread
Spring
1. The season of the year comprising the transition period from winter to summer occurring when the sun is approaching the summer solstice. In the Northern Hemisphere, spring customarily includes the months of March, April and May.

2. In hydrologic terms, an issue of water from the earth; a natural fountain; a source of a reservoir of water.
Spring Tide
A tide higher than normal which occurs around the time of the new and full moon.
Sprinkle
Very light rain showers. Precipitation measurement is a trace.
SPS
Severe Weather Potential Statement
St Lawrence Freeze-Up Outlook
A National Weather Service forecast product to keep mariners informed of the projected freeze-up date of ice the St. Lawrence River.
Stair Stepping
In hydrologic terms, the process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous forecast was prepared.
Standard Atmosphere
A hypothetical vertical distribution of atmospheric temperature, pressure, and density that, by international agreement, is taken to be representative of the atmosphere for purposes of pressure altimeter calibrations, aircraft performance calculations, aircraft and missile design, ballistic tables, etc.
Standard Synoptic Times
The times of 0000, 0600, 1200, and 1800 UTC. Also known as the main synoptic times.
State Forecast Product
This National Weather Service product is intended to give a good general picture of what weather may be expected in the state during the next 5 days. The first 2 days of the forecast is much more specific than the last 3 days. In comparison with the Zone Forecast Product, this product will be much more general.
State Weather Roundup
This is a National Weather Service tabular product which provides routine hourly observations within the state through the National Weather Wire Service (NWWS). It gives the current weather condition in one word (cloudy, rain, snow, fog, etc.), the temperature and dew point in Fahrenheit, the relative humidity, wind speed and direction, and finally additional information (wind chill, heat index, a secondary weather condition). These reports are broken up regionally. When the complementary satellite product is not available, reports from unaugmented ASOS stations will report "fair" in the sky/weather column when there are few or no clouds (i.e., scattered or less) below 12,000 feet with no significant weather and/or obstructions to visibility.
Station Pressure
The absolute air pressure at a given reporting station. The air pressure is directly proportional to the combined weight of all air in the atmosphere located in a column directly above the reporting site. Consequently, the station pressure may vary tremendously from one location to another in mountainous regions due to the strong variation of atmospheric pressure with height. Vertical variations of pressure range up to 150 mb per mile whereas horizontal variations are usually less than .1 mb per mile.
Steepness
In marine terms, on a buoy report, wave steepness is the ratio of wave height to wave length and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio, the wave becomes unstable and begins to break.
Stepped Leader
A faint, negatively charged channel that emerges from the base of a thunderstorm and propagates toward the ground in a series of steps of about 1 microsecond duration and 50-100 meters in length, initiating a lightning stroke.
Stoplogs
In hydrologic terms, large logs, timbers or steel beams placed on top of each other with their ends held in guides on each side of a channel or conduit providing a temporary closure versus a permanent bulkhead gate.
Storm Relative Mean Radial Velocity Map
(SRM): This WSR-88D radar product depicts a full 360º sweep of radial velocity data with the average motion of all identified storms subtracted out. It is available for every elevation angle sampled. It is used to aid in displaying shear and rotation in storms and storm top divergence that might otherwise be obscured by the storm's motion, investigate the 3-D velocity structure of a storm, and help with determining rotational features in fast and uniform moving storms.
Storm Total Precipitation
This radar image is an estimate of accumulated rainfall since the last time there was a one-hour, or more, break in precipitation. It is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event and is available only for the short range (out to 124 nm). To determine accumulated precipitation at greater distances you should link to an adjacent radar.
Straight-Line Hodograph
The name pretty well describes what it looks like on the hodograph. What causes this shape is a steady increase of winds with height (vertical wind shear). This shape of hodograph favors multicell thunderstorms.
Stratopause
The boundary between the stratosphere and mesosphere.
Stratosphere
The region of the atmosphere extending from the top of the troposphere to the base of the mesosphere, an important area for monitoring stratospheric ozone.
Stratospheric Ozone
In the stratosphere, ozone has beneficial properties where it forms an ozone shield that prevents dangerous radiation from reaching the Earth's surface. Recently, it was discovered that in certain parts of the world, especially over the poles, stratospheric ozone was disappearing creating an ozone hole.
Sub-synoptic Low
Essentially the same as mesolow.
Subtropical Cyclone
A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.
Subtropical Depression
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
Subtropical Jet
(Abbrev. STJ) - this jet stream is usually found between 20° and 30° latitude at altitudes between 12 and 14 km.
Subtropical Storm
A subtropical cyclone in which the maximum 1-minute sustained surface wind is 34 knots (39 mph) or more.
Sudden Impulse (SI+ or SI-)
In solar-terrestrial terms, a sudden perturbation of several gammas in the northward component of the low-latitude geomagnetic field, not associated with a following geomagnetic storm. (An SI becomes an SC if a storm follows.)
Sudden Ionospheric Disturbance (SID)
In solar-terrestrial terms, HF propagation anomalies due to ionospheric changes resulting from solar flares, proton events and geomagnetic storms.
Summation Principle
This principle states that the sky cover at any level is equal to the summation of the sky cover of the lowest layer plus the additional sky cover provided at all successively higher layers up to and including the layer in question.
Sun Pillar
A bright column above or below the sun produced by the reflection of sunlight from ice crystals.
Sun Pointing
Alignment of the radar antenna by locating the position of the sun in the sky, which has an exactly known position given the radar's location and the present time. This may be necessary to verify that when we think we're pointing "north", we actually are! The sun's signal is usually several dB above the background noise, and this technique is also sometimes used to examine the receiver sensitivity.
Sunspot
In solar-terrestrial terms, an area seen as a dark spot on the photosphere of the sun. Sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere.
Sunspot Group Classification
  • A: A small single unipolar sunspot or very small group of spots without penumbra.
  • B: Bipolar sunspot group with no penumbra.
  • C: An elongated bipolar sunspot group. One sunspot must have penumbra.
  • D: An elongated bipolar sunspot group with penumbra on both ends of the group.
  • E: An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 10 deg. but not 15 deg.
  • F: An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 15 deg.
  • H: A unipolar sunspot group with penumbra.
Sunspot Number
In solar-terrestrial terms, a daily index of sunspot activity (R), defined as R = k (10 g + s) where S = number of individual spots, g = number of sunspot groups, and k is an observatory factor.
Super Typhoon
Typhoon having maximum sustained winds of 130 knots (150 mph) or greater.
Supercell
Short reference to Supercell Thunderstorm; potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours.
Supercell Thunderstorm
Potentially the most dangerous of the convective storm types. Storms possessing this structure have been observed to generate the vast majority of long-lived strong and violent (F2-F5) tornadoes, as well as downburst damage and large hail. It is defined as a thunderstorm consisting of one quasi-steady to rotating updraft which may exist for several hours. Supercells usually move to the right of the mean wind. These are called "Right Movers" and they are favored with veering winds. Occasionally, these thunderstorms will move to the left of the mean wind. These thunderstorms are called "Left Movers". These supercells typically don't last as long as their "Right Mover" cousins and they usually only produce large hail (greater than 3/4 inch in diameter) and severe wind gusts in the excess of 58 miles an hour. Left Movers are favored when you have backing winds.

Radar will observe essentially one long-lived cell, but small perturbations to the cell structure may be evident. The stronger the updraft, the better the chance that the supercell will produce severe (hail greater than 3/4 inch in diameter, wind gusts greater than 58 miles an hour, and possibly a tornado) weather.

Severe supercell development is most likely in an environment possessing great buoyancy (CAPE) and large vertical wind shear. A Bulk Richardson Number of between 15 and 35 favor supercell development. Typically, the hodograph will look like a horse shoe. This is due to the wind speed increasing rapidly with height and the wind direction either veering or backing rapidly with height.
Supercool
To cool a liquid below its freezing point without solidification or crystallization.
Supercooled Liquid Water
In the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.
Superrefraction
Bending of the radar beam in the vertical which is greater than sub-standard refractive conditions. This causes the beam to be lower than indicated, and often results in extensive ground clutter as well as an overestimation of cloud top heights.
Surcharge Capacity
In hydrologic terms, the volume of a reservoir between the maximum water surface elevation for which the dam is designed and the crest of an uncontrolled spillway, or the normal full-pool elevation of the reservoir with the crest gates in the normal closed position.
Surface impoundment
In hydrologic terms, an indented area in the land's surface, such as a pit, pond, or lagoon.
SWP
On a buoy report, Swell Period is the time (usually measured in seconds) that it takes successive swell wave crests or troughs pass a fixed point.
SYNOP
Synoptic - relating to the general weather pattern over a wide region, such as areas of high and low pressure or frontal boundaries, as opposed to mesoscale or smaller features such as a thunderstorm.
Synopsis
A broad discussion of the weather pattern expected across any given area, generally confined to the 0-48 hour time frame.
Synoptic Code
Rules and procedures established by the World Meteorological Organization(WMO) for encoding weather observations.
Synoptic Scale
The spatial scale of the migratory high and low pressure systems of the lower troposphere, with wavelengths of 1000 to 2500 km.
Synoptic Track
Weather reconnaissance mission flown to provide vital meteorological information in data sparse ocean areas as a supplement to existing surface, radar, and satellite data. Synoptic flights better define the upper atmosphere and aid in the prediction of tropical cyclone development and movement.
Synoptic Weather
Weather occurring over a wide region on time scales exceeding 12 hours.
Synthetic Aperture Radar (SAR)
A radar mounted on a moving platform (aircraft or satellite) used for imaging. Since the radar antenna moves a significant distance between transmission and receiving the signal back, the radar antenna acts as if it is larger than its physical dimensions, providing better range and azimuth resolution.
Synthetic Aperture Radar River Ice Surveillance (SARRIS)
An experimental river ice mapping experiment using Synthetic Aperture Radar (SAR).
TEMP
Temperature- A measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.
Temperature
(Abbrev. TEMP)- The temperature is a measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.
Temperature Inversion
(surface-based or elevated) : a layer of the atmosphere in which air temperature increases with height. When the layer's base is at the surface, the layer is called a surface-based temperature inversion; when the base of the layer is above the surface, the layer is called an elevated temperature inversion.
Temperature Recovery
The change in temperature over a given period of time. Generally, the period between late evening and sunrise. Windy or cloudy conditions will tend to produce slow temperature recovery, while clear, calm weather can cause rapid recovery.
TEMPS
temperatures
Thermograph
An instrument that measures and records air temperature.
Thermosphere
The atmospheric shell extending from the top of the mesosphere to outer space. It is a region of more or less steadily increasing temperature with height, starting at 70 or 80 km.
Tidal Piling
Occurs when unusually high water levels occur as the result of an accumulation of successive incoming tides that do not completely drain due to opposing strong winds and/or waves.
Tide Prediction
The computation of tidal highs and lows at a given location resulting from the gravitational interactions between the earth and primarily the moon and sun.
Tilted Updraft
A thunderstorm updraft which is not purely vertical but instead exhibits a slanted or tilted character. It is a sign of vertical wind shear, a favorable condition for severe storm development.
Tipping-Bucket Rain Gage
A precipitation gage where collected water is funneled into a two compartment bucket; 0.01, 0.1 mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it actuates an electric circuit.
TOP
Cloud Top
Topography
The shape of the land.
TPC
(Tropical Prediction Center) - An NCEP center which produces marine offshore and high seas forecasts south of 30N in the Eastern Pacific, Gulf of Mexico and Caribbean.
TPW
Total Precipitable Water
Transpiration
Water discharged into the atmosphere from plant surfaces.
Transport Wind
The average wind over a specified period of time within a mixed layer near the surface of the earth.
Trapper
A valley or basin in which cold air becomes trapped or pooled.
Triple Doppler
Since any wind has three components (say, in the x, y and z directions), and a single radar measures in only one direction (radial), a single radar cannot give the 3D winds everywhere it samples. However, if three different radars view a storm from three different locations, the 3 measured radial velocities can be transformed into the actual 3D wind field.
Triple Point
The intersection point between two boundaries (dry line, outflow boundary, cold front, etc.), often a focus for thunderstorm development. Triple point also may refer to a point on the gust front of a supercell, where the warm moist inflow, the rain-cooled outflow from the forward flank downdraft, and the rear flank downdraft all intersect; this point is a favored location for tornado development (or redevelopment).
TROP
Tropopause
Tropical Advisory
Official information issued by tropical cyclone warning centers describing all tropical cyclone watches and warnings in effect along with details concerning tropical cyclone locations, intensity and movement, and precautions that should be taken. Advisories are also issued to describe: (a) tropical cyclones prior to issuance of watches and warnings and (b) subtropical cyclones.
Tropical Analysis and Forecast Branch
One of three branches of the Tropical Prediction Center (TPC). It provides year-round products involving marine forecasting, aviation forecasts and warnings (SIGMETs), and surface analyses. The unit also provides satellite interpretation and satellite rainfall estimates for the international community. In addition, TAFB provides support to NHC through manpower and tropical cyclone intensity estimates from the Dvorak technique.
Tropical Cyclone
A warm-core, non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters with organized deep convection and a closed surface wind circulation about a well-defined center.
Tropical Cyclone Plan of the Day
A coordinated mission plan that tasks operational weather reconnaissance requirements during the next 1100 to 1100 UTC day or as required, describes reconnaissance flights committed to satisfy both operational and research requirements, and identifies possible reconnaissance requirements for the succeeding 24-hour period.
Tropical Cyclone Position Estimate
The National Hurricane Center issues a position estimate between scheduled advisories whenever the storm center is within 200 nautical miles of U.S. land-based weather radar and if sufficient and regular radar reports are available to the hurricane center. As far as is possible, the position estimate is issued hourly near the beginning of the hour. The location of the eye or storm center is given in map coordinates and distance and direction from a well-known point.
Tropical Cyclone Update
This brief statement is issued by the National Hurricane Center in lieu of or preceding special advisories to inform of significant changes in a tropical cyclone or the posting or cancellation of watches and warnings.
Tropical Depression
A tropical cyclone in which the maximum 1-minute sustained surface wind is 33 knots (38 mph) or less.
Tropical Disturbance
A discrete tropical weather system of apparently organized convection--generally 100 to 300 mi in diameter--originating in the tropics or subtropics, having a nonfrontal migratory character and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.
Tropical Storm
A tropical cyclone in which the maximum 1-minute sustained surface wind ranges from 34 to 63 knots (39 to 73 mph) inclusive.
Tropical Storm Summary
Written by the Hydrometeorological Prediction Center* (HPC) after subtropical and names tropical cyclones have moved inland and advisories have been discontinued. These advisories will be terminated when the threat of flash flooding has ended or when the remnants of these storms can no longer be distinguished from other synoptic features capable of producing flash floods. Storm summaries will not be issued for storms that enter the coast of Mexico and do not pose an immediate flash flood threat to the coterminous United States. They will be initiated when and if flash flood watches are posted in the United States because of an approaching system. Storm summaries will continue to be numbered in sequence with tropical cyclone advisories and will reference the former storm's name in the text. Summaries will be issued at 0100, 0700, 1300, and 1900 Eastern Daylight Time (EDT). The only exception will be the first one in the series may be issued at a nonscheduled time.
Tropical Storm Warning
An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are expected somewhere within the specified coastal area within 36 hours.
Tropical Storm Watch
An announcement that tropical storm conditions (sustained winds of 39 to 73 mph) are possible within the specified coastal area within 48 hours.
Tropical Wave
(formerly known as inverted trough) - A trough or cyclonic curvature maximum in the trade wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere or may be the reflection of an upper tropospheric cold low or an equatorward extension of a mid-latitude trough.
Tropical Weather Discussion
These messages are issued 4 times daily by the Tropical Analysis and Forecast Branch (TAFB) to describe significant synoptic weather features in the tropics. One message will cover the Gulf of Mexico, the Caribbean, and the Atlantic between the equator and 32 degrees North and east of 140 degrees West. Plain language is used in these discussions.
Tropical Weather Outlook
This outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two. In the Atlantic, the outlook is transmitted daily at 0530, 1130, 1730, and 2230 Eastern local time. In the eastern Pacific, it is transmitted daily at 0100, 0700, 1300, and 1900 Eastern local time. For the Central Pacific, transmission times are 1000 and 2200 UTC. Existing tropical and subtropical cyclones are mentioned, as are depressions not threatening land. Given for each system are its location, size, intensity, and movement. For the first 24 hours of a depression or tropical cyclone, the outlook includes a statement identifying the AFOS and World Meteorological Organization (WMO) headers for the advisory on it.
Tropical Weather Summary
The National Hurricane Center issues a monthly summary of tropical weather is included at the end of the month or as soon as feasible thereafter, to describe briefly the past activity or lack thereof and the reasons why.
Tropics
Areas of the Earth within 20° North and South of the equator.
Tropopause
The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).
Tropopause Jet
Type of jet stream found near the tropopause. Examples of this type of jet are the subtropical and polar fronts.
Troposphere
The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height (except, perhaps, in thin layers - see inversion, cap), vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).
TRPCL
Tropical
Turning Point
In hydrologic terms, a temporary point whose elevation is determined by additions and subtractions of backsights and foresights respectively.
Type I, II, III, IV
(In solar-terrestrial terms) -Emissions of the sun in radio wavelengths from centimeters to dekameters, under both quiet and disturbed conditions. Type I. A noise storm composed of many short, narrow-band bursts in the metric range (300 - 50 MHz). Type II. Narrow-band emission that begins in the meter range (300 MHz) and sweeps slowly (tens of minutes) toward deka- meter wavelengths (10 MHz). Type II emissions occur in loose association with major FLAREs and are indicative of a shock wave moving through the solar atmosphere. Type III. Narrow-band bursts that sweep rapidly (seconds) from decimeter to dekameter wavelengths (500 - 0.5 MHz). They often occur in groups and are an occasional feature of complex solar ACTIVE REGIONs. Type IV. A smooth continuum of broad-band bursts primarily in the meter range (300 - 30 MHz). These bursts are associated with some major flare events beginning 10 to 20 minutes after the flare maximum, and can last for hours
Typhoon
A tropical cyclone in the Western Pacific Ocean in which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater.
Typhoon Season
The part of the year having a relatively high incidence of tropical cyclones. In the western North Pacific, the typhoon season is from July 1 to December 15. Tropical cyclones can occur year-round in any basin.
UCP
(Unit Control Position): The WSR-88D radar operator uses this to control the entire radar system. One of the main things that the radar operator will do at the UCP is change volume scan strategies of the antenna. These volume scan strategies tell the radar how many elevation angles will be used during a single volume scan (a volume scan is the completion of a sequence of elevation angles), and the amount of time it will take to complete that sequence of elevation cuts, each one being a single rotation of the antenna's 1 degree beam at selected elevation angles. The WSR-88D uses 3 scan strategies. They are the following: 14 elevation angles in 5 minutes (this is used during severe weather situations), 9 elevation angles in 6 minutes (this is used when there is precipitation within 248 nautical miles of the radar), and 5 elevation angles in 10 minutes (this is used when there is no precipitation within 248 nautical miles). The radar operator at the UCP can also adjust the radar products and help the users out with their communication problems.
Unit Control Position
The WSR-88D radar operator uses this to control the entire radar system. One of the main things that the radar operator will do at the UCP is change volume scan strategies of the antenna. These volume scan strategies tell the radar how many elevation angles will be used during a single volume scan (a volume scan is the completion of a sequence of elevation angles), and the amount of time it will take to complete that sequence of elevation cuts, each one being a single rotation of the antenna's 1 degree beam at selected elevation angles. The WSR-88D uses 3 scan strategies. They are the following: 14 elevation angles in 5 minutes (this is used during severe weather situations), 9 elevation angles in 6 minutes (this is used when there is precipitation within 248 nautical miles of the radar), and 5 elevation angles in 10 minutes (this is used when there is no precipitation within 248 nautical miles). The radar operator at the UCP can also adjust the radar products and help the users out with their communication problems.
Unit Hydrograph
The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period
Universal Geographic Code
(UGC) - UGC's, (e.g. ANZ300 for Western Long Island Sound) are used in many National Weather Service text products to provide geographical information. This allows users easy automated processing and redistribution of the information. More specifically, the purpose of the UGC are to specify the affected geographic area of the event, typically by state, county (or parish), or unique NWS zone (land and marine). The only exception to the above is to define the weather synopsis part of certain marine products.
Up-valley Wind
A diurnal thermally driven flow directed up a valley's axis, usually occurring during daytime; part of the along-valley wind system.
Updraft
A small-scale current of rising air. If the air is sufficiently moist, then the moisture condenses to become a cumulus cloud or an individual tower of a towering cumulus or Cb.
Upper Level
In weather observing, the term applies to the portion of the atmosphere that is above the lower troposphere, generally 850 hPa and above.
Upper Level Disturbance
A disturbance in the upper atmospheric flow pattern which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air, and sometimes a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.
Upper Level System
A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere. This term sometimes is used interchangeably with impulse or shortwave.
Upper-air Weather Chart
Weather maps that are produced for the portion of the atmosphere above the lower troposphere, generally at and above 850 mb. Isolines on these maps usually represent the heights of a constant pressure surface, such as the 500 mb surface.
UPR
Upper
Upslope Flow
Same as Orographic Lifting; air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.
Upslope Fog
A fog that forms when moist, stable air is carried up a mountain slope.
UPSLP
Upslope
Upstream
Towards the source of flow, or located in the area from which the flow is coming.
Upstream Slope
The part of the dam which is in contact with the reservoir water. On earthen dams, this slope must be protected from the erosive action of waves by rock riprap or concrete.
UPSTRM
Upstream
Upwelling
In ocean dynamics, the upward motion of sub-surface water toward the surface of the ocean. This is often a source of cold, nutrient-rich water. Strong upwelling occurs along the equator where easterly winds are present. Upwelling also can occur along coastlines, and is important to fisheries and birds in California and Peru.
VAD Wind Profile
A radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets). This product often is referred to erroneously as a VAD.
Vapor Pressure
The partial pressure of water vapor in an air-water system.
Variable Infiltration Capacity (VIC)
A large-scale, semi-distributed hydrologic model that solves full water and energy balances. As such, it shares several basic features with other land surface models that are commonly coupled to global circulation models.
VCP
Volume Coverage Pattern - A volumetric sampling procedure designed for the surveillance of one or more particular meteorological phenomena. Clear Air Mode uses VCP 31 and 32. Each has a Volume Scan consisting of 5 elevation angles (0.5 to 4.5 degrees) in ten minutes. VCP 31 has a long pulse length and provides a better signal-to-noise ratio permitting lower reflectivity returns to be detected. VCP 32 has a short pulse length which provides for larger unambiguous velocity values. Precipitation Mode uses VCP 11 and 21. VCP 11 provides better vertical sampling of weather echoes near the antenna and is usually preferred in situations where convective precipitation is within 60 nmi of the antenna. VCP 11 Volume Scan consists of 14 elevation angles (0.5 to 19.5 degrees) in 5 minutes. VCP 21 has a slower antenna rotation rate and provides better velocity and spectrum width estimates beyond 60 nmi. VCP 21 Volume Scan consists of 9 elevation angles (0.5 to 19.5 degrees) in 6 minutes.
Velocity Azimuth Display
A WSR 88-D product which shows the radar derived wind speeds at various heights. This radar product shows the wind speeds from 2,000 to 55,000 feet above the ground. VAD and EVAD (Extended VAD) are methods of guessing the large scale two-dimensional winds from one-dimensional radial velocity data. They are essentially multivariate regressions which fit a simple, large scale wind model to the observed winds. EVAD also estimates the large scale horizontal divergence and particle fall speed. See VWP.
VIP
Video Integrator and Processor, which contours radar reflectivity (in dBZ) into six VIP levels:

  • VIP 1 (Level 1, 18-30 dBZ) - Light precipitation.
  • VIP 2 (Level 2, 30-38 dBZ) - Light to moderate rain.
  • VIP 3 (Level 3, 38-44 dBZ) - Moderate to heavy rain.
  • VIP 4 (Level 4, 44-50 dBZ) - Heavy rain.
  • VIP 5 (Level 5, 50-57 dBZ) - Very heavy rain; hail possible.
  • VIP 6 (Level 6, >57 dBZ) - Very heavy rain and hail; large hail possible.
Virtual Potential Temperature
The virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb. This is defined by Poisson's equation.
Virtual Temperature
The virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and humidity.
Visibility Protection Program
The program specified by the Clean Air Act to achieve a national goal of remedying existing impairments to visibility and preventing future visibility impairment throughout the United States.
Visual Spectrum
The portion of the electromagnetic spectrum to which the eye is sensitive, i.e., light with wavelengths between 0.4 and 0.7 micrometers. Compare shortwave radiation and longwave radiation.
Volume Velocity Processing
A way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data. Essentially a multivariate regression which fits a simple wind model to the observed radial velocities. Very similar to VAD and EVAD, except it uses different functions for the fit.
Voluntary Observing Ship Program
(VOS) - An international voluntary marine observation program under the auspices of the World Meteorological Organization (WMO). Observations are coded in a special format known as the ships synoptic code, or "BBXX" format. They are then distributed for use by meteorologists in weather forecasting, by oceanographers, ship routing services, fishermen, and many others.
VVP
Volume Velocity Processing - a way to guess the large-scale 2-dimensional winds, divergence and fall speeds from one-dimensional radial velocity data. Essentially a multivariate regression which fits a simple wind model to the observed radial velocities. Very similar to VAD and EVAD, except it uses different functions for the fit.
VWP
VAD Wind Profile - a radar plot of horizontal winds, derived from VAD data, as a function of height above a Doppler Radar. The display is plotted with height as the vertical axis and time as the horizontal axis (a so-called time-height display), which then depicts the change in wind with time at various heights. This display is useful for observing local changes in vertical wind shear, such as backing of low-level winds, increases in speed shear, and development or evolution of nearby jet streams (including low-level jets). This product often is referred to erroneously as a VAD.
Watch Status Reports
This product lets the NWFO know of the status of the current severe weather watch (Tornado or Severe Thunderstorm). During the severe weather watch, the Storm Prediction Center (SPC) will issue these reports periodically. These reports will describe, in plain language, the current evaluation of the severe weather situation and whether the watch will expire or be reissued. A status report is not issued if a cancellation or replacement has been issued at least 1 hour prior to the expiration time of the original watch.
Water Pollution
The alteration of the constituents of a body of water by man to such a degree that the water loses its value as a natural resource.
Water Supply Outlook
A seasonal volume forecast, generally for a period centered around the time of spring snowmelt (e.g., April-July). The outlooks are in units of acre-feet and represent the expected volume of water to pass by a given point during a snowmelt season. The outlook categories include Most Probable, Reasonable Maximum, and Reasonable Minimum.
Water Vapor Plume
This appears in the water vapor satellite imagery. It is a plume-like object that extends from the Intertropical Convergence Zone (ITCZ) northward or southward into the higher latitudes. It is usually located over a 850 to 700 mb theta-e ridge axis. As a result, it is a favored location for the development of a Mesoscale Convective Complex (MCC). Researchers have found it to be a favored region for very heavy rain. It is thought that the ice crystals located in this plume help thunderstorms to become highly efficient rainfall producers. In North America, this is sometimes called the "Mexican Connection".
Waterspout
In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters.

The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.
Wave Period
Time, in seconds, between the passage of consecutive wave crests past a fixed point.
Wave Spectrum
The distribution of wave energy with respect to wave frequency or period. Wave spectra assist in differentiating between wind waves and swell.
Wave Steepness
The ratio of wave height to wavelength and is an indicator of wave stability. When wave steepness exceeds a 1/7 ratio; the wave typically becomes unstable and begins to break.
WDSPRD
Widespread
Weighing-Type Precipitation Gage
A rain gage that weighs the rain or snow which falls into a bucket set on a platform of a spring or lever balance. The increasing weight of its contents plus the bucket are recorded on a chart. The record thus shows the accumulation of precipitation.
Wet Floodproofing
In hydrologic terms, an approach to floodproofing which usually is a last resort. Flood waters are intentionally allowed into the building to minimize water pressure on the structure. Wet Floodproofing can include moving a few valueable items to a higher place or completely rebuilding the floodable area. Wet floodproofing has an advantage over other approaches: no matter how little is done, flood damage will be reduced. Thousands of dollars in damage can be avoided just by moving furniture and appliances out of the flood-prone area.
Wet-Bulb Temperature
The lowest temperature that can be obtained by evaporting water into the air.
WFP
Warm Front Passage
Whitecap
The breaking crest of a wave, usually white and frothy.
Widespread
Areal coverage of non-measurable, non-convective weather and/or restrictions to visibility affecting more than 50 percent of a forecast zone(s).
Wind Couplet
An area on the radar display where two maximum wind speeds are blowing in opposite directions.
Wind Shear Profile
The change in wind speed and/or direction usually in the vertical. The characteristics of the wind shear profile are of critical importance in determining the potential for and type of severe weather.
Wind Speed
The rate at which air is moving horizontally past a given point. It may be a 2-minute average speed (reported as wind speed) or an instantaneous speed (reported as a peak wind speed, wind gust, or squall).
Winter Pool
The pool, or height of the water surface, of a reservoir during the winter. This pool is usually a specific height and is maintained for the control of late winter and spring flooding.
Wrapping Gust Front
A gust front which wraps around a mesocyclone, cutting off the inflow of warm moist air to the mesocyclone circulation and resulting in an occluded mesocyclone.
WSPD
On a buoy report, the wind speed (m/s) averaged over an eight-minute period for buoys and a two-minute period for land stations. Reported Hourly.
WTMP
On a buoy report, the sea surface temperature (Celsius).
WWP
On a buoy report, Wind Wave Period is the time (in seconds) that it takes successive wind wave crests or troughs to pass a fixed point.
XCPT
Expecting
XPC
Expect
ZFP
Zone Forecast Product
Zurich Sunspot Classification
In solar-terrestrial terms, a sunspot classification system that has been modified for SESC use.
Z\/R Relationship
An empirical relationship between radar reflectivity factor z (in mm^6 / m^3 ) and rain rate ( in mm / hr ), usually expressed as Z = A R^b; A and b are empirical constants.

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