National Weather Service United States Department of Commerce

Weather Glossary: S's

S-Band Radar
These were in use as network radars in the National Weather Service prior to the installation of the WSR 88-D radars. They were 10-centimeter wavelength radars.
Sacramento Soil Moisture Accounting Model (SACSMA)
A model which simulates the movement and occurrence of water in and on top of the ground.
A satellite-based part of the Global Maritime Distress and Safety System (GMDSS) for automatically disseminating safety information, including weather warnings and forecasts, to mariners almost anywhere on the world's oceans.
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.

The scale does not address the potential for other hurricane-related impacts, such as storm surge, rainfall-induced floods, and tornadoes. It should also be noted that these wind-caused damage general descriptions are to some degree dependent upon the local building codes in effect and how well and how long they have been enforced.

Saffir-Simpson Hurricane Intensity Scale
Scale Number
Winds Damage
mph knots km/h
5 ≥ 157 ≥ 137 ≥ 252 Catastrophic damage will occur.
4 130 - 156 113 - 136 209 - 251 Catastrophic damage will occur.
3 111 - 129 96 - 112 178 - 208 Devastating damage will occur.
2 96 - 110 83 - 95 154 - 177 Extremely dangerous winds will cause extensive damage.
1 74 - 95 64 - 82 119 - 153 Very dangerous winds will produce some damage.
Sample and Hold
The process of sampling (measuring) the signal strength at a particular point in space (i.e., at a range gate).
Sandstorm (SS)
Particles of sand carried aloft by strong wind. The sand particles are mostly confined to the lowest ten feet, and rarely rise more than fifty feet above the ground.
Santa Ana Wind
A strong, hot, dry foehn-like wind that blows from the north, northeast, or east into southern California.
SAWRS (Supplementary Aviation Weather Reporting Station)

The SAWRS program addresses the concerns of users who depend on weather observations for air operations. If the cooperator is collocated with a commissioned automated system, they ensure continuity during outage periods of the automated system.

The requirement for a SAWRS arises from the FAA validated need for observations to satisfy FAR 121 or 135 operations or for the safe conduct of other aircraft. The classification of a SAWRS operation is directly related to the automated observing platforms. There are three distinct SAWRS classifications:

  1. SAWRS Indicates manual weather observations are the primary source of reporting the weather at the airport.
  2. Backup SAWRS Indicates automated observations taken by a commissioned version 3 of an AWOS, better known as an AWOS III, is the primary source of reporting the airport's weather. AWOS III systems may be FAA owned units or privately non-federal units owned by airports or local governments.
  3. SAWRS-II Indicates automated observations taken by a commissioned Automated Surface Observing System (ASOS), is the primary source of reporting the airport's weather. An ASOS unit is federally owned by the Federal Aviation Administration or the National Weather Service.
CAPE calculated using a Surface based parcel.
Lifted Index (LI) calculated using surfaced based parcel.
Scattered (SCT)
  1. An official sky cover classification for aviation weather observations, descriptive of a sky cover of 3/8 to 4/8. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.
  2. A National Weather Service convective precipitation descriptor for a 30%, 40%, and 50% chance of measurable precipitation (> Trace). See Probability of Precipitation (PoP).
The change in direction, frequency, or polarization of electromagnetic waves. See also Back scatter.
Scud (or Fractus)
Small, ragged, low cloud fragments that are unattached to a larger cloud base and often seen with and behind cold fronts and thunderstorm gust fronts. Such clouds generally are associated with cool moist air, such as thunderstorm outflow.
Sea Breeze
A current of air flowing inland, associated with warmer surface temperatures inland than at sea. Often shows up as a long, thin radar feature as insects get caught up in the frontal region. Temperature and moisture gradients across the front may also contribute to its reflectivity.
This term is used in National Weather Service Marine Forecasts to describe the combination or interaction of wind waves and swells (combined seas) in which the separate components are not distinguished. This includes the case when swells are negligible or are not considered in describing sea state.
Sea Level Pressure
The pressure value obtained by the theoretical reduction or increase of barometric pressure to sea-level.
Second-Day Feet (SDF)
The volume of water represented by a flow of one cubic foot per second for 24 hours; equal to 86,400 cubic feet. This is used extensively as a unit of runoff volume.
Sector Visibility
The visibility in a specific direction that represents at least a 45° arc of a horizontal circle.
Sectorized Hybrid Scan
A single reflectivity scan composed of data from the lowest four elevation scans. Close to the radar, higher tilts are used to reduce clutter. At further ranges, either the maximum values from the lowest two scans are used or the second scan values are used alone.
Sediment Storage Capacity
The volume of a reservoir planned for the deposition of sediment.
The interstitial movement of water that may take place through a dam, its foundation, or abutments.

A standing wave oscillation in any enclosed lake which continues after the forcing mechanism has ceased. In the Great Lakes, this forcing mechanism may be either strong winds along the axis of a lake, a pressure jump, or down draft winds associated with fast moving squall lines over the lake.

In either case, water is piled up at one end. The water sloshes from one end of the lake to the other causing fluctuations of perhaps several feet before damping out.

SELS - SEvere Local Storms Unit
The former name of the Operations Branch of the Storm Prediction Center (SPC) in Norman, OK.
Serial Derecho

It consists of an extensive squall line which is oriented such that the angle between the mean wind direction and the squall line axis is small. A series of LEWPs and bow echoes move along the line. The downburst activity is associated with the LEWPs and bows.

A serial Derecho tends to be more frequent toward the north end of the line during the late winter and spring months. It occurs less frequently than its cousin the progressive derecho.

It is associated with a linear type mesoscale convective system that moves along and in advance of a cold front or dry line. These boundaries are often associated with a strong, migratory surface low-pressure system and strong shortwave trough at 500 mb (strong dynamic forcing).

Lifted Indices are typically -6 or lower and the advection of dry air in the mid-troposphere (9,800 to 23,000 ft / 3,000 to 7,000 km above ground) by relatively strong winds leads to high convective instability and increased downdraft potential.

The bow echoes move along the line in the direction of the mean flow, often southwest to northeast. These storms move at speeds exceeding 35 knots (64 km/h). Squall line movement is often less than 30 knots (56 km/h).

Service Hydrologist
The designated expert of the hydrology program at a local weather forecast office.
The direction towards which a current is headed. For example, a current moving from west to east is said to be set to east.
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 Icing
The rate of ice accumulation on an aircraft is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary. This standard of reporting this type of icing was based on a recommendation set forth by the subcommittee for Aviation Meteorological Services in the Office of the Federal Coordinator for Meteorology in November 1968.
Severe Storm
A storm with a tornado, hail 1" (25 mm) or greater, or wind gusts 50 knots (58 mph / 93 km/h) or greater, or all three.
Severe Supercell Thunderstorm

It is 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 (EF2-EF5) 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 (1" in diameter or greater) and severe wind gusts in the excess of 50 knots (58 mph / 93 km/h). 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 1" in diameter, wind gusts greater than 58 mph (93 km/h), 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.

Severe Thunderstorm

A thunderstorm that produces either of the following: winds of 50 kt (58 mph / 93 km/h) or greater (these speeds can result in structural or tree damage), hail 1" (25 mm) in diameter or larger, or a tornado. Lightning frequency is not a warning criterion for issuing a severe thunderstorm warning.

Severe thunderstorms can result in the loss of life and property. They can also produce a tornado with little or no advanced warning. A table of hail sizes can be found in this glossary, under the definition of hail. See approaching (severe).

Severe Thunderstorm Warning (SVR)

This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail 1" (25 mm) or larger in diameter and/or winds equal or exceed 50 kt (58 mph / 93 km/h); therefore, people in the affected area should seek safe shelter immediately.

Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criterion for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect.

Like a Tornado Warning, the Severe Thunderstorm Warning is issued by your National Weather Service Forecast Office (NWFO). Severe Thunderstorm Warnings will include where the storm was located, what towns will be affected by the severe thunderstorm, and the primary threat associated with the severe thunderstorm warning.

If the severe thunderstorm will affect the nearshore or coastal waters, it will be issued as the combined product--Severe Thunderstorm Warning and Special Marine Warning.

If the severe thunderstorm is also causing torrential rains, this warning may also be combined with a Flash Flood Warning. After it has been issued, the issuing weather forecast office will followed up periodically with Severe Weather Statements.

These statements will contain updated information on the severe thunderstorm and they will also let you know when warning is no longer in effect. Severe weather reports will either be reported in one the following 3 products:

  1. Another severe weather warning (Tornado or Severe Thunderstorm),
  2. In a Severe Weather Statement, or
  3. In a Local Storm Report.
Severe Thunderstorm Watch (WWA)

This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces 1" (25 mm) hail or larger in diameter and/or winds equal or exceed 50 kts (58 mph / 93 km/h).

The size of the watch can vary depending on the weather situation. They are usually issued for a duration of four to eight hours. They are normally issued well in advance of the actual occurrence of severe weather.

During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.

A Severe Thunderstorm Watch is issued by the Storm Prediction Center in Norman, Oklahoma. Prior to the issuance of a Severe Thunderstorm Watch, SPC will usually contact the affected local National Weather Service Forecast Office and they will discuss what their current thinking is on the weather situation.

Afterwards, SPC will issue a preliminary Severe Thunderstorm Watch and then the affected NWFO will then adjust the watch (adding or eliminating counties/parishes) and then issue it to the public by way of a Watch Redefining Statement.

During the watch, the local forecast office will keep the you informed on what is happening in the watch area and also let the you know when the watch has expired or been canceled.

Severe Weather Potential Statement (SPS or HWO)
This statement is issued 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 Statement (SVS)
A National Weather Service product which provides follow up information on severe weather conditions (severe thunderstorm or tornadoes) which have occurred or are currently occurring.
A transient electric or magnetic field generated by any feature of lightning discharge (entire flash).
Variation in wind speed (speed shear) and/or direction (directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances.
Sheet Flow
Flow that occurs overland in places where there are no defined channels, the flood water spreads out over a large area at a uniform depth. This also referred to as overland flow.
SHEF (Standard Hydrologic Exchange Format)
A documented set of rules for coding data for operational day-to-day use in a form for both visual and computer recognition.
A software decoder for SHEF Data.
Shelf Cloud

A low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms).

Unlike the roll cloud, the shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm). Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn. It is accompanied by gusty, straight-line winds and is followed by precipitation.

The process whereby waves coming into shallow waters are slowed by bottom friction and become closer together and steeper.
Shore Ice
An ice sheet in the form of a long border attached to the bank or shore. See border ice.
Short-Fuse Warning
A warning issued by the NWS for a local weather hazard of relatively short duration. Short-fuse warnings include tornado warnings, severe thunderstorm warnings, and flash flood warnings. Tornado and severe thunderstorm warnings typically are issued for periods of an hour or less, flash flood warnings typically for three hours or less.
Short Term Forecast
This National Weather Service narrative summary describes the weather in the local area and includes a short-range forecast (usually not more than 6 hours). This product will be updated more frequently when it is used during active weather. This product is also sometimes referred to as a nowcast.
Shortwave (or Shortwave Trough)
A disturbance in the mid or upper part of the atmosphere which induces upward motion ahead of it. If other conditions are favorable, the upward motion can contribute to thunderstorm development ahead of a shortwave.
Showalter Index (SWI)

It is a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI.

It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.

Showalter Index values
Showalter Index Thunderstorm Consideration
< -6 Extremely unstable - good strong thunderstorm potential
-4 to -6 Very unstable - good heavy thunderstorm potential
0 to -3 Unstable - thunderstorms probable
3 to 1 Thunderstorm possible - strong trigger needed
Shower (SH)
It implies short duration, intermittent, and scattered precipitation (rain, snow, ice pellet) of a more unstable, convective nature.
A secondary energy maximum located outside the main radar beam. Typically, it contains a small percentage of energy compared to the main lobe, but it may produce erroneous echoes.
Side Channel Spillway
A spillway whose crest is roughly parallel to the channel immediately downstream of the spillway.
Significant Wave Height
The average height (trough to crest distance) of the one-third highest waves. An experienced observer will most frequently report heights equivalent to the average of the highest one-third of all waves observed.
Single Cell Thunderstorm

This type of thunderstorm develops in weak vertical wind shear environments. On a hodograph, this would appear as a closely grouped set of random dots around the center of the graph. They are characterized by a single updraft core and a single downdraft that descends into the same area as the updraft.

The downdraft and its outflow boundary then cut off the thunderstorm inflow. This causes the updraft and the thunderstorm to dissipate. Single cell thunderstorms are short-lived. They only last about 1/2 hour to an hour. These thunderstorms will occasionally become severe, but only briefly. In this case, they are called Pulse Severe Thunderstorms.

Siphon Spillway
A spillway with one or more siphons built at crest level. This type of spillway is sometimes used for providing automatic surface-level regulation within narrow limits or when considerable discharge capacity is necessary within a short period of time.
Term used in conjunction with "forecast" or "warning" to convey the fact that a hydrologic (stream) forecast is produced for an individual stream gage location as opposed to a general area (e.g., a city, zone, or county) as is commonly done in many types of weather forecasts.
Sky Condition
Used in a forecast to describes the predominant/average sky condition based upon octants (eighths) of the sky covered by opaque (not transparent) clouds.
Predominant/average sky condition
Sky Cover (in eighths) Description
8/8 Cloudy
5/8 to 7/8 Mostly Cloudy or Considerable Cloudiness
3/8 to 4/8 Partly Cloudy or Partly Sunny
1/8 to 2/8 Mostly Clear or Mostly Sunny
0/8 Clear or Sunny
Sleet (PL)
Describes solid grains of ice formed by the freezing of raindrops or the refreezing of largely melted snowflakes. These grains usually bounce upon impact with the ground or pavement. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of 1/2" (13 mm) or more. See Ice Pellets.
Slight Chance
A National Weather Service precipitation descriptor for a 20 percent chance of measurable precipitation (0.01" / 0.25 mm). When the precipitation is convective in nature, the term widely scattered is used. See Precipitation Probability (PoP).
Slight Risk of Severe Thunderstorms
An area of organized severe storms, which is not widespread in coverage with varying levels of intensity. See high risk, moderate risk, enhanced risk and marginal risk.
Sling psychrometer
A psychrometer in which the wet and dry bulb thermometers are mounted upon a frame connected to a handle. The psychrometer may be whirled by hand in order to provide the necessary ventilation.
Small Craft Advisory

This is issued by the National Weather Service to alert small boats to sustained (more than 2 hours) hazardous weather or sea conditions. These conditions may be either present or forecasted.

The threshold conditions for it are usually sustained winds of 18 kt (21 mph / 34 km/h) (less than 18 knots in some dangerous waters) to 33 kt (38 mph / 61 km/h) inclusive or hazardous wave conditions (such as 4 feet/1.2 meters or greater).

Along the coastal regions of the East Coast, Gulf of Mexico, and West Coast, this advisory relates to conditions out to as much as 100 nautical miles (185 kilometers) of shore (coastal waters). As a result, these will be only issued in the Coastal Marine Forecast


Mariners learning of this advisory are urged to determine immediately the reason by turning their radios to the latest marine broadcast. Decisions as to the degree of the hazard will be left to the boater, based on experience and size and type of boat. There is no legal definition for a "small craft".

Small Stream Flooding
Flooding of small creeks, streams, or runs.
Originally smog meant a mixture of smoke and fog. Now, it means air that has restricted visibility due to pollution or pollution formed in the presence of sunlight--photochemical smog.
Smoke (FU)
A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled a great distance (25-100 miles/40-160 kilometers or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.
Smoke Management
Conducting a prescribed fire or slash burn with firing techniques and meteorological conditions that keep the smoke's impact on the environment with acceptable limits.
Snow (SN)
Snowfall intensity based upon visibility
Snow Intensity Visibility
Miles Meters
Heavy < ¼ < 400
Moderate ¼ to ½ 400 to 800
Light > ½ > 800

Precipitation of snow crystals, mostly branched in the form of six-pointed stars. It usually falls steadily for several hours or more.

Qualifiers, such as occasional or intermittent, are used when a steady, prolonged (for several hours or more) fall is not expected. Like drizzle, its intensity is based on visibility.

The amount of snow that falls is highly dependent upon temperature. For example, at 10°F (-12°C), one inch (25 mm) of precipitation will produce 30" (76 cm) of snow. At 20°F (-7°C), one inch of precipitation will produce 20" (51 cm) of snow. At 30°F (-1°C), one inch of precipitation produces 10" (25 cm) of snow.

At freezing (0°C), one inch of precipitation will produce approximately 6" (15 cm) of snow.

Snow Advisory
This product is issued by the National Weather Service when a low-pressure system produces snow that may cause significant inconveniences, but do not meet warning criteria and if caution is not exercised could lead to life threatening situations. The advisory criteria vary from area to area.
Snow and Blowing Snow Advisory
This product is issued by the National Weather Service during situations that cause significant inconveniences, but do not meet warning criteria and if caution is not exercised could lead to life threatening situations. The advisory criteria vary from area to area.
Snow Core
A sample of either freshly fallen snow, or the combined old and new snow on the ground. This is obtained by pushing a cylinder down through the snow layer and extracting it.
Snow Density
The mass of snow per unit volume which is equal to the water content of the snow divided by its depth.
Snow Depth
The combined total depth of both the old and new snow on the ground.
Snow Flurries
They are intermittent light snowfalls of short duration (generally light snow showers) with no measurable accumulation.
Snow Grains (SG)
Precipitation of very small, white, and opaque grains of ice. They can be distinguished from ice pellets, because ice pellets bounce and snow grains do not bounce at all.
Snow Pack
The combined layers of snow and ice on the ground at any one time. It is also called snowcover.
Snow Pillow
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.
Snow Pellets (GS)
Precipitation of white, opaque grains of ice. The grains are round or sometimes conical. Diameters range from about 0.08" to 0.2" (2 mm to 5 mm).
Snow Shower (SHSN)
It is a moderate snowfall of short duration. Some accumulation is possible.
Snow Squalls (SQSN)
They are intense, but limited duration, periods of moderate to heavy snowfall. They are accompanied by strong, gusty surface winds, and possibly lightning (generally moderate to heavy snow showers). Snow accumulations may be significant.
Snow Stake
A 1¾" square, semi-permanent stake, marked in inch increments to measure snow depth.
Snow Stick
A portable rod used to measure snow depth.
Snow Water Equivalent
The water content obtained from melting accumulated snow.
A flat, solid, white material, such as painted plywood, approximately two feet square, which is laid on the ground, or snow surface by weather observers to obtain more accurate measurements of snowfall and water content.
Snowmelt Flooding
Flooding caused primarily by the melting of snow.
The total snow and ice on the ground, including both the new snow and the previous snow and ice which has not melted.
Soil Moisture
Water contained in the upper part of the soil mantle. This moisture evaporates from the soil and is the used and transpired by vegetation.
A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature, etc.
Southern Oscillation (SO)

A "see-saw" in surface pressure in the tropical Pacific characterized by simultaneously opposite sea level pressure anomalies at Tahiti, in the eastern tropical Pacific and Darwin, on the northwest coast of Australia.

The SO was discovered by Sir Gilbert Walker in the early 1920's. Walker was among the first meteorologists to use the statistical techniques to analyze and predict meteorological phenomena. Later, the three-dimensional east-west circulation related to the SO was discovered and named the "Walker Circulation".

The SO oscillates with a period of 2-5 years. During one phase, when the sea level pressure is low at Tahiti and High at Darwin, the El Niño occurs.

The cold phase of the SO, called La Niña, is characterized by high pressure in the eastern equatorial Pacific, low in the west, and by anomalously cold sea surface temperature (SST) in the central and eastern Pacific. This is called El Niño Southern Oscillation or ENSO.

Space Weather Prediction Center (SWPC)

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.

SWPC'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 Mesoscale Discussions (SWO)
A mesoscale discussion issued by the Storm Prediction Center in Norman, Oklahoma as a routine/daily, but nonscheduled, short-term (0-6 hours) product to communicate the current judgment of the SPC to the user community. It also provides guidance on other short-term mesoscale phenomenon that may be of significance (for example heavy snow potential, the formation of dense fog, etc.).
Spearhead Echo
A radar echo associated with a Down burst 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 its 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)

This is issued by the National Weather Service for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots or more that is not covered by existing marine warnings.

These are tone alerted on NOAA Weather Radio. Boaters will also be able to get this information by tuning into Coast Guard and commercial radio stations that transmit marine weather information.

Special Tropical Disturbance Statement (DSA)
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).
Special Weather Statement (SPS)
This is used by the National Weather Service to provide additional information about expected or ongoing significant weather changes not covered in other statements. This would include non-severe convective, winter weather, and non-precipitation events.
Specific Humidity
In a system of moist air, the ratio of the mass of water vapor to the total mass of the system.
Spectrum Width (SW)

This WSR-88D radar product depicts a full 360° 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 (SCS)

This WSR-88D radar product displays a vertical cross section of spectrum width on a grid with heights up to 70,000 feet (21 kilometers) on the vertical axis and distance up to 124 nm (230 km) 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 (230 km) of the radar that are less than 124 nm (230 km) 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 (32 km/h) at 10,000 feet (3,000 meters) increasing to 50 mph(80 km/h) at 20,000 feet(6,100 meters). Speed shear is an important factor in severe weather development, especially in the middle and upper levels of the atmosphere.
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. Some various types of spillways include:
  • Auxiliary or Emergency Spillway - A secondary spillway designed to operate only during exceptionally large flood flows. Allows inflows from large storms to be released from the reservoir before the water level raises high enough to overtop the dam.
  • Fuse Plug Spillway - An auxiliary or emergency spillway comprising a low embankment or a natural saddle designed to be overtopped and eroded away during flood flows.
  • Primary (or Principal) Spillway - The spillway which would be used first during normal inflow and flood flows.
  • Shaft or Morning Glory Spillway - A vertical or inclined shaft into which flood water spills and then is conducted through, under, or around a dam by means of a conduit or tunnel. If the upper part of the shaft is splayed out and terminates in a circular horizontal weir, it is termed a "bellmouth" or "morning glory" spillway.
Spillway Crest
The elevation of the highest point of a spillway.
Slang for a small-scale vortex initiation, such as what may be seen when a gustnado, landspout, or suction vortex forms.
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.
Spot Forecasts
These are NWS site-specific fire weather forecasts. They are issued upon request of User Agencies for wildfires, prescribed burns, or special projects.
Spray (PY)
An ensemble of water droplets torn by the wind from the surface of the of an extensive body of water, generally from crests of waves, and carried a short distance into the air.
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.
Squall (SQ)
A strong wind characterized by a sudden onset in which the wind speed increases at least 16 knots and is sustained more than 22 knots or more for at least one minute.
Squall Line
A line or narrow band of active thunderstorms. The line may extend across several hundred miles. It forms along and ahead of an advancing cold front.
An acronym for Storm-relative Helicity.
Stability Index
The overall stability or instability of a sounding is sometimes conveniently expressed in the form of a single numerical value. Used alone, it can be quite misleading, and at times, is apt to be worthless. The greatest value of an index lies in alerting the forecaster to those soundings which should be examined more closely.
An atmospheric state with warm air above cold air which inhibits the vertical movement of air.
Stable Air
Air with little or no tendency to rise, that is usually accompanied by clear dry weather.
Staccato Lightning
A Cloud to Ground (CG) lightning discharge which appears as a single very bright, short-duration stroke, often with considerable branching.
Staff Gage
A vertical staff graduated in appropriate units which is placed so that a portion of the gage is in the water at all times. Observers read the river stage off the staff gage.
The level of the water surface above a given datum at a given location along a river or stream.
Stair Stepping
The process of continually updating river forecasts for the purpose of incorporating the effects rain that has fallen since the previous forecast was prepared. The goal of using QPF is to minimize "stair-stepping.".
State Maximum/Minimum Temperature and Precipitation Table (STP)

This tabular product is issued by the National Weather Service once in the morning and evening. The morning product will contain the current weather conditions, yesterday's daytime high temperature, the 12-hour low temperature ending at a specified time, and 24-hour precipitation ending at a specified time from available reporting stations within the state or NWFO forecast area.

The evening product will contain the same information; however, the daytime high temperature will be today's high temperature instead of yesterday's high temperature. In the winter time, this product will contain the snow depth in inches if it is available.

State Weather Roundup (SWR)
This is a National Weather Service tabular product which provides routine hourly observations within the state through the National Weather Wire Service. It gives the current weather condition in one word (cloudy, rain, snow, fog, etc.), the temperature and dew point, 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 (3,650 meters) with no significant weather and/or obstructions to visibility.

Station Identifier
A group of four alphabetic characters used to identify a location that makes weather observations.
Station Pressure
The pressure that is read from a barometer but is not adjusted to sea level.
Stationary Front
A front that barely moves with winds blowing in almost parallel, but in opposite directions on each side of the front. Occasionally, these fronts can cause widespread flooding, because showers and thunderstorms moving along them will continue to move across the same area. This weather situation is called "train echoing".
Steam Fog

It forms as cold air moves over warm water. Water evaporates from the warm water surface and immediately condenses in the cold air above. Heat from the water warms the lower levels of the air creating a shallow layer of instability. It rises like smoke from the warm surface.

The low-level convection can become quite turbulent. Steam fog is most common in Arctic regions where it is called "Arctic Sea Smoke", but it can and does occur occasionally at all latitudes.

Steering Winds (or Steering Currents)
A prevailing synoptic scale flow which governs the movement of smaller features embedded within it.
Stepped Leader
A leader which initiates the very first stroke and establishes the channel for all subsequent streamers of a lightning discharge.
Stilling Basin
A basin constructed to dissipate the energy of fast-flowing water (e.g., from a spillway or bottom outlet), and to protect the streambed from erosion.
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.
Any disturbed state of the atmosphere, especially affecting the Earth's surface, and strongly implying destructive and otherwise unpleasant weather. Storms range in scale from tornadoes and thunderstorms through tropical cyclones to widespread extratropical cyclones.
Storm Data (SD)
This National Climatic Data Center (NCDC) monthly publication documents a chronological listing, by states, of occurrences of storms and unusual weather phenomena. Reports contain information on storm paths, deaths, injuries, and property damage. The December issue includes annual tornado, lightning, flash flood, and tropical cyclone summaries.
Storm Hydrograph
A hydrograph representing the flow or discharge of water past a point on a river.
Storm Motion
The speed and direction at which a thunderstorm travels.
Storm Prediction Center (SPC)
A national forecast center in Norman, Oklahoma, which is part of NCEP. The SPC is responsible for providing short-term forecast guidance for severe convection, excessive rainfall (flash flooding), and severe winter weather over the contiguous United States.
Storm Relative
Measured relative to a moving thunderstorm, usually referring to winds, wind shear, or helicity.
Storm Relative Mean Radial Velocity Map (SRM)
This WSR-88D radar product depicts a full 360° 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 Relative Mean Radial Velocity Region (SRR)

This WSR-88D radar product depicts a 27 nm by 27 nm (50 km by 50 km) region of storm relative mean radial velocity centered on a point which the operator can specify anywhere within a 124 nm (230 km) radius of the radar. The storm motion subtracted defaults to the motion of the storm closest to the product center, or can be input by the operator.

It is used to examine the 3-dimensional storm relative flow of a specific thunderstorm (radar operator centers product on a specific thunderstorm; aid in displaying shear and rotation in thunderstorms and storm top divergence that might otherwise be obscured by storm motion; and gain higher resolution velocity product.

Storm Scale
Referring to weather systems with sizes on the order of individual thunderstorms. See synoptic scale and mesoscale.
Storm Surge

A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Most hurricane deaths are caused by the storm surge. It can be 50 or more miles (80 kilometers or more) wide and sweeps across the coastline around where the hurricane makes landfall.

The maximum rises in sea-level move from under the storm to the right of the storm's track, reaching a maximum amplitude of 10 to 30 feet (3 to 9 meters) at the coast. The storm surge may even double or more in height when the hurricane's track causes it to funnel water into a bay.

The storm surge increases substantially as it approaches the land because the normal water depth decreases rapidly as it approaches the beaches. The moving water contains the same amount of energy; thus, resulting in an increase of storm surge. Typically, the stronger the hurricane, the greater the storm surge.

Storm Tide
The actual sea level resulting from astronomical tide combined with the storm surge. This term is used interchangeably with "hurricane tide".
Storm Total Precipitation (STP)

This WSR-88D radar product displays the total precipitation (in inches) as a graphical image. It displays hourly precipitation total (in inches) as a graphical image. Currently, this product is done in a polar format with resolution 1.1 nm (2 km) by 1 degree. It will reset after one hour of no precipitation.

It is used to monitor total precipitation accumulation; observe short term trends of precipitation tracks with time lapse of this product; and estimate total basin runoff and ground saturation.

Storm Track
The path that a low-pressure area follows.
Storm Warning
A warning of 1-minute sustained surface winds of 48 kt (55 mph or 88 km/h) or greater, either predicted or occurring, not directly associated with tropical cyclones.
Stormwater Discharge
Precipitation that does not infiltrate into the ground or evaporate due to impervious land surfaces, but instead flows onto adjacent land or water areas and is routed into drain/sewer systems.
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.
Straight Line Winds
Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.
Descriptive of clouds of extensive horizontal development, as contrasted to the more narrow and vertically developed cumuliform type. Stratiform clouds cover large areas but show relatively little vertical development. Stratiform precipitation, in general, is relatively continuous and uniform in intensity (i.e., steady rain versus rain showers).
Stratiform Rain
Horizontally widespread rain, uniform in character, typically associated with macroscale fronts and pressure systems.
Stratiform Rings and Bands
These occur between the active convective bands of a hurricane outside of the eye wall. Inner stratiform bands often exhibit the bright band aloft, a VIP Level 2, and in the lower layers typically show a VIP Level 1.
Stratiform Snow
Same as for stratiform rain except precipitation is in the form of snow.
Stratocumulus (Sc)

It has globular masses or rolls unlike the flat, sometimes definite, base of stratus. This cloud often forms from stratus as the stratus is breaking up or from spreading out of cumulus clouds. They usually consist of mainly water vapor and are located between the ground and 6,500 feet (1980 meters).

Stratocumulus often reveals the depth of the moist air at low levels, while the speed of the cloud elements can reveal the strength of the low-level jet.

The layer of the atmosphere above the troposphere, where temperature increases with height.
Stratus (St)
It is a low, uniform sheet-like cloud. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. It usually is located between the ground and 6,500 feet (1980 meters). It usually consists of mainly water vapor. Fog is a stratus cloud with its base located at the ground.
Stream Gage
A site along a stream where the stage (water level) is read either by eye or measured with recording equipment.
A channel of very high ion density which propagates through the air by the continual establishment of an electron avalanche ahead of its tip.
Water flowing in the stream channel. It is often used interchangeably with discharge.
Grooves or channels in cloud formations, arranged parallel to the flow of air and therefore depicting the airflow relative to the parent cloud. Striations often reveal the presence of rotation, as in the barber pole or "corkscrew" effect often observed with the rotating updraft of a Low Precipitation (LP) storm.
The change from ice (a solid) directly to water vapor (a gas) without going through the liquid water phase. It is the opposite of Deposition.
The bending of the radar beam in the vertical which is less than under standard refractive conditions. This causes the beam to be higher than indicated, and lead to the underestimation of cloud heights.
1. The slow sinking of air usually associated with high pressure areas. It is usually over a broad area.
2. Sinking down of part of the earth's crust due to underground excavation, such as the removal of groundwater.
Subsidence Inversion
It is produced by adiabatic heating of air as it sinks and is associated with anticyclones (high pressure) and/or stable air masses. These inversions form between sinking heated air and air below and they are characterized by temperature increase with height through the inversion, while above the inversion, the temperature cools almost dry adiabatically. The dew point temperature, relative humidity, and mixing ratio values all decrease with height through the inversion.
Subsurface Storm Flow
The lateral motion of water through the upper layers until it enters a stream channel. This usually takes longer to reach stream channels than runoff. This also called interflow.
A location where observations are taken or other services are furnished by people not located at NWS offices who do not need to be certified to take observations.
Sub-Synoptic Low
Essentially the same as mesolow.
Subtle Heavy Rainfall Signature ("SHARS")

This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2° per 12 hours). The average surface temperature is 68°F (15°C) with northeasterly winds.

The average precipitable water (P) value is equal to or greater than 1.34" (34 mm) and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2° to 3° north or northeast of the 850 mb low.

Subtropical Cyclone
A low-pressure system that develops over subtropical waters that initially has a non-tropical circulation, but in which some elements of tropical cyclone cloud structure are present. Subtropical cyclones can evolve into tropical cyclones. Subtropical cyclones are generally of two types:
  • Cold Low Type - This type has a circulation extending from the surface to the upper troposphere, with the maximum sustained low-level winds typically extending to a radius of 100 miles or more from the center.
  • Mesoscale (Sub-Synoptic Scale) Cyclone Type - This type develops in or near a dying frontal zone with horizontal wind shear. This low is compact and develops a tight pressure gradient with the maximum sustained low-level winds, which can reach hurricane intensity, typically located less than 30 miles (48 kilometers) from the center.

    The whole storm circulation may initially be no more than 100 miles (161 kilometers) in diameter. These lows are typically short-lived and spend their lives usually over water. They may be cold core or warm core. This strange hybrid was once referred to as a "neutercane" after being discovered by satellite imagery.

Subtropical Depression
A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 33 kt (38 mph or 62 km/h) or less.
Subtropical Storm
A subtropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) is 34 kt (39 mph or 63 km/h) or more.
Subtropical Jet
This jet stream is usually found between 20° and 30° latitude at altitudes between 39,000 and 46,000 feet (12 and 14 kilometers).
Suction Vortex (sometimes Suction Spot)
A small but very intense vortex within a tornado circulation. Several suction vortices typically are present in a multiple-vortex tornado. Much of the extreme damage associated with violent tornadoes (F4 and F5 on the Fujita scale) is attributed to suction vortices.

A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers).

Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS.

Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm.

Storms exhibiting these characteristics often are called classic supercells; however HP storms and LP storms also are supercell varieties.

Supercooled Liquid Water
In the atmosphere, liquid water can survive at temperatures colder than 32°F (0°C); many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.
The condition which occurs in the atmosphere when the relative humidity is greater than 100%.
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
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 Based Convection
Convection occurring within a surface-based layer, i.e., a layer in which the lowest portion is based at or very near the earth's surface. Compare with elevated convection.
Surface Impoundment
An indented area in the land's surface, such as a pit, pond, or lagoon.
Surface Pressure
The pressure that is read from a barometer but is not adjusted to sea level.
Surface Runoff
The part of runoff, caused by precipitation and/or snowmelt, that moves over the soil surface to the nearest stream channel. Rain that falls on the stream channel is often lumped with this quantity.
Surface Water
Water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.
Sustained Wind
Wind speed determined by averaging observed values over a 2-minute period.
Severe WEAther Threat Index (SWEAT Index)

A stability index developed by the Air Force which incorporates instability, wind shear, and wind speeds. The index combines the effects of low-level moisture (850 mb dew point), convective instability (Total Totals (TT) Index), jet maxima (850 mb and 500 mb wind speed), and warm air advection (veering directional shear between 850 mb and 500 mb).

It was designed to discriminate between ordinary and severe convection by incorporating thermodynamic information (850 mb dew point and Total Totals Index) and kinematic information (low- and mid-level flow characteristics showing strong wind fields and veering directional shear).

SWEAT Index - Severe Weather ThrEAT index as follows: SWEAT = (12 Td 850) + (20 [TT-49]) +(2 f 850) + f 500 + (125 [s+0.2]) where

  • Td 850 is the dew point temperature at 850 mb,
  • TT is the total-totals index,
  • f 850 is the 850-mb wind speed (in knots),
  • f 500 is the 500-mb wind speed (in knots), and
  • s is the sine of the angle between the wind directions at 500 mb and 850 mb (thus representing the directional shear in this layer).

SWEAT values of about 250-300 or more indicate a greater potential for severe weather, but as with all stability indices, there are no magic numbers. The SWEAT index has the advantage (and disadvantage) of using only mandatory-level data (i.e., 500 mb and 850 mb), but has fallen into relative disuse with the advent of more detailed sounding analysis programs.

When there are no opaque (not transparent) clouds. Same as Clear.
A rise in water level caused by strong wind or fluctuations in the atmospheric pressure. This term is usually applied to a sharp set-up effect at the downwind end of the lake, but thunderstorms can cause brief local storm surges in bays and harbors. See Set-up.
Sustained Overdraft
Long-term withdrawal from the aquifer of more water than is being recharged.
Snow Water Equivalent.
Wind-generated waves that have traveled out of their generating area. Swells characteristically exhibit smoother, more regular and uniform crests and a longer period than wind waves.
SWODY1 (sometimes pronounced swoe-dee)
The Day 1 Severe Weather Outlook outlines areas in the continental United States where severe thunderstorms may develop during the next six to 30 hours. The Convective Outlook is issued five times daily.
The Day 2 Severe Weather Outlook is very similar to the Day 1 Outlook. It is issued only twice a day.
The Day 3 Severe Weather Outlook is very similar to the Day 2 Outlook. It is issued daily.
The fear of ice or frost. See also Cheimaphobia, Cheimatophobia, Cryophobia, Psychorophobia.
Symmetric Double Eye
A concentrated ring of convection that develops outside the eye wall in symmetric, mature hurricanes. The ring then propagates inward and leads to a double-eye. Eventually, the inner eye wall dissipates while the outer intensifies and moves inward.
Synoptic Chart
Chart showing meteorological conditions over a region at a given time; weather map.
Synoptic Scale (or Large Scale)
The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning a continent. Also called cyclonic scale. Compare with mesoscale and storm-scale.
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.
The instance (new moon or full moon) when the earth, sun, and moon are all in a straight line.