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Weather Glossary: O's

Occluded Front (Occlusion)
A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air is colder than the air ahead of it, the front is called a cold occlusion. When the air behind the front is milder than the air ahead of it, it is called a warm occlusion. These processes lead to the dissipation of the front in which there is no gradient in temperature and moisture.
Occluded Mesocyclone
A mesocyclone in which air from the rear-flank downdraft has completely enveloped the circulation at low levels, cutting off the inflow of warm unstable low-level air.
Ocean Prediction Center (OPC)

This is one of 9 centers that comprises the National Centers for Environmental Prediction (NCEP). The Ocean Prediction Center (OPC) is an integral component of NCEP. OPC is located at the NOAA Center for Weather and Climate Prediction in College Park, MD.

The primary responsibility is the issuance of marine warnings, forecasts, and guidance in text and graphical format for maritime users. Also, the OPC quality controls marine observations globally from ship, buoy, and automated marine observations for gross errors prior to being assimilated into computer model guidance.

In addition, OPC coordinates with the National Hurricane Center (NHC) with forecast points for tropical cyclones in the Atlantic Ocean E of 65°W.

Offshore/Onshore Flow
Offshore flow occurs when air moves from land to sea. It is usually associated with dry weather. Meanwhile onshore flow is when air over the water advances across land. It usually indicates an increase in moisture.
Offshore Forecast (OFF)
This marine forecast is designed to serve users who operate beyond the coastal waters where it usually requires more than a day or more of sailing to and from port. These users are mainly commercial fishermen and merchant shipping and, to a lesser extent, government and research vessels and large recreational craft.
Offshore (Open) Waters
  1. The waters extending from 5 miles (8 kilometers) to the midpoint of the Great Lakes.
  2. That portion of the oceans, gulfs, and seas beyond the coastal waters extending to a specified distance from a coastline, to a specific depth contour, or covering an area defined by a specific latitude and longitude points.
A reverse curve, shaped like an elongated letter S. The downstream faces of overflow dams are often made to this shape. (From the French word Ogive).
The Office of Hydrology, located in Silver Spring, MD.
The fear of rain or of being rained on.

A term used to describe vertical motion in the atmosphere. The "omega equation" used in numerical weather models is composed of two terms, the "differential vorticity advection" term and the "thickness advection" term. Put more simply, omega is determined by the amount of spin (or large-scale rotation) and warm (or cold) advection present in the atmosphere.

On a weather forecast chart, high values of omega (or a strong omega field) relate to upward vertical motion in the atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results.

Omega High
A blocking ridge of high pressure that forms in the middle or upper troposphere. It looks like the Greek letter omega.
One-Hour Rainfall Rate (OHP)
This WSR-88D radar product displays hourly precipitation total (in inches) as a graphical image. This product is done in polar format with resolution 1.1 nm by 1 degree and it requires 54 minutes of precipitation for initial product. The product is then updated every volume scan for the most recent hourly precipitation accumulation. It is used to:
  1. Assess rainfall intensities and amounts, and
  2. Aid in forecast procedures for flash flood watches and warnings, various statements, and river forecasts.
One Percent Chance Flood (One Hundred Year Flood)

Flood magnitude that has one chance in 100 of being exceeded in any future 1-year period. The occurrence of floods is assumed to be random in time, or regularity of occurrence is implied. The exceeding of a 1-percent chance is no guarantee, therefore, that a similar size flood will not occur next week.

The risk of experiencing a large flood within time periods longer than 1 year increases in a nonadditive fashion. For example, the risk of exceeding a 1-percent chance flood one or more times during a 30-year period is 25 percent and during a 70-year period is 50 percent.

On-site Meteorological Services

Meteorological services provided at or near the site of a wildland fire or major project site, normally, but not necessarily, utilizing a mobile fire weather support unit.

NWS personnel provide forecasts, summaries, and updates directly to the official having overall planning responsibility for the fire or project. These services are usually provided on a reimbursable basis.

Open Lake
The Great Lakes waters beyond 5 nautical miles from shore.
  1. An opening with closed perimeter, usually sharp edged, and of regular form in a plate, wall, or partition through which water may flow, generally used for the purpose of measurement or control of water.
  2. The end of a small tube, such as a Pilot tube, piezometer, etc.
Related to, or caused by, physical geography (such as mountains or sloping terrain).
Orographic Lifting (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.
Orphan Anvil
Slang for an anvil from a dissipated thunderstorm, below which no other clouds remain.
Outburst Flood
See Jokulhlaup.
Outer Convective Bands
These bands occur in advance of main rain shield and up to 300 miles (500 kilometers) from the eye of the hurricane. The typical hurricane has 2 or 3 bands and sometimes more of these bands which are comprised of cells resembling ordinary thunderstorms. Wind gusts are usually higher in these bands than in the Pre-Hurricane Squall Line.
Outflow Boundary

A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a temperature drop.

Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).

Outflow Channel
A natural stream channel which transports reservoir releases.
An opening through which water can be freely discharged from a reservoir.
Outlet Discharge Structure
Protects the downstream end of the outlet pipe from erosion and is often designed to slow down the velocity of released water to prevent erosion of the stream channel.
It is used to indicate that a hazardous weather or hydrologic event may develop. It is intended to provide information to those who need considerable lead time to prepare for the event.
Overcast (OVC)
An official sky cover classification for aviation weather observations, when the sky is completely covered by an obscuring phenomenon. This is applied only when obscuring phenomenon aloft are present--that is, not when obscuring phenomenon are surface-based, such as fog.
Radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. (The latter area is known as a weak-echo region, or WER.) The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).
Overland Flow
The flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a watercourse, it becomes runoff.

A weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface. Embedded thunderstorms sometimes develop in such a pattern; severe thunderstorms (mainly with large hail) can occur, but tornadoes are unlikely.

Overrunning often is applied to the case of warm air riding up over a retreating layer of colder air, as along the sloping surface of a warm front. Such use of the term technically is incorrect, but in general it refers to a pattern characterized by widespread clouds and steady precipitation on the cool side of a front or other boundary.

The failure of the radar to detect a target due to the radar beam passing above the target.
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 or a cyclic storm.
A nearly colorless (but faintly blue) gaseous form of oxygen, with a characteristic odor like that of weak chlorine. Its chemical formula is O3. It is usually found in trace amounts in the atmosphere, but it is primarily found at 30,000 to 150,000 feet (9,000 to 46,000 meters) above the ground.


Its production results from photochemical process involving ultraviolet radiation. Because it absorbs harmful radiation at those heights, it is a very beneficial gas. However, photochemical processes involving industrial/vehicle emissions can produce ozone near the ground. In this case, it can be harmful to people with respiratory or heart problems.

Ozone Action Day
A message issued by the Department of Natural Resources (DNR) through the National Weather Service when ozone levels may reach dangerous levels the next day. This message encourages you to prevent air pollution by postponing the use of lawn mowing, motor vehicles, boats, as well as filling their vehicle gas tanks.
Ozone Advisory
It is issued by the Department of Natural Resources (DNR) through the National Weather Service when ozone levels reach 100. Ozone levels above 100 are unhealthy for people with heat and/or respiratory ailments.