Observation Well:  A non-pumping well used for observing the elevation of the water table or piezometric surface.

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.

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 National Weather Service 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 lessor extent, government and research vessels and large recreational craft.

Offshore (Open) Waters: 1) The waters extending from 5 miles 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.

Ogee:  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).

OH:  The Office of Hydrology, located in Silver Springs, MD.

Ohm's Law:  I = E / R, where I is current (amperes), E is electromotive force (i.e., voltage) and R is resistance (ohms)

OmbrophobiaThe fear of rain or of being rained on.

Omega High: A blocking ridge of high pressure that forms in the middle or upper troposphere. It looks like the Greek letter omega.

OML:  An Operations Manual Letter. These serve as updates to policy and procedure for the National Weather Service Operations Manual (WSOM).

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 ServicesMeteorological 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.

Open Great Lakes Marine ForecastsThis type of marine forecast is issued every 6 hours for the open waters of The Great Lakes and all of Lake St. Clair.  The term open waters refers to those areas of The Great Lakes which are beyond 5 nautical miles from shore.  The only exception to this rule in on Lake St. Clair.  The reason for this is that the open waters on this lake would be such a small area that it would be ridiculous to split the lake up in this way. This forecast will usually cover a 36 hour period.  It contains the expected wind direction (to 8 points of the compass) and speed, waves heights, and significant weather conditions.  They will also include information regarding structural icing and lake ice.  If these conditions are expected to vary across one of The Great Lakes, the lakes will be broken up into pieces.  This forecast is also used to let the mariner know when either a Gale or Storm Warning has been issued on the lake. The range of waves in this forecast indicate the heights to the nearest foot at the end of the fetch (downwind end of the lake--that area where the greatest significant waves are expected).  The range in the forecast is intended to show the uncertainty of precise values in the downwind area.  It is not intended to indicate a range of wave heights over the entire lake area.  The mariner should infer the wave heights in other areas of the lake based on his/her position and wind direction.

This product is currently issued by:  NWFO Buffalo, New York (Lake Ontario); NWFO Cleveland, Ohio (Lake Erie); NWFO Chicago, Ohio (Lake Michigan); NWFO Marquette, Michigan (Lake Superior) and NWFO Detroit/Pontiac Michigan (Lake Huron).  This product is issued 4 times a day and it covers three periods (about 36 hours).  The following table shows the times that this product is issued and what periods that they cover:

Open Great Lakes Forecasts Issuance Time

Issuance Time

Periods Covered

Early Morning (3-4 AM)

Today, Tonight, Tomorrow

Late Morning (9-10 AM)

This Afternoon, Tonight, Tomorrow

Late Afternoon (3-4 PM)

Tonight, Tomorrow, Tomorrow Night

Evening (9-10 PM)

Overnight, Tomorrow, Tomorrow Night


Operational Mode: A combination of scanning strategies and product mixes tailored to one or more meteorological situations.

Orifice: 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 Pitot tube, piezometer, etc.

Orographic:  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.

Oscillator:  The general term for an electric device that generates alternating currents or voltages. The oscillator is classified according to frequency of the desired signal.

Outburst Flood See Jokulhlaup.

Outer Convective Bands: These bands occur in advance of main rain shield and up to 300 miles from the eye of the hurricane. The typical hurricanes 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 drop in temperature. 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.

Outlet:  An opening through which water can be freely discharged from a reservoir.
Outlet Discharge StructureProtects 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.

Outlook: 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.

Outlook for Freeze-up on the St. Lawrence Sea Way:  In late autumn, NWFO Buffalo, New York will issue an outlook for freeze-up on the St. Lawrence at Massena.  Other ice products are normally not needed for Lake Ontario and the seaway since the Welland Canal and the seaway close in December.

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.

Overdetermined Multiple Doppler AnalysisA multiple Doppler analysis technique in which there are more radars than the desired number of wind components to be retrieved. The wind retrieval can then be performed as an 'optimization' to reduce the retrieval errors.

Overhang:  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.
Overrunning:  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.

Overshooting: 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.

Ozone: 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 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 "heads-up" 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 residents 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.  

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