This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its
territories.

Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.

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OBS
Observation(s)

OBSC
Obscure

Obscuration
Any atmospheric phenomenon, except clouds, that restricts vertical
visibility (e.g., dust, rain, snow, etc.).

Obscuring Phenomena
Any atmospheric phenomenon, except clouds, that restricts vertical
visibility (e.g., dust, rain, snow, etc.).

Observation Well
In hydrologic terms, a non-pumping well used for observing the elevation
of the water table or piezometric surface.

Occluded Front
A composite of two fronts, formed as a cold front overtakes a warm or quasi-stationary front. Two types of occlusions can form depending on
the relative coldness of the air behind the cold front to the air ahead
of the warm or stationary front. A cold occlusion results when the
coldest air is behind the cold front and a warm occlusion results when
the coldest air is ahead of the warm front.

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.

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.

OCFNT
Occluded Front - a composite of two fronts, formed as a cold front
overtakes a warm or quasi-stationary front. Two types of occlusions can
form depending on the relative coldness of the air behind the cold front
to the air ahead of the warm or stationary front. A cold occlusion
results when the coldest air is behind the cold front and a warm
occlusion results when the coldest air is ahead of the warm front.

OCNL
Occasional

OEODM
The Office of Equal Opportunity and Diversity Management. Provides a
dual role. Advises and assists the Assistant Administrator in carrying
out the National Weather Service's (NWS) responsibilities relative to
Civil Rights laws, Executive Orders, regulatory guidelines, and other nondiscrimination laws within the Federal Government. Advises and
assists the Assistant Administrator in carrying out the NWS policy of
diversity management by fostering an inclusive workforce, building an environment that respects the individual and offering opportunities for
all employees to develop to their full potential. Cultural Diversity is
the mixture of differences and similarities each employee brings to the workplace to accomplish the goals of the NWS.

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.

Offshore Breeze
A wind that blows from the land towards a body of water. Also known as
a land breeze.

Offshore Flow
Occurs when air moves from land to sea, and is usually associated with
dry weather.

Offshore Waters
That portion of the oceans, gulfs, and seas beyond the coastal waters
extending to a specified distance from the coastline, to a specified
depth contour, or covering an area defined by specific latitude and
longitude points.

Offshore Waters Forecast
(OFF) - A National Weather Service marine forecast product for that
portion of the oceans, gulfs, and seas beyond the coastal waters
extending to a specified distance from the coastline, to a specified
depth contour, or covering an area defined by specific latitude and
longitude points.

OFSHR
Offshore

OH
Office of Hydrology

OHD
Overhead

Okta
Used for the measurement of total cloud cover. One okta of cloud cover
is the equivalent of 1/8 of the sky covered with cloud.

OLR
Outgoing Longwave Radiation

Omega
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 (UVV) in the
atmosphere. If this upward vertical motion is strong enough and in a sufficiently moist airmass, precipitation results.

Omega High
A warm high aloft which has become displaced and is on the polarward
side of the jet stream. It frequently occurs in the late winter and
early spring in the Northern Hemisphere. The name comes from its
resemblance to the Greek letter, Omega, when analyzed on upper air
charts. It is an example of a blocking high.

Onshore Breeze
A wind that blows from a body of water towards the land. Also known as
a seabreeze.

Onshore Flow
Occurs when air moves from sea to land, and is usually associated with increased moisture.

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 30°N.

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.

Orifice
In hydrologic terms,

(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.

ORIG
Original

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.

Oscillation
A shift in position of various high and low pressure systems that in
climate terms is usually defined as an index (i.e., a single numerically-derived number, that represents the distribution of
temperature and pressure over a wide ocean area, such as the El
NiÑo-Southern Oscillation, North Atlantic Oscillation, and Pacific
Decadal Oscillation).

OTLK
Outlook

OTR
Other

OTRW
Otherwise

Outer Convective Band
Bands in a hurricane that occur in advance of main rain shield and up
to 300 miles from the eye of the hurricane. The typical hurricane has
two or three bands (and sometimes more) which are comprised of cells
resembling ordinary thunderstorms. Wind gusts are usually higher in
these bands than in the Pre-Hurricane Squall Line.

Outflow
Air that flows outward from a thunderstorm.

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
In hydrologic terms, a natural stream channel which transports
reservoir releases.

Outgoing Longwave Radiation
Outgoing Longwave Radiation is a polar satellite derived measurement
of the radiative character of energy radiated from the warmer earth
surface to cooler space. This measurement provides information on
cloud-top temperature which can be used to estimate tropical
precipitation amounts which is important in forecasting weather and
climate.

Outlet
In hydrologic terms, an opening through which water can be freely
discharged from a reservoir.

Outlet Discharge Structure
In hydrologic terms, 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.

Outlook
An outlook 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.

OVC
Overcast- 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.

Overcast
(Abbrev. 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.

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
In hydrologic terms, 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 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.

OVNGT
Overnight

OVR
Over

OVRN
Overrun

OVRNGT
Overnight

Ozone
A form of oxygen, O3. A powerful oxidizing agent that is considered a
pollutant in the lower troposphere but an essential chemical in the stratosphere where it protects the earth from high-energy ultraviolet
radiation from the sun.

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.

Ozone Hole
A severe depletion of stratospheric ozone over Antarctica that occurs
each spring. The possibility exists that a hole could form over the
Arctic as well. The depletion is caused by a chemical reaction
involving ozone and chlorine, primarily from human produced sources,
cloud particles, and low temperatures.

Ozone Layer
An atmospheric layer that contains a high proportion of oxygen that
exists as ozone. It acts as a filtering mechanism against incoming
ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above
the earth's surface.
--- SBBSecho 3.14-Win32
* Origin: The Thunderbolt BBS - tbolt.synchro.net (618:250/33)