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Questions about what weather terms mean? We've got answers.

Advisory: Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.

Alberta Clipper: A fast moving low pressure system that moves southeast out of Canadian Province of Alberta through the Plains, Midwest, and Great Lakes region usually during the winter. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures.

Atmospheric Pressure: The pressure exerted by the earth's atmosphere at any given point, determined by taking the product of the gravitational acceleration at the point and the mass of the unit area column of air above the point.

Black Ice: Slang reference to patchy ice on roadways or other transportation surfaces that cannot easily be seen.

Blowing Snow: Blowing snow is wind-driven snow that reduces surface visibility. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds. Blowing snow is usually accompanied by drifting snow.

Bow Echo: A radar echo which is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation exhibits cyclonic rotation.

CAPE: Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.
Clear Slot: With respect to severe thunderstorms, a local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.

Climate: The composite or generally prevailing weather conditions of a region, throughout the year, averaged over a series of years.
Comma Echo: A thunderstorm radar echo which has a comma-like shape. It often appears during latter stages in the life cycle of a bow echo.

Condensation: In general, the physical process by which a vapor becomes a liquid or solid; the opposite of evaporation, although on the molecular scale, both processes are always occurring.

CPC: Climate Prediction Center

ECMWF: European Center for Medium-Range Weather Forecasts, also known as Euro. Operational references in forecast discussions typically refer to the ECMWF's medium-range numerical forecast model, which runs out to 10 days.

EDT: Eastern Daylight Time.

EST: Eastern Standard Time.

GEM: Global Environmental Multiscale. One of the operational forecast models run at the CMC. The GEM is run twice daily, with forecast output out to 240 hours.

GFS: Global Forecast System. One of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 384 hours.

Mesoscale Discussion: When conditions actually begin to shape up for severe weather, SPC (Storm Prediction Center) often issues a Mesoscale Discussion statement anywhere from half an hour to several hours before issuing a weather watch.

NAM: North American Mesoscale. The NAM is run four times per day (00,06,12,18Z), all cycles run to 84 hours.


SPC: Storm Prediction Center

Trace: An unmeasurable or insignificant quantity. A precipitation amount of less than 0.2 mm.


Typhoon: A tropical cyclone in the Western Pacific Ocean in which the maximum 1-minute sustained surface wind is 64 knots (74 mph) or greater.

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