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Guide to winter weather definitions




When you listen to a typical forecast and you hear there is a chance of snow, do you know what the National Weather Service (NWS) is saying? This is a guide that explains the different winter weather terms used by the NWS.

The word snow in a NWS forecast, without a qualifying word such as occasional or intermittent, means that the fall of snow will be of a steady nature and will probably continue for several hours without letup. Accumulation will be fairly uniform over a wide area, and the expected accumulation will be given in the forecast.

Snow flurries are defined as snow falling for short duration at intermittent periods. Although visibility may be reduced at times, any accumulation will be small, thus accumulation will not be mentioned in the forecast.

Snow showers and snow squalls are brief, intense falls of snow and are comparable to summer rain showers. Squalls are accompanied by gusty surface winds. Accumulation will vary greatly from one area to another, just as summer thunderstorms will drop significant rain in one area, and bypass others. Accumulation will be an average expected for the area, with higher amounts very possible.

Blowing and drifting snow generally occur together and result from strong winds and falling snow, or loose snow on the ground. Blowing snow is defined as snow lifted from the surface by the wind and blown about to a degree that horizontal visibility is greatly restricted. Drifting snow is used to indicate that strong winds will blow the snow into significant drifts.

Blizzards are the most dramatic and perilous of all winter storms, characterized by strong winds of at least 35 miles an hour, bearing blowing snow that will be reducing visibility to less than 1/4 mile for at least three hours. This can be new snow or snow already on the ground being picked up by the strong winds.

Freezing rain or freezing drizzle is rain or drizzle occurring when surface temperatures are below freezing. The moisture falls in liquid form, but freezes upon impact, resulting in a coating of an icy glaze on all exposed objects. This can range from a thin glaze to ice of several inches thick. A heavy accumulation of ice, especially when accompanied by high winds, devastates trees and transmission lines.

Sleet (ice pellets) can be easily identified as frozen raindrops which bounce when hitting the ground or other objects. Sleet does not stick to trees or power lines, but sleet in sufficient depth does cause hazardous driving conditions.

Any road-icing condition is extremely hazardous, as most drivers and pedestrians understand. Sometimes, precipitation does not occur, yet icing becomes a serious tragedy. Sometimes, the moisture within a heavy fog will freeze on road surfaces, or snow on roadways may become melted with traffic, which then refreezes as ice, and is polished by the automobile tires, turning the roadway into a veritable skating rink. This condition is known as black ice and any driver during the winter season should be on the alert for its development.

When the NWS feels that winter weather will threaten an area, then watches or warnings will be issued. A winter storm watch means that severe winter weather conditions may affect your area. This will include the elements of freezing rain, sleet, or heavy snow, which may occur separately or in combination. A winter storm warning means that severe winter weather conditions are imminent. A blizzard warning means that considerable falling and/or blowing snow and sustained winds of at least 35 miles per hour are expected for several hours.

For more information on winter weather, visit the National Weather Servicee at www.weather.gov; the Federal Emergency Management Agency at www.fema.gov; or the Red Cross at www.redcross.org.


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