Below are the definitions of winter weather products issued by the National Weather Service including watches, warnings, and advisories. We have also included the definitions of some general winter weather terms.
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National Weather Service Winter Watches, Warnings, and Advisories
Blizzard Warning: Sustained winds or frequent gusts of 35 mph occurring in combination with considerable falling and/or blowing snow for a period of at least three hours. Visibilities will frequently be reduced to less than one-quarter mile and temperatures will often remain extremely cold in a blizzard.
Heavy Snow Warning: Snow accumulations are expected to approach or exceed six inches in 12 hours but will not be accompanied by significant wind. A heavy snow warning could also be issued if eight inches or more of accumulations are expected in a 24 hour period. In addition, during a heavy snow warning, freezing rain and sleet are not expected.
Ice Storm Warning: A significant coating of ice, one-quarter inch or more, is expected.
Wind Chill Warning: Life-threatening wind chills of minus 50 or lower are expected.
Winter Storm Watch: A significant winter storm may affect your area, but its occurrence, location and timing are still uncertain. A winter storm watch is issued to provide 12 to 36 hours notice of the possibility of severe winter weather. A watch will often be issued when neither the path of a developing winter storm nor the consequences of the weather event are as yet well defined. Ideally, the winter storm watch will eventually be upgraded to a warning when the nature and location of the developing weather event becomes more apparent. A winter storm watch is intended to provide enough lead time so those who need to set plans in motion can do so.
Winter Storm Warning: Issued when hazardous winter weather is occurring, imminent or likely. A warning is used for winter weather conditions posing a threat to life and property. A winter storm warning are usually issued for heavy snow approaching or exceeding six inches, ice accumulations, dangerous wind chills, or a combination of the three. Warnings can be issued for lesser amounts of snow, say 3 to 6 inches, if the snow occurs with strong winds in excess of 20 miles an hour and/or significant sleet or heavy ice accumulations from freezing rain. Expected snow accumulation during a winter storm warning is four inches or more in 12 hours or six inches or more in 24 hours at low, flat areas such as the Plains or South. For mountainous areas less than or equal to 7,000 feet, a snowfall of six inches or more in 12 hours or 10 inches or more in 24 hours would prompt a warning. For elevations greater than 7,000 feet, snowfall of eight inches or more in 12 hours or 12 inches or more in 24 hours would qualify for a warning.
Blizzard: Winds of 35 mph or more along with considerable falling and/or blowing snow reducing visibility to less than one-quarter mile for three or more hours. Extremely cold temperatures often are associated with dangerous blizzard conditions, but are not a formal part of the definition. The hazard created by the combination of snow, wind and low visibility significantly increases, however, with temperatures below 20 degrees.
Blowing Snow: Wind driven snow that reduces visibility to six miles or less causing significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind.
Drifting Snow: Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility.
Flurries: Very light snow falling for short durations.
Freeze: Occurs when the surface air temperature is expected to be 32 degrees Fahrenheit or below over a widespread area for a significant period of time.
Freezing rain or drizzle: Occurs when rain or drizzle freezes on surfaces such as trees, cars and roads, forming a coating or glaze of ice. Temperatures above the ground are warm enough for rain to form, but surface temperatures are below 32 degrees Fahrenheit, causing the rain to freeze on impact. Even small accumulations of ice can be a significant hazard.
Frost: Describes the formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the earth's surface falls below 32 degrees Fahrenheit, but because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with air temperatures in the middle 30s.
Graupel: Small pellets of ice created when supercooled water droplets coat, or rime, a snowflake. The pellets are cloudy or white, not clear like sleet, and often are mistaken for hail.
Heavy snow: Depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours.
Ice storm: An ice storm is used to describe occasions when damaging accumulation of ice are expected during a freezing rain situation. Significant accumulations of ice are defined as one-quarter inch or greater. This can cause trees, utility and power lines to fall down causing the loss of power and communication.
Sleet: Rain drops that freeze into ice pellets before reaching the ground. Sleet usually bounces when hitting a surface and does not stick to objects. However, it can accumulate like snow and cause a hazard to motorists. Heavy sleet occurs when a half of an inch of sleet accumulates.
Snow showers: Snow falling at varying intensities for brief periods of time. Some accumulation is possible.
Snow squalls: Intense, but of limited duration, periods of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possible lightning.
Whiteout: A condition caused by falling and/or blowing snow that reduces visibility to nothing or zero miles; typically only a few feet. Whiteouts can occur rapidly often blinding motorists and creating chain-reaction crashes involving multiple vehicles. Whiteouts are most frequent during blizzards.
Wind chill: The effect of the wind on people and animals. Heat produced by the body radiates out of our skin and into the surrounding air. When there is no wind, this thin layer of heat partially insulates us from the full effect of the cold surrounding us. When conditions are windy, this layer of heat is swept away from the body and thus the full impact of the cold air is felt on our skin. Thus, even though the actual air temperature is the same, it "feels" colder. The wind chill temperature gives a comparison to what it would feel like at a lower temperature if there were no wind. Wind chill only applies humans and animals; it has no effect on inanimate objects such as vehicles.
Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society