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Winter advisories, warnings alert to rain, ice, snow

Thu., Nov. 12, 2009

With the official start to winter more than a month away, some folks are already beginning to ask, “Where’s the snow?”

Last month, our weather pattern did change to the wetter side of the meteorological scale, but the air mass was too warm and produced only trace amounts in the lower elevations near Spokane. However, Coeur d’Alene has received 0.4 inches of snow for the season as of early Tuesday. By the end of this week, it’s possible we may see some measurable snow in many valley locations.

As Pacific storms roll through our region, there will be a number of weather advisories and warnings issued by the National Weather Service in Spokane. The criteria for issuing some of the advisories or warnings can depend on the time of year and/or the elevation. During the upcoming holiday season, it’s a good idea to know what’s ahead, especially when planning to hit the roadways to go to the mountains or visit friends and relatives.

For much of the Inland Northwest, the mountains refer to any elevation above 3,000 feet. A winter weather advisory, snow advisory, winter storm warning, and heavy snow warning are usually the most common statements issued by the National Weather Service.

A winter weather advisory is issued when a combination of winter weather hazards is expected. This would include a precipitation mix of snow, sleet or freezing rain, or a combination of snow and strong winds. The advisory is upgraded to a winter storm warning if snowfall in the valleys is expected to exceed 4 inches in a 12-hour period in addition to the sleet, freezing rain or wind.

In the mountains, the expected snowfall must exceed 8 inches in that time frame to prompt a warning. If the precipitation is expected to be all snow, a snow advisory is issued when 2 to 4 inches is likely in a 12-hour period. When more than 4 inches of snow is forecast for the valleys (8 inches in the mountains) in a 12-hour period, we’ll see a heavy snow warning.

For early- or late-season storms in the mountains, like in April or October, lesser snow amounts also can prompt warnings.

Another type of advisory one might see is freezing rain or sleet advisory. These are issued any time the surface becomes hazardous due to those types of precipitation. When more than a half inch of sleet is expected, a heavy sleet warning is issued. An ice storm warning is issued when the area is threatened by more than a quarter inch of ice. A blizzard warning is rare in the Inland Northwest, but is issued when visibility due to blowing snow is reduced to a quarter mile or less and winds are 35 mph or stronger. Also, a blizzard can occur even after it has stopped snowing, especially if there are strong winds that reduce visibilities.

I’ll have an update on our winter weather forecast next week.

Contact Randy Mann at randy@longrangeweather. com.

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