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. By the time this article appears, we will likely see some of these advisories and warnings as a series of storm systems are expected to bring plenty of snow to our area.
The criteria for issuing some of the advisories or warnings can depend on the time of year and/or the elevation. This holiday season, it’s a good idea to know the weather forecast, especially if traveling the mountain roadways. With additional storms coming, I expect to see more advisories or warnings issued as the cool and wet La Niña sea-surface temperature event continues to maintain its strength. I would also expect that approximately 70 percent of our total winter of 2011-’12 snowfall will be measured prior to the third week of January during the 60-day span from now to Jan. 19.
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 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 2-4 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 can also prompt warnings.
Another type of advisory is for freezing rain or sleet. 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 Empire, 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 visibility.