With the official start to winter on Tuesday, many folks have become concerned about the big thaw earlier this week. Much of the snow that fell in late November has melted and many are asking, “Are we going to have a white Christmas?”
It now appears that colder air from the north will settle in over our region between now and the end of December. Therefore, what falls from the sky should be in the form of snow. I do expect a series of smaller-type storms to bring us occasional snow, so the chances are still good that we’ll enjoy a white Christmas across the Inland Northwest.
As Pacific storms roll through our region, there will be a number of weather advisories and warnings issued by the National Weather Service. We’ve already seen a variety of these advisories and warnings, including a blizzard warning last month.
The criteria for issuing some of the warnings can depend on the time of year and/or the elevation. During this holiday season, it’s a good idea to know what’s in store, especially if one is planning to hit the roadways to go to the mountains or visit friends and relatives. With additional storms coming, I do expect more advisories or warnings over the next several weeks.
For much of the Inland Northwest, “the mountains” refers 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 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 can also prompt warnings.
Another type of advisory one might see is the 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 visibility.