Weather warnings can be confusing
Wintry weather is finally getting a foothold here in North Idaho, and with it comes a host of different weather advisories and warnings. Now seems like a good time to go over the ones you’ll most often see and hear about on TV when wicked winter weather is approaching.
One of the reasons the different warnings can be confusing, is that the criteria for issuing some of them can depend on the time of year and/or the elevation. You almost need a cheat sheet to remember what means what. For the purposes of these advisories, mountains refer to any elevation above 3,000 feet. Winter weather advisory, snow advisory, winter storm warning and heavy snow warning seem to be the four most common weather service issuances, and can be the most confusing.
A winter weather advisory is issued when a combination of winter weather hazards is expected. This would include mixed precipitation such as 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 forecast to be all snow, a snow advisory is issued when 2 to 4 inches is expected 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, a heavy snow warning is issued. For early or late season storms in the mountains (i.e. October or April) lesser snow amounts can prompt warnings.
Another type of advisory you might see is the freezing rain or sleet advisory. These are issued any time the ground 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. Though rare, a blizzard warning is issued when visibility due to blowing snow is reduced to a quarter-mile or less and winds are 35 mph or stronger. A blizzard can occur even after it has stopped snowing, because the wind and visibility are the determining factors.
Speaking of wind, we seem to have had more of that than snow lately. A wind advisory is issued when sustained winds of at least 30 mph or gusts of 45 mph or greater are expected. The advisory gets upgraded to a high wind warning when sustained winds of at least 40 mph or wind gusts of 58 mph or greater are occurring or forecast to occur.
North Idaho saw 50-plus mph winds on several occasions this past week. Neither of the strong wind events were attributed to thunderstorms. Instead, they were caused in part by a strong pressure gradient, which is the change in air pressure from one place to another. Stronger gradients mean stronger winds. High winds at the surface can also occur when the faster winds aloft (i.e. the jetstream) are transported downward by sinking air behind a cold front. A combination of these two factors led to the destructive winds in the past week, including a 63-mph gust measured at the Pullman Airport last Monday.