High pressure doesn’t always mean fair weather

Dry weather over the Thanksgiving holiday was good news for travelers, but was not so great for those folks hoping to spend some of their time off on area slopes. The first significant snow of the season didn’t arrive until this past Monday, bringing a total of nearly 4 inches to Coeur d’Alene with even higher totals in the mountains. Spokane saw quite a bit less with just over an inch, while Hauser and Sandpoint were on the high side with reports of 5 and 6 inches respectively.

Round two of the week’s wintry weather arrived Wednesday night and Thursday, bringing another good dose of snow to the region. (Snow totals from that storm were not available by the column deadline). Despite all the recent snows along with the healthy rains earlier in the month, Coeur d’Alene still has a precipitation deficit of more than 3 inches for 2007. When all the final numbers are tabulated, though, we will likely come in above normal for precipitation in November.

Before the snows had arrived, it was high pressure that had dominated the area. And though you might have come to associate the term “high pressure” with fair weather, that is not always the case, and certainly wasn’t the case last weekend. During the cold season, high pressure can bring two kinds of weather.

Over the Thanksgiving holiday, the air was stagnant and gray, with very little sun. Temperatures moved very little from day to night. In fact, Sunday and Monday, the difference between the high and low temperature in Coeur d’Alene was only 3 degrees. When we do get sun with high pressure, it is usually accompanied by bitter cold, as a chilly, dry, northerly flow scours out the moisture that might lead to foggy skies and other low clouds.

As last Monday’s high pressure moved eastward, with sub-freezing air firmly entrenched across North Idaho, low pressure moved in and the snows came. With the area snows came a host of winter weather warnings and advisories from the National Weather Service.

At one point, Spokane was under a snow advisory, Coeur d’Alene was under a heavy snow warning, and parts of western Montana and central Idaho were under a winter storm warning. Why all the different terminology for the same storm, you might ask? Criteria for certain types of warnings/advisories differ based on whether the area affected is in the mountains (elevations greater than 3,000 feet) or the valleys.

Snow advisories are issued (for valley locations only) when snowfall totals are expected to be between 2 and 4 inches in a 12-hour period. If more than 4 inches of snow are expected, a heavy snow warning is issued instead. In the mountains, the forecast must call for at least 8 inches of snow in 12 hours in order for a heavy snow warning to be issued.

If mixed precipitation is expected, such as sleet or freezing rain, or a combination of snow and wind with accumulations between 2 and 4 inches, then a winter weather advisory is issued. The advisory is upgraded to a winter storm warning if snow totals greater than 4 inches in a 12-hour period are forecast, with accompanying mixed precipitation, wind or both. Knowing the differences between all the advisories and warnings will help you to better prepare for what Mother Nature throws your way.

We are currently dealing with some pretty chilly temperatures to start the month of December. Folks who were in the area back in 1985, however, may recall an even more frigid Dec. 1, when temperatures in Coeur d’Alene dropped to 5 below zero.