We may end the month with the first colder-than-normal December since 2010. (George Jiang)

It’s a really warm week to be starting the month of December, but here we are. This warmth should continue through the first week of December, but then a new cold, wintry pattern gets going by late next week.

In fact, the change could be so dramatic, we may end the month with the first colder-than-normal December since 2010.

In all, we are forecasting average temperatures of 37-39 degrees, about 1-2 degrees colder than normal. We think total liquid precipitation will be around 2 to 3 inches, which is below the 3.05 inch normal.

The Washington 30-year (1981-2010) temperature average for December is 39.7 degrees, and we’ve been colder than that just six times here in the 2000s (most recently in 2010).  The three coldest Decembers of the 2000s were all La Niña events like this year, however, some warmer cases were too (like 2011, 2008, 2007).

What about snow?

Washington’s 30-year (1981-2010) average snow in December is a mere 2.3 inches, and we’ve only gotten above that about 30 percent of the time since the year 2000. The last time we did it was in 2009 (although 2010’s 2.1 came close!).

With a colder pattern arriving middle to late month, our odds for snow increase compared to normal, but our La Niña background state may favor a drier lean as shown on the CFS modeling below.  We’ll go the ultimate “hedgecast” of 1 to 3 inches for December, which is *still* snowier than last year’s mere trace outcome.

Impressive cold pattern emerging. Despite a warm-dominated first week, we are monitoring a very amplified cold pattern emerging by Thursday of next week.

A strong high pressure ridge is poised to build up the western coast of North America right up into Alaska and across much of the Polar regions. This sets up the “sliding board” for significant cold weather outbreaks across the Midwest, East and South.  Despite the warm first week, we believe colder weather from late next week into the Christmas time frame should balance the month toward the colder side.

This type of pattern could persist for at least 2-3 weeks.  Sometimes they can hold for up to 4-6 weeks, but this year’s weak La Niña status argues for more variability, keeping confidence lower on bigger duration.

CFS model forecasts cold and dry. The National Weather Service CFS long-range model jumps around a lot but has recently begun to pick up on the upcoming December pattern change.  It features slightly colder weather in the East (about 1F colder than normal for Washington), but it also favors a drier East Coast, limiting our snow opportunities.

National Weather Service offers limited guidance for Washington.  The National Weather Service forecast maps shown below will be updated again on Thursday evening.  I believe they may go colder over more of the United States given the incoming colder pattern — especially Midwest, East and South.  However, they may hold the drier lean for the East and South, which is a situation commonly found with La Niña- even weak ones like ours now.  Their updates below from Nov. 16 only favor “equal chances” of any result for Washington.