Meteorological winter officially began for the Northern Hemisphere yesterday, December 1.  I think it was obvious in the last 10 days of November that we were getting close; we’ve already seen snow down to 1,500 feet once plus a very chilly and foggy pattern for the valleys on Thanksgiving and Black Friday.

December has begun with a strong ridge in the jet stream over the western US, while a cold trough further east is bringing a blast of winter cold and snow to the Midwest, Great Lakes and East Coast:

However, in winter this pattern brings a paradox to the valleys and basins of the West:  “fake cold,” more properly known as a wintry inversion.  In spite of very warm air in the middle levels of the atmosphere (5,000-10,000 ft.), the long nights and weak sun allow for cool air to pool in the lower elevations of the Pacific Northwest.   The result is that a much warmer than normal airmass can actually result in below normal temps at the surface!

Last Thursday and Friday we struggled to get out of the low 40s under the stubborn clouds (normal high is about 48-50°).  The last 3 days, east winds have pushed away the clouds and allowed daytime temps to rise to near normal…but now the nights are very cold instead. Last night PDX dropped to 27° and Vancouver to 24°; that’s the coldest we’ve gotten since last January!  Again, quite paradoxical when you consider that this same pattern causes heatwaves in spring and summer.

The dry pattern is expected to continue through Thursday, with some light rain activity likely Friday through Sunday.  That will mix out some of the inversion and allow temperatures to warm up, especially at night.

As of now the models are not showing a really active or stormy pattern for at least the next 10 days, and no arctic air or low elevation snow either. We deserve a break after our mostly wet November, but at some point we’re going to want rain and mountain snow to return. Stay tuned.