Well…that happened quicker than I might have predicted. PDX saw its first snowfall of the winter today, a whopping 0.1″!  It’s actually the first time the snow station on Parkrose has ever measured any snow on December 4.  We had a fairly marginal situation for snow today in the Metro area, with just some cool/dry east wind from the Gorge to provide evaporative cooling in the lower levels of the air. Temps were in the 33-35° range for most of the day, which limited accumulations to Trace-1″ most low elevation spots.  Parts of the eastern suburbs (Gresham/Troutdale) and the hilltops were cool enough to pick up 1-2″, and totals as high as 4″ were reported in the Gorge proper.

I missed most of the Portland/Vancouver action but got to see it in Tacoma instead!  A slushy inch had fallen in the south part of Tacoma as well as the northern hills.  Of course the Puget Sound already saw some minor snow events over the past few days.  Here is a photo of the snow at my house circa 2pm, courtesy of my roommate Megan:

Tacoma had about a full inch on the south end, here is an image I took at South Park:

Today’s snowfall is a stark reminder that you don’t need a really cold airmass to get frozen precip at the surface. Snow is possible with temps of -4°C or even -2°C at the 5000-foot level, provided the lower layers are able to stay cool.  Here is the atmospheric profile near Portland for 10am today:

Notice how the red temperature curve runs nearly parallel to the dotted line from 800mb down to 900mb elevation, and doesn’t cross the freezing line until the last 1000′ or so.  Easterly flow below the 850mb level meant that these low levels were able to cool just enough to bring the snow to low elevation. Without this effect, a -4°C airmass would only bring the snow down to about 2,000′.

Looking ahead…we have very quiet weather tomorrow through Wednesday. Then another chilly North Pacific system comes in Thursday morning. The mid-level airmass will be somewhat cooler (between -5° and-6°), BUT there won’t be any cool east wind to drag the snow level down this time.  The air will be sweeping in directly off the ocean. In this scenario you need 850mb temps close to -7 in order to overcome the low-level warming effects of the Pacific. So snow on Thursday should be confined to the coast range and Cascade foothills, or maybe a dusting atop the Tualatin Hills if we get lucky.

-Karl