Merry Christmas!  As I write this around 10am, it’s 37 degrees outside in my Orchards neighborhood with light rain.  I don’t see any signs of slush mixing in…but I imagine a few hundred feet up in the hills a few miles east, there’s a wintry mix or maybe even snow.

This morning’s 12z GFS showed an 850mb temp of -5.5C for 10am, and a “hole” in shower activity over Portland and Vancouver.  In fact it was dry for most of the morning until a few minutes ago.  That -5.5C is about 2 degrees Celsius too warm for sticking snow at the lowest elevations, at least with a westerly or southerly wind like we have right now.

(all maps in this blog post credited to TropicalTidbits.com)

Colder air should move in this evening, along with heavier shower activity.  By 10pm the 850s should be down to -7.5, putting us solidly in the “snow zone.”  Arctic air is spilling out over the eastern Pacific, which could create bands of “ocean-effect snow” for the Coast Range and even some coastal towns.  These ocean-effect bands can result in fairly heavy dumps of wet onshore snow for the immediate coast.  But in the valleys, the rainshadow effects of the ‘Stupid Coast Range’ (SCR) can result in minimal shower activity.  We will have to watch this closely, as the GEM model thinks the SCR effect will be pretty strong for Multnomah and Clark counties.  But the GFS clearly shows valley snow this evening, for an official White Christmas – even if it comes a few hours too late to enjoy in a proper fashion:

We stay in a showery pattern until Monday morning, as colder and drier air gradually filter down from the north.  By 10am Monday the GFS maps look like this:

An 850 temp of -11C over Portland.  That boundary between dark blue and pale whitish blue, is the -11.5 mark BTW.  Some arctic air in the Columbia Basin too…Monday is going to be very cold across the region!  Probably highs only in the upper 20s to near 30 for Portland-Vancouver.

The GFS and GEM models, as I mentioned above, disagree as to how much snow makes it over the Coast Range tonight, tomorrow and tomorrow night.  Here are the two snow maps, GFS first.  Lighter blue colors over the Willamette Valley indicate moderate snow totals of 2-5″, while the darker blue suggests only light dustings less than 2″.

The National Weather Service has a Winter Storm Warning for NW Oregon and SW Washington, due to predicted accumulations of 2-8″ between 4pm today and 4am Monday.  Higher snow totals expected in the hills.  I think a lot of that will depend on the wind direction.  If it stays strongly onshore, the Coast and Coast Range get pummeled while we only see an inch or so in the valleys.  But if we can get a little more northerly or easterly flow at the surface, it should perk up the inland snow shower activity.

I’m not going to go into Tuesday and beyond…we don’t yet have a clear idea where any “slider lows” might set up.  Position of the low pressure centers makes a big difference who gets the snow, when we’re in this pattern.  But it is slated to remain on the cold side of normal for at least the next 7-10 days across the Pacific Northwest, with multiple opportunities for snow possible.

-Karl