Are an Arctic Blast and Snow Coming to the Northwest?

The media is beginning to talk about it and the emails are coming in, so perhaps it is time to talk about the potential for cold air and snow later this weekend and early next week.

My general policy is to wait until such events are about 120 hour out, because their predictability is often very low beyond that.  And although some models have been indicating a cold event for a while, their forecasts have been inconsistent--suggesting large uncertainty.  So now that we are getting close enough to see "the whites of their eyes", what do the latest model predictions indicate.

First, snowstorm 101.  It is hard to snow around western Washington and Oregon.  We can be warm and wet, and sometimes cool and dry.  But only rarely are we wet and cold--which you need for snow.    The trick is to get moisture off the ocean and cold from off the land, without letting the ocean warmth turn it to rain.    I did a paper on NW snowstorms with Garth Ferber, Mike Patnoe, and Gary Lackmann.   We found that most snowstorm events looked very similar, with an upper level (500hPa) trough moving southward along the eastern flanks of a large north-south ridge in the northeastern Pacific.  Like this.  The trough provides lift to produce clouds and precipitation.

 At the surface, snowstorms are usually are accompanied by cold, high pressure over British Columbia and a low center over SW Washington.  This pattern pulls cool air in from BC at low levels with moisture moving off the ocean, circling around the low, and the riding over the cool air from B.C.   It all has to come together just right.

So what are the latest WRF model runs showing?  Here is the 500 hPa upper level map for 4 PM Sunday.   Nice, strong trough, but a bit more extended east-west than the canonical snow trough.  But perhaps close enough.

The surface map for 1 PM on Sunday shows a strong cold front coming in off the ocean.  The action starts about then, including the potential for a convergence zone snow event.  Strong winds too.

 One day later, it is a different world.  VERY cold air and intense pressure gradients in southern BC.  A low off of SW Washington. This is getting close enough to the typical snow pattern to worry.  The low is not super intense though.  But any precipitation that falls will be snow...the question is how much precipitation will occur at this time.

 By Tuesday morning at 4 AM cold air has spread southward and the low has moved south with it...expect cold and dry with such a pattern.

Looking at several other models (including the ECMWF) it really looks like frigid cold air is quite certain.  But the run to run consistency of the details (strength and position of the trough and surface low) are quite variable, indicating lots of uncertainty.

Now the snow forecasts.  Here is the total snowfall for the 24h ending 4 AM on Monday.  Lots of snow over the mountains (foot plus over the Cascades), with several inches over Seattle to Everett.  This lowland band is associated with a convergence zone and the leading edge of arctic air coming out of the Fraser gap.   Note snow on the NW side of the Olympics--classic with Fraser outflow.

 You want to see the Fraser gap winds?  Here is the forecast of sustained winds at 10 PM on Monday.  Strong NW flow heading towards the San Juans and Victoria, and then on to the Olympics.

I can't stress enough that there is still considerable uncertainty about the lowland snowfall, in contrast to the Cascades, for which a few feet is relatively certain.

These kinds of situations are reminiscent of the details of setting off explosions.   Although you need to have  a match, explosives, and dry conditions, there is no certainty everything will come together to get the big bang if the components.  We have all the ingredients...now the question is how they will come together.  And to get significant lowland snow over western Washington, the requirements are exacting and relatively rare. 

What about Thanksgiving Day?  Mainly dry, but cloudy.  Perhaps a few sprinkles late in the day.  Friday will be cloudy with a few light showers.  The big action will be on Sunday and early Monday...unless the models change, again.  You coming home from east of the Cascade crest?  Perhaps better to return on Saturday.

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