The Detailed Snow Forecast for Sunday and Monday

We are now close enough in time that we can apply some of our most powerful prediction tools to the snow forecast problem for Sunday/Monday.  Plus, we are close enough so that uncertainties have lessened.

Before I talk about that, let me note that as expected the Cascades are being hit very hard with a large snowfall, with Snoqualmie Pass receiving about 2 feet of snowfall during the past day.  Snoqualmie was closed for a while, and both Snoqualmie and Stevens require chains at this time.

But let's talk about the situation in the lowlands.   It is clear that the Arctic Front will push southward across western Washington Sunday afternoon and Monday morning, with snow associated with its leading edge and when cold air is forced up terrain.  The snow action should be over by 4 AM Monday morning.

Below is the latest super high resolution forecast of total snowfall for the 48-h period ending 4 AM Monday.   There is, of course, a huge amount of snow in the mountains, particularly over the western slopes of the Cascades and Olympics, reaching 2-3 feet.

Another focus of snow is from Bellingham northward, across the San Juans and Victoria and then to the northern slopes of the Olympics.  This band is associated with the cold outflow of the Fraser River Valley, providing cold air for snow when Sunday's weather system moves through.  More snow occurs as the cold air is forced to rise on the northern slopes of the Olympics.   Another region of snow (.5 to 3 inches) is found from Seattle southwards, where the cold northerly flow converges with westerly/southwesterly flow from off the Pacific.  So at this point, south Seattle looks more heavily effected.

Another very powerful tool is the large high-resolution ensemble (many forecasts) system run at the UW.  This is the best such system in the country...and something the National Weather Service should be doing. 

Below is the ensemble snow forecast for Seattle Tacoma Airport.  The black line shows the average of the many (25) forecasts.  So what can you conclude?  Virtually no chance of snow in Seattle before 3 PM (00Z/13), but considerable uncertainty for amounts after that.   The average is about .6 inches and no member had more than 1.5 inches. Most of the action is between 4 PM and 11 PM.

So no big snowfall, but enough to whiten things up. 

An important issue is road temps.  The City of Seattle and WSDOT collects such information and is available on the City of Seattle SnowWatch site.

The latest values (11AM Sunday) show road temperatures (in the boxes), in the low to mid-40s.  This is good-- the light snow will tend to melt.  If Seattle's Department of Transportation (SDOT) does a  bit of pretreatment with salt in vulnerable locations (like bridges and elevated roadways), then everything should be fine for the commute on Monday.  The timing of the snow will give them several hours to make sure all the roads are fine. And salt will help prevent refreeze as the temperatures cool.

And talking of temperatures, here is the ensemble prediction for Sea-Tac.  Wow.  Rapid cooling Sunday evening (about 15-20 degrees!), with subfreezing temperatures occurring for several days.  This is going to be quite a shock.  And as I noted before, the City must get all the homeless folks off the street.