Massive Snowfall and Heavy Precipitation Will Pummel the Cascades This Week

This is going to be a week of superlatives over the region, as heavy precipitation will produce both river flooding and a huge snowfall over regional terrain.   Enough snow so that skiers and ski areas  will be jubilant, and enough water that fears of lack of drought this summer will be far less.

First, the big picture.  Below is the forecast precipitation over the next 7 days. Precipitation totals are amazing, teaching 10 inches near the crests of the Cascades and Olympics.

The snowfall  total over Washington high terrain is amazing (bel0w), reaching about 4 feet near the crest by next Saturday afternoon. Snow descends down to lower elevations in eastern WA and over Whatcom County.

Now, the details.  Here are the ensemble forecasts (forecast model run many times) from the National Weather Service GEFS system for precipitation at Stampede Pass at 4000 ft on the eastern side of the Cascade crest.  The gray lines are from the many forecasts, with the black line being the average.  But the ensemble is at relatively low resolution (~35 km grid spacing). A higher resolution simulation of the same system is also shown (blue), and brings much more.   The ensembles have substantial precipitation tonight and tomorrow, with steady accumulation through January 13th.  The high resolution simulation, has much more--around 8 inches.  The lack of variability in the ensembles suggests great confidence in the wet forecast.

But what about snow?  Now things get more complicated.  The precipitation will start as snow, but quickly transition to rain, except at the highest elevations (above roughly 5-6 thousand feet).  But by Tuesday the air will cool and the Cascade snowfest will begin, with the ensembles going for about 30 inches, and the high resolution run goes nuts, hitting nearly 70 inches.

The European Center solutions also bring huge amounts of snow to our mountains, as shown below:

So by next weekend, our ski season will be set, and the water supply in much better shape (although the last few weeks have been good). Seattle's reservoirs are already at above-normal levels.  I can image what they will be like in a week.

But there is more.  Much more.  Very cold air will be moving southward into Washington next Sunday (see sea level pressure and low-level temperature forecast for 4 PM next Sunday).  Plenty cold enough for snow.  All that is needed is moisture.   Gasoline without the match.  Will watch it carefully.