Marginal Lowland Snow, Again.

Folks... this is getting tiring.  Another situation in which lowland temperatures are marginal for snow, making the forecast exquisitely difficult and elevation dependent.  With rapidly increasing day length and strengthening sun, we are almost out of the lowland snow season (which is really over by March 8-10th).

Later tonight and tomorrow morning, an upper level  (500 hPa) trough of low pressure will pass over the Northwest, producing upward motion and precipitation (see forecast map for 10 AM tomorrow).

Associated with the upper trough there will be a surface low, that will be along the Washington coast around 10 AM (see below).   The trouble is that temperatures are not that cold over the region and there is not a good push of cold air out of British Columbia. Marginal for snow near sea level.

But we do have cool air over us now, with the freezing level around 1300 ft (see below).  The snow level is generally about 1000 ft below that, so snow could fall to 300 ft. And precipitation can cool the atmosphere, thus bringing wet snow to sea level if the intensities are large enough.

So  what are the model's saying?  Well, the super high-resolution UW WRF model is predicting a few inches of snowfall away from the water around central Puget Sound by 4 PM Sunday (keep in mind that snow DEPTH would be less). Lots of snow in the Cascades (1-1.5 ft).

The vaunted European Center model for the same period?  Less snow near sea level, with increasing amounts (few inches) towards the Cascade foothills.  Somewhat different details though.

What about the ensemble (many model) forecasts? A lot of uncertainty, with varying amounts.  The ensemble average is about 1.25 inches in Seattle (SeaTac) and 2.00 inches at Paine Field in Everett.

So what is the bottom line?   Precipitation will start tonight around 10PM-midnight and will continue until mid-morning.   Above 400 feet, there will be a few inches of wet snow, increasing with higher elevation.  Below that, it will be a mixed bag.  Perhaps 0-1 inch near sea level, increasing to a few inches as one goes higher.  

But please note there is a lot of uncertainty.  At sea level it could be all rain or you might get 3-4 inches.

Later in the day, temperatures will warm into the 40s with onshore flow behind the low and warming by the sun.  So driving conditions will improve rapidly.

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The Northwest Weather Workshop

And don't forget...if you want to attend the big weather meeting of the year...the Northwest Weather Workshop on March 3-4 in Seattle...you have to register before.  The agenda and more information (including how to register) is found here: https://www.atmos.washington.edu/pnww/