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 upper trough will be a surface low, that will be along the Washington coast.   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.

But we do have cool air over us now, with the freezing level around 1300 ft.  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.

So  what are the model's saying?  Well, the super high-resolution UW WRF model is predicting a few inches of snowfall over 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). But it is spotty and less near the water.

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 (any model) forecasts? A lot of uncertainty, with varying amounts.  The ensemble average is about 1.25 inches in Seattle 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 .5-1 inch near sea level, increasing to a few inches as one goes higher.  

But please not there is a lot of uncertainty.  Sea level could be all rain or 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.

from Cliff Mass Weather Blog http://ift.tt/2lW0Ws5