Here we go again. Another difficult to forecast snow event, with temperature sclose to the margin over the western WA lowlands. A situation where a number of our models suggest several inches over much of the lowlands west of the Cascades, with those poor suffering folks in Portland getting hit the hardest.
To "warm up", here are the forecast snowfall totals for the 24h ending 4 PM Sunday and 4 PM Tuesday from the UW WRF system . On Sunday there is LOTS of snow over the southern WA and northern WA Cascades....feet.
The snow around Puget Sound begins late Sunday and early Monday in the UW model forecasts. The situation late Sunday is close to the canonical snow set-up, but not ideal. As shown by the forecast surface map, very cold air and high pressure is over British Columbia, and a low center is just off of the Columbia River bars. Such a pattern helps drive cold air into western WA.
At the same time an upper trough is approaching from the west, providing upward motion that drives clouds and precipitation. This trough extends a bit too southward to optimize snow over western WA: the strongest lift is over western Oregon at this time.
The above is just ONE solution and there is considerable uncertainty. We can get an idea of the amount of uncertainty from our ensemble (many forecast) prediction systems.
The U.S. global ensemble (GEFS) snow prediction for Seattle is shown below with the average indicated by the black line and the high resolution GFS member by blue. The ensemble mean shows a lot of uncertainty in the forecasts (big range of possibilities with the average getting to around 3-4 inches and a range from zero to 8 inches). Snow starts late Sunday and continues for several days.
Why is there substantial uncertainty? Because the air temperatures are marginal for snow and the models have variations in the forecast details.
Keep tuned...the uncertainty should decline during the next day. But it looks like the most likely forecast is 2-5 inches of snow over Seattle, with about 20% chance of little snow or over 6 inches.