Some forecasts are very hard and this is one of them.
The lowlands of western Washington have two shots at snow: Saturday morning/early afternoon and Sunday afternoon/evening/Monday AM.
The several weather forecasting models we have available are not in agreement on snow amounts and distributions, with temperatures being right on the edge for rain versus snow.
I am going to give my best shot at this, but it is important to check the forecasts often, since the situation should clarify as we get closer to the events.
Saturday's Potential Snow
Virtually all the models are in agreement that a weak disturbance will approach western Washington from the northwest on Saturday, with precipitation reaching the western interior just after daybreak. (see map for 10 AM Saturday morning below)
The mountains are guaranteed snow..up to around a foot in the central Cascades.
The bigger issue is Sunday. As the Saturday disturbance passes through, colder air will return to the lowlands on Sunday AM. At the same time, a warm front (with plenty of precipitation) is approaching, with precipitation reaching western Washington late Sunday afternoon. With low pressure with the warm front and higher pressure over BC, cold air will push southward, particularly through the Fraser River Gap (see figure). Here is the regional sea level pressure and temperature UW WRF forecast for 7 PM Sunday. Blue indicates cold temps, yellow warm temps.
The problem is that some models...like the gold standard European Center model...are a bit warmer, resulting in no snow near sea level on Sunday. Folks...we are right on the edge and small differences in the model solution will make all the difference in rain versus snow. Large uncertainty.
We will be watching the models and observations over the weekend...and hopefully the forecast will become more definitive in time. My gut feeling is that Sunday night/Monday morning will whiten up the Seattle area...but we will see.