Well, its pretty definite now...we are going to experience an unusually cold period for this time of the year...with a real threat of snow showers on Sunday.
Lets start with right now. A cold front is now approaching..and there are clouds and some light showers associated with it. But the flow is sufficiently westerly that central Puget Sound is being rain shadowed (see images). The showers will move through tomorrow am, with improving weather during the day.
On Friday, an upper ridge will move in and it will by dry and sunny. But the fun begins on Saturday as a major Pacific ridge shifts eastward and an upper trough moves southward down its eastern flanks (image). Behind the trough much colder air will flood the region on Saturday into Sunday. On Saturday, the associated showers will probably be rain...but on Sunday things get interesting. The winds will be westerly, so there will be rain shadowing, but the models are insistent about the creation of a Puget Sound convergence zone...and if they are right it will be cold enough for snow (see model snow forecast).
The cold air will hold in for several days...and there will be a substantial pressure difference across the Fraser River Valley...so Bellingham and NW Washington could be raked by strong NE winds and colder temps than anywhere else. This air is cold enough to allow temps to drop into the 20s at night...so there could well be a serious frost that has the potential to damage sensitive plants...particularly those that are budding and vulnerable...so get ready to protect them.
A positive part of the forecast is that the Cascades should enjoy plenty of snow tonight and over the weekend...with up to foot possible tonight and tomorrow.