Things have changed. The forecast models, which indicated substantial uncertainty yesterday, appear to be locked on to a solution--one in which a strong low center will move SOUTH of Seattle, not north, as previously predicted (see attached sea level pressure forecast for tomorrrow at 4 PM). This will have a huge impact on the forecast tomorrow. Forget strong southerly winds along our coast and in Puget Sound. Forget the strong westerly surge in the Strait of Juan de Fuca. Thats history now. Just so you can appreciate how difficult this forecast problem is.... consider that the low doesn't even exist now...but will form shortly. That this low is forecast to amplify explosively. That a 100 mile error in its position can radically change the forecast. But as I will note, there is no doubt about cold air coming...the big issue is snow.
So this is how I see it. Today will be cloudy and dry. It better not rain...the weather radar is broken (I swear that hardware knows when challenging weather is coming.... similar to the fact the copy machines malfunction when you REALLY need them). The low will develop rapidly tomorrow, spreading rain over the area tomorrow, starting in the late morning . When the low was going north of us we didn't have to worry about snow, because there would be strong southerly and southwesterly flow. Now we have to worry.
I think it will be too warm for snow over much of the lowlands initially...except for one place...southeast of the Olympics around the lower hood canal and the Kitsap. If you live there, better be prepared. Second, as the air gets colder later in the day there is the possibility of snow reaching the ground where showers are strongest--and the higher elevation you are the more probably it gets. However, initially there will be strong easterly flow that should keep the wesern side of the Sound snowfree. As the low moves east northerly flow will increase..resulting in upslope flow and potential snow immediately NE of the Olympics...like the Sequim area (the opposite of the normal rainshadow there). Then after midnight a puget sound convergence zone could form...providing light snow over central puget sound. We will get a better idea about the above threats as the higher resolution runs become available later this morning. On Saturday, strong northeasterly flow will be pouring through the Fraser River Valley into Bellingham and NW Washington (see second figure of near surface wind speeds). Very strong, cold, dry air.
The mountains will get snow...but less than the old storm track. Perhaps a foot in the pass. And this may not be enough to initiate downhill skiing.
No matter what, our region will switch to unusually cold, dry air. It will be like moving to Vermont or the upper midwest. Highs will only reach the low to mid 30s from Sunday to Tuesday....but it will be dry. Even sun. No seasonal affective disorder. But you need to protect pipes and plants.