It is an interesting observation that the heaviest precipitation in the mountains and worst flooding in area rivers can be associated with relatively dry, balmy weather in Seattle and the central PS lowlands. To get heavy, unusual precipitation in the mountains you need a good upward component along the western mountain slopes. If the winds are mainly southerly the flow is mainly parallel to the mountains (except for the southern Olympics) and you don't get big-time orographic rainfall. But westerly winds are another story....strong westerly winds, particularly with a warm subtropical origin, can give massive amounts of precipitation...often rain...that can add up to 10-20 inches over a few days. And that is what is going to happen over the next 48h hrs. The irony of this, is that westerly winds produce a nice rain shadow east of the Olympics that can protect Seattle and neighboring regions. That is why Seattle will get wet, but not flooded.
To show you what I mean, take a look at a recent latest radar image (attached). Look how dry it is over central PSound. But if you head east you hit the rain by North Bend.
The precipitation is now mainly over the northern portion of the State, but the current of moist flow will settle south this afternoon. I have attached the model forecast 24-h precipitation for two periods...ending 4 AM tomorrow and Thursday morning. One is struck by two things...the tremendous rainfall in the mountains (reds are 5-10 inches). Clearly, some mountain locations will get 10-15 inches. The other feature is the rainshadow over Puget Sound...particularly today. (Note..the first figure is from a higher resolution simulation). You can also see a rainshadow to the lee of the mountains of Vancouver Island over Georgia Strait. The air coming into the region will be very warm, with freezing levels rising to 6-7 thousand feet. The snowpack can absorb some of the water, but plenty will go into the rivers. That plus the melting of the huge low-level snowpack has led the NWS to put up a flood watch for many of the local rivers. If you live in an area that often floods...you should prepare...now.