One thing that meteorologists look at carefully is the trend in the model solutions for the same period...in the biz we call this dmodel dt (those who know calculus will understand the terminology). Anyway, the trend of the high resolution forecasts is for more rain from this event.
Here are the 48h precipitation totals ending 4 AM on Monday. You will notice some increases over the central Puget Sound (like Seattle) and over the central Washington Cascades. If this is right and 2-5 inches falls over the lowlands, there will certainly be some flooded roadways and perhaps some slides. Want to stay dry (or nearly dry)?..head for Port Townsend! South of town the forecasts are for roughly .10 inch of rain. You could play golf in that! At the same time south Seattle could be getting 20-30 times more! Just amazing. And there could be 100 times more on the windward side of the Olympics.
The National Weather Service is on top of this and their River Forecast Center is now predicting more rivers to flood...here is the latest flood forecast map:
Red indicates flooding forecast and orange is bank full. The Green Basin will be getting more precipitation, but not enough to cause problems for the dam. So this will be a serious event..the kind we get once every 1-2 years roughly. If you are in one of the typically flood prone locations it is time to take action to protect your possessions and people's safety. And it is never a good idea to drive through more than a few inches of water.
And as forecast by our models, snow has begun on the eastern slopes of the Cascades (here is an image on I90 at Rocky Canyon west of Ellensburg.