I don't want to overhype this event--it should not be one of the "great" flooding events--such as December 2007, January 2009, or November 2006, but it will probably bring enough precipitation to cause flooding on a number of local rivers and to cause localized urban flooding, particularly south of Seattle. The latest infrared satellite figures says a lot--a current of warm, moist air is headed our way.
Here is something I haven't shown before--the 48hr precipitation total, in this case ending 4 AM Monday. 5-10 inches on the windward slopes of the mountain in "favored" locations, with over an inch in the lowlands from Seattle south.
You will note the profound rain shadowing over western Washington--with less than a third of an inch NE of the Olympics. A substantial gradient of precipitation over Seattle! Seattle Public Utilities will undoubtedly be dealing with more incidents on the south side of the city.
How much uncertainty is there in this forecast? Much less than for the snow predictions of two weeks ago (thank goodness!), but the amounts could vary substantially from the above. For example, there are often waves on the fronts associated with such warm currents and such waves are very hard to predict--thus, there could be some modulation of the timing and distribution of the rain that the latest model runs do have have correctly. A coastal radar could have given us some short-term (0-9 hr) information about such waves, but that will have to wait until next year.
The National Weather Service River Forecast Center in Portland uses observed rainfall and model predictions to provide forecasts of levels of major NW rivers. The latest is shown below. Red indicates basins with predicted flooding, orange indicates basins where the rivers are running bank full.
The weather should be decent over the area until lunchtime, by which time light rain should have made it to Puget Sound land--a good period to rake those leaves from the street drains and clear out your gutters.
There is a lot more to talk about.
There should be snow tomorrow afternoon on the eastern Cascade slopes as the precipitation comes in and cold air holds for a few hours. Strong winds along the coast and over NW Washington on Sunday. And substantial avalanche danger Sunday and Monday. Cross Cascade travel later on Saturday and Sunday will not be great...and road closures for avalanche work is probably inevitable. And there is a real risk of freezing rain in the passes later on Saturday and early Sunday before everything turns to rain.
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