Weather Underground reports that:
A front bringing heavier rain will arrive late Saturday... with significant precipitation continuing Sunday and Monday.Because of the long dry spell, forecasters say, flooding is unlikely. But rivers are expected to rise sharply.
Rainfall amounts during the period from Saturday afternoon through Monday evening will likely be around 3 to 8 inches in the mountains... with the heaviest precipitation over the Olympics and north Cascades. The snow level will be mainly around 10000 feet.
Rainfall over the western Washington lowlands will probably range from 2 to 4 inches along the coast and 1 to 2 inches over the interior lowlands.
Drivers should also be extra cautious. During dry weather, oil that has dripped off cars and trucks onto roadways doesn't have a chance to be washed away. The rains will spread that oil, making roads extra-slick for the first couple days of rain.