Let's look at recent radar imagery every two hours.
5 PM: lots of showers on the coast and offshore.
7 PM: the coastal showers moved inland and substantial thunderstorms hit eastern WA.
9 PM: Puget Sound gets moistened and a very heavy show (red color) is moving through south of Yakima
The 12-h totals as of 9 PM, modest precipitation is found around the region. Few hundredths to a few tenths in general.
The approach of the trough/low provided the lift that released the instability. I flew in tonight from California and was impressed that some of the convection looked like it reached 20-25 thousand feet. Substantial for these parts.