Yesterday (Saturday) the area experienced an event more common to the eastern U.S.: a summer convective squall.
Forced by an short-wave trough aloft that was moving northward, the action occurred around noon to 3 PM, as the fast moving line of convection brought heavy rain, lightning/thunder to some, and winds gusting up to around 40 mph on the east side of Puget Sound. Roughly 10,000 customers lost power, including 6,000 customers in Mukilteo, Monroe and Granite Falls served by Snohomish County PUD and a number of trees were downed including one on I5 (see picture above). The raindrops was far bigger than normal, a sign of the convective origin of the rain...and the top of the thunderstorms exceeded 25,000 ft...very high for western WA but wimpy back east.
The Granite Falls area got hit particularly hard. Here is a very professional video on the storm by Mark Horner (click on image to view)
Here is the radar sequence, and keep in mind the red indicates POURING rain or hail.
At 11:19 AM...showers were south of Seattle and moving north. Some yellows (moderate rain)
This band extended across the Cascades into eastern Washington. You can see that by following a sequence of images showing the lightning at 11:30 AM, 1 PM, AND 3 PM. The lightning was pretty much limited to the eastern portion of western WA, the Cascades, and eastern WA.
Here is what happened when the line passed the UW (where it was only of moderate strength). Winds gusted to 25 knots (around 30 mph), precipitation picked up, temperature fell from the low 60s to the low 50s, and pressure abruptly increased and then steadily rose after that.