Let's look at the satellite imagery to see this happen from space. At 8:45 AM, western Washington and the coast experienced fog and low clouds, while most of eastern Washington was clear. With a cool surface and lower atmosphere, there was no convection.
Over eastern Washington the convection did not lay down much rain, but some of the western Washington showers provided a few downpours. Most of this convection was fairly shallow (less than 10,000 ft high), but could produce some modest bumps coming into or out of a few airports. The pilot report (PIREP) at 10:45 AM at Fairchild AFB outside of Spokane confirmed this.
SKA UA /OV GEG235020/TM 1745/FLUNKN/TP DH8D/TB LGT BLO 060
Finally, it would like to see a wonderful video of the atmosphere going from stable (low clouds and fog) to unstable (convective clouds and showers), check out this video from the top of my building today (click on image or link):
Convection is a sign of spring around here, just like hay fever and mowing your lawn.