The thunderstorms were associated with the most pronounced Puget Sound convergence zone in a long time. You remember the convergence zone, when northerly and southerly flow clash over Puget Sound producing clouds and precipitation? Here is radar image at 4:46 PM, near the time of the big thunderstorm over Seattle. The heaviest precipitation was associated with the red colors, which was associated with the pellet size small hail over the city.
Total precipitation from the convergence zone showers was respectable, with the Seattle RainWatch web site suggesting some locations got as much as 1-1.5 inches (see below). The precipitation extended from the eastern slopes of the Olympics past the Cascade crest.
But in Seattle the show was thrilling to the weather hungry in the populace. The small hail from the thunderstorm turned the ground white.
But the real thrill was the sound of the innumerable hail pellets striking the windows. Click on the image below and enjoy. A second video below that will add to your meteorological pleasure.