Seattle can be a great place to ride, even if you live in the east, since you can fly in and rent a bike within a few miles of the airport. For all you switchback road lovers there's the Hwy 410 switchback near Mt Rainier. See more info and the 3 routes on our Western Routes - Seattle page.
It does snow some in December and January, and rain can be an issue. However, the average rainfall is only one inch in the months of July & August, with average high temps in the mid-70's and lows in the mid-50's.
Sounds like ideal summer riding weather to me.