Ken's description seems spot-on with my experience. It's relatively rare to get summer/early-fall storms on the west side, and when they do happen it's almost always because weather has pushed over the crest from the interior. I believe these are the weather patterns that give the desert southwest (e.g., Phoenix) its summertime wet season.
It's rare, but not unheard of, for a summer storm to make it inland from the Pacific side, as occurs the rest of the year. Pacific weather systems are usually pushed far north that time of year. When it does happen, you can get two feet of snow, pronto, or at least a vigorous soaking.
When I think back, my raingear stays stowed on 90% of my Sierra summer/fall trips (but I always carry it).