The weather is great here in the Summer. From June-August Seattle on average gets less daily precip than NE states (especially this year since the NE just had a miserable rainy summer). Checking the weather was just something I didn't do this summer. And even if you do get some precip, it won't be for a whole week. But yeah, if you are looking for the absolute best good weather window then late July/early August is it, but I wouldn't get too hung up on it.
RE: Oregon Coast - there are many state park campgrounds along the coast. I would suggest making reservations down there in the summer or else it might be a gamble - the campgrounds along the coast fill up well, especially the popular ones.