I am one of the fortunate people who have lived most of my life in the Pacific Northwest. My work also allowed me to live and work in many areas of both states.
Despite our soggy reputation, we in the northwest enjoy a pretty mild climate year round, with what the garden books call a "brief but definate summer." August and September should be good for your trip. We ususally get a "Labor Day Storm" in early September, just enough to let us know summer is winding down.
As someone already said, the Cascade Mountains divide both states into roughly western 1/3 wet side and eastern 2/3 dry side. The east side is generally higher, drier, hotter in the summer and colder in the winter, with the possibility of morning frost in the higher areas in Sept.
The coast should have temps in the upper 60s/low 70s, with possible morning fog or drizzle with clear afternoons. Some tourists consider this cold, natives think it is swimsuit weather.
You didn't say what you were interested in seeing, so I won't bore you with generalities, but feel free to respond or PM me for my suggestions. However, since you are coming this far, if you plan on traveling the coast I would strongly suggest you get into northern California to see the coast redwoods, they are magnificent.
It's good you have about thirty days for each state. I've lived in both states for over thirty years each and still have a lot to see.
Not to brag, but the northwest has it all, beaches, prairies, mountains, lakes, desert, wheat fields and rain forests, cities and wilderness areas
Enjoy your trip, and stay longer nest time.
