If you're looking to escape the rain, the Pacific Northwest is probably not the best vacation climate. As far as drinking underage in the US, there's plenty of underage drinkers here, not hard to get away with in a private, closed environment like an of-age friend's house. Also take a look at this:
There is not a hard 21 or nothing rule in America, conditions and exceptions vary by stateIf I were you, I would hook up with someone you already know in America and go from there.