Yep, mid-May is iffy in the West-of-Cascades Northwest. You can get some real nice days, sunny, highs in the 70s. Or you can get rained on with highs in the 50s to 60s (which will definitely feel colder when you're wet.) Since I live in the Northwest I haven't let it stopped me from touring in May. However, if you really want to bet on drier weather, I'd recommend going east of the Cascades. Though saying that, my last two east-of-Cascades tours in May/early June did see significant amounts of rain. Be prepared for rain no matter where you end up in this part of the world.