I just want to say that usually this is the dry season in Washington. However, right now its cloudy and there are thunderstorms in the mountain areas. Also, cool misty conditions can be deceptive--it feels like you are not getting wet, but if hiking hard you can still get hypothermia, even though it's not that cold.
You definitely want some sort of raingear that at least is going to last you through Washington. There are limited shopping opportunities on the trail, so if going with something relatively cheap maybe buy two and put one in a mail drop. I haven't tried the umbrella routine yet, but I'm thinking seriously about it.