I am no expert. I think rubber boas are gonna be way less active this time of the year and therefor much harder to find. but if you must try i would look for riparian areas within 400 yards of water and start looking around the largest rotted logs you can find, when i go looking for bottae that is where i usualy find them. Make sure when you start flipping large logs make sure you can return them to there original position, if you don't find a boa the first time you definatly won't find one the second!
where are you located in washington and where do you plan on going? I think the population of bottae is greater west of the cascades personally. If you start finding alot of boas i would also be looking for ringnecks and sharptails.
heres a good site if you don't already know about it
www.dnr.wa.gov/refdesk/herp/index.html
nate