I'm just north of Seattle, so we haven't had any summer yet either. My girls are almost 18 weeks old and they are out in the rain right now. If it really starts pouring they wander inside the covered run for a while, but mostly they don't mind the rain. A good portion of their open run is under some big douglas firs, so they aren't directly in it all the time, but I've seen them out in the open grass during the rain. Sorry you've lost some of your flock.
