Cliff and tweeters,

The mew gull subspecies that occurs in the NW (and in most of North America) is L.c. brachyrhyncus. It used to be called the short-billed gull (which is what "brachyrhyncus" means). A few of the nominate subspecies, L.c. canus, are occuring each winter in Newfoundland, and I know that there are other records of this subspecies for the northeastern United States. I don't think it likely that this subspecies occurs here in Washington. (But see comment below about vagrancy.)

As for the Siberian subspecies, L.c. kamtchatschensis, there are records for the Bering Sea, and it may be annual in very small numbers in Alaska. I don't have any of the literature at hand, so I could be overestimating its occurence in Alaska.
I believe there was a report of kamtchatschensis in Oregon last winter (Oregon birders, correct me if I'm wrong.) I don't know if it was accepted by their records committee or not.
I know of no records of kamtchatschensis from Washington, but given that gulls are prone to wandering (there is a record of glaucous-winged gull in Europe!) and that slaty-backed gulls (another Siberian gull) are starting to be found in the lower 48 (Washington, Oregon, Colorado and Texas, among other places), it would not be out of the question for one to turn up in the NW.
Mike Donahue
Seattle