You get more sun on the east side of the Cascades but it comes with colder temperatures. Your best bet for sun in the winter is to go to the rain shadow of the Olympics (north and northeast of the range) - places like Port Angeles, Sequim, Port Townsend, Whidbey Island.

We do actually get SOME sun here and there during winter, it's not terrible. The bigger issue than the rain is the fact that we're pretty far north so sunset is at 4pm. Definitely take vitamin D.