But I don't see a problem with photographing wet landscapes. Most of the time you get steady light rain, rather than roaring torrential storms; you should think about how to keep your equipment dry, you might not want to go for long muddy hikes (the locals generally do, though), but rain in the Northwest doesn't bring things to a halt.
Take fast film and/or a tripod. A cloudy day in a closed forest is pretty dark. (I've always had trouble getting black and white images to work well in those forests, by the way, though some people do a great job. The challenge is all the different shades of green; to your eye they look very different, but on film they tend to merge into a general indistinct grey. If you're shooting b&w in there, keep that in mind. A green filter helps.)
East of the mountains, Oregon and Washington are both considered "high desert"---though as a Californian I wouldn't really call it "desert", more like "semi-arid scrub". Rocks and grassland, not sand, and with plenty of vegetation in most areas (the main exceptions being the lava fields). The roads, signs, and maps are good, but there is nothing to see out there except landscape features and you'll struggle to find anything you'd recognize as "civilization" outside of a few large towns like Bend and Spokane. By all means explore out there, but take food along and expect to drive for hours.
Really, people forget how BIG the western US states are. The state of Oregon, by itself, is roughly the same size as Oman (but road travel is faster, according to Google). A photo trip from Portland out into the Columbia Gorge and back is an all-day affair. Don't try to take too much in too quickly.