Here in NW Oregon, too--in fact, it's more like 9 months--but that's the rainy season. Will you really be doing extensive camping in heavy snow more than 4-5 months of the year? Tarptents do just fine keeping the rain off. You need the heavy winter tent only when you are in a place/time where it's possible to get heavy snow or consistently below-freezing conditions (mountains or east of the Cascades). You stated that you're mostly concerned about our NW winter rains. If you're going to do lots of snow camping, that's a different story.

I'd still suggest renting a winter tent until you have more experience snow-camping. I also strongly recommend going with experienced folks, preferably in an instructional setting, before you try to snow-camp on your own. There are lots of dangers like avalanches that you need to learn about.