I'm in Grants Pass, on the I-5 corridor. Looks like our weather is probably pretty similar.
I have this style of pens and really like them. I've had them about a year and a half. I plan on doing something similar when we move, I like them that much.
With my climate, I really don't see the need for a conventional "coop". In my mind a coop has a few purposes....first is predator protection. I've got that covered. How you address that is up to you.
Second is protection from elements. My elements aren't that bad. Like you said, teens in the winter. Birds handle that just fine, even my bantams. My pens are in a very sheltered area, between a hay shed, a fence and a pump house. So, no direct winds. That's going to be your biggest challenge, I'd think. Finding a protected area, building or planting a wind barrier. But as long as they're out of the wind, and dry, they can easily handle our weather. Even my littles---I use the middle pen as a grow-out pen.
We started with just regular duty tarps on the top. They didn't last well at all. We had some sheet metal, so used that to cover the pens. When we move and build again, I'll look into the heavy duty poly tarps/boat covers/whatever....something heavier duty and made to last.
A lot of what we deal with is water, like you said...rain. Be aware those tarp like covers are just going to dump the rain right down along the side of the pen. You'll wind up with all that water going into the pen and making a lake. I don't know if there's some type of gutter system you could run along the sides, that might be something to look into. It would be cool to run it to a rain barrel, maybe something you could use to then water the birds? I've seen some folks put "wings" on the sides of hoop pens, but I don't know if that was just for shade in the summer or if it helped shed water.
Here's a link to my pens....
https://www.backyardchickens.com/t/1003092/my-breeding-pens