Not that familiar with conditions in the PACNW. However, here in Minnesota (which I imagine has colder winters), I fish trout 12 months of the year.
Most of our streams have a lot of spring water, don't get much below 40 degrees, and don't freeze over. So if you can deal with the air temp and snow cover, you can fish open water stream trout.
Fish do slow down when water temps are in the high 30s to low 40s. They tend to bunch up and hunker down in deeper and slower water than at other times of the year. It's a great opportunity to practice and improve your nymph fishing. Fish, typically, won't move far off the bottom or out of their lane. So you usually need to get a deep, dead drift and detect subtle strikes. However, if you can find fish and work out the drift and strike detection, you can sometimes catch quite a few fish out of a single run / pool. On occasion, I've run into short hatches of midges and Baetis (early or late in the winter) and caught fish on dry flies in the winter. Always fun to see midges sprinkled like black pepper on bank side snow.
If you've got open water (and it's legal), I would get out there and fish. Get a stream thermometer and use it to find slightly warmer water. Work on your deep nymphing technique (which will pay dividends all year). Don't count on it, but you might even run into a midge or Baetis hatch and catch fish on dry flies.
