I commute year round in the PNW and it is fine. The #1 key is proper gear.

1) Merino wool. Yes, it is expensive but if properly taken care of will last you 10+ years. Merino wool wicks moisture away from your body. There is no better fabric that can keep you warm than merino. Get merino leggings and a few base layers, socks, etc.

2) Full fenders for your MTB

3) Showers Pass jacket

4) Gloves

5) No matter how cold it gets in the PNW don't put 5+ layers on, you'll regret it. If you are using merino you will only need 2 + your rain jacket.

6) The more you ride the easier that hills becomes. You don't have to race up that hill. Take your time, breathe, and you can get up there with minimal sweating.