No, the weather in the PacNW is quite mild, frankly not too much different than the UK. It may rain, but I really don't think you will have any problems except for rain. Timberline should still be without snow at that point. Mult. falls rarely freezes over and when it does, the road along the Columbia River Gorge that you take to get there would be closed. But this is very rare and weather like this doesn't usually happen until Jan/Feb.