That cold is a bit unusual for December, but not at all for January or February. The key is that I was in the mountains. I realize the Appalachians aren't much compared to the Rockies, but even in South Carolina they pop over 1000 meters and their valleys can easily collect a LOT of cold.

In the Smokies, where they top out at just over 2000 meters, they can catch an incredible amount of precipitation and (I'm told) have a climate very similar to low country Vermont.