Hey Stuart
Theres an article here that's is for members only concerning a trip to the flagstaff mountains in AZ, with conditions at 25* and 2 inches of snow, but the author does not state the month. The author Carol was able to stay comfortable with a sub 5lb base weight, so it seems as if you can stay warm, and you have a shelter that can withstand a little bit of snow, as some tarps can while pitched well you probably could do quite fine, but I stress again that I don't know the seasonal lows in December, have you tried looking at weather archives.