I lived in Boulder for awhile. The temperature in the winter is entirely dependent on the wind direction. West winds are downslope and can be relatively warm, it can also get quite cold/snowy when the wind is from the north or northeast as we saw in the last Utah-Colo game. Boulder is much more vulnerable to arctic air pushing south in the Plains than Utah. The averages reflect both extremes, so that said, the average high for Dec 11 is 45 and the average low is 21.