In the midst of real winter-like air, the front range of the Rockies is rather mild. Denver was at 60° at 5 PM CST Thursday, much warmer than surrounding areas. The reason is down-sloping winds, which creates compressional warming. The image above shows how this works. Westerly jet stream winds rise over the Rockies, then descend on the lee side, warming the temperature in the process as the air pressure increases. Thus, with westerly winds, the front range becomes rather warm.