Officially Denver's Coldest Night Of The Season

As of this post, DIA has fallen to zero degrees. That makes today, December 17, the coldest so far this season. The coldest temperatures of the night normally happen just before the sun comes up. That's because it's the most time that has elapsed since the sun's rays have heated the earth. Tonight's temperature trend will be a little different.

Several factors are working together to cause the temperatures to INCREASE around midnight and beyond. That means the coldest weather happens sometime between 10 p.m. and midnight. A surface low pressure system over Manitoba, Canada has been the main source of today's bitter cold. But the northerly winds on the west side of this low will weaken tonight as it slowly moves further east. This opens the door for a new airmass to take over and bring much warmer temperatures with it.

Higher up in the atmosphere, the jet stream is forming a ridge over the western United States. The Front Range of Colorado is right in the middle of these upper level winds, and the westerly flow will cause surface temperatures to warm. As the fast-moving air approaches the Continental Divide from the west, it gets forced up and over the mountain tops, then quickly races down the eastern slopes and into metro Denver. When air rises vertically in the atmosphere, it cools down. But when it descends through the atmosphere, it warms up. That's the "downslope" effect we talk about, and will gradually cause our overnight temps to climb BEFORE sunrise.

We dropped to 3 degrees above zero on November 27, the only other time it was close to this cold. Tired of this Arctic blast? Good news comes this weekend with a 30 degree rebound for Denver.