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40 Degrees Colder By Thanksgiving

By Justin McHeffey

DENVER (CBS4) - Enjoy the abnormally warm weather Monday and Tuesday, our temperatures may be 40 degrees colder by Friday!

Why the abrupt change in our forecast? Our present airmass has origins in the desert southwest and northern Mexico, also known as "Continental Tropical."

Later this week, very cold air originating in the Arctic will descend upon the Front Range of Colorado – and bring Denver its coldest temperatures of the season.

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The arrival of this air is expected by noon on Wednesday, so our high temperatures will likely occur in the morning. A stiff north wind will mark the cold front's passage and bring increasing clouds through the evening. At this point, it looks like the foothills, plains, and metro area will have chances of precipitation Wednesday night through Thanksgiving.

Because the upper level winds (around 20,000 feet) will still be blowing in from the southwest, there may be some freezing drizzle Wednesday night. This is due to mild air "overrunning" the very cold, dense air coming into the picture from Canada. If this happens, the falling snow will melt within the mild layer, then refreeze on ground and road surfaces chilled by the Arctic air.

Once the upper winds turn colder by Thursday any falling moisture will likely be all snow.

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This will also be a weather situation that brings colder conditions to Denver as compared to the high country. The depth of the Arctic air only extends a few thousand feet into the atmosphere, so points west of the Continental Divide will not be affected by the bitter cold. However, there's enough lift within the jet stream and moisture available to bring modest snow accumulations to the mountains. 2-5" will be found through Thanksgiving in the northern and central mountains. Since we'll have some time to wait for the freezing drizzle to change over to snow in the metro area, we should only end up with a trace to 2" in the city.

The headline Thursday will definitely be our cold temperatures, but be careful driving to celebrate the holiday! A glaze of ice on the roadways is still possible even if this system isn't a huge snow-maker.

Justin McHeffey provides nightly reports from the Mobile Weather Lab. He travels Colorado in search of Mother Nature's most powerful and beautiful conditions. Like his Facebook page Meteorologist Justin McHeffey and follow him on Twitter @WeatherMcHeffey.

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