Christmas Eve may feature temperatures 20 to 40 degrees below normal around Denver and Bismarck. By Christmas Day, such cold could envelop Minneapolis and Chicago. Subzero nighttime lows are a good bet.

The exact timing, intensity and extent of the cold this weekend and early next week are still in question, however. The European model shows the bulk of the cold centered over the Rockies and the Dakotas Christmas Day, with much above-normal temperatures along the East Coast. For example, it predicts highs of 71 in Washington and 62 in New York City on Dec. 25.

But the American model places the core of the cold farther east by Christmas Day, focused on the Great Lakes region. It suggests cold weather could even be knocking on the East Coast’s door while the West Coast thaws. Washington’s high is predicted to be only 37 degrees.

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Given the differences in the models, we have most confidence in abnormally cold weather from the Central Plains to the western Great Lakes around Christmas with decreasing confidence to the west and east.

Who will see a white Christmas?

Areas that are colder than normal also have the best chance for snow to fall in the days leading up to Christmas and then remain on the ground.

Those areas with the best chance of a white Christmas include the Rockies, the Dakotas, Minnesota, northern Wisconsin and northern Michigan, and the interior of New England. These are the same places that historically have the highest odds of snow cover Christmas Day.

However, some places farther south that less frequently have Christmas snow may have elevated chances given the Arctic invasion, including parts of the southern and central plains into the central Midwest.

A front along the leading edge of the Arctic air is likely to produce some snow on its backside, and some modeling suggests Oklahoma, Kansas, Missouri, and Illinois could see some light snow on Christmas Eve, although the front’s position could change in future forecasts.

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What’s causing the cold?

The cold is expected to invade the middle of the nation as the jet stream surges north over western Canada and Alaska, building a bulging high pressure zone known as a ridge. The middle of the United States will sit on the downhill side of that ridge, forming a deep trough.

“It’s like the giant ridge over the North Pacific creates a slide” for frigid air from the polar vortex to spill down, said Ryan Maue, meteorologist with weather.us. “The trajectory of the upper level vortex anomalies is straight out of the Arctic.”