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Major cold blast to crash into middle of nation by Christmas

FILE PHOTO: Pronounced sundogs appear on each side of the sun over Fargo as unseasonably cold temperatures form ice crystals suspended in the atmosphere, thereby refracting the sun's light. File photo. Michael Vosburg / Forum Photo Editor

A major cold wave, the most intense of the season so far, will drop out of Canada and into Lower 48 states late this week. By Christmas Day, a biting chill is likely to stretch from the Rockies to the Great Lakes. But relatively mild weather may cover the East Coast.

The polar vortex, the slithering zone of frigid air encircling the Arctic, will cut some of this cold air lose - first arriving in Montana and the Dakotas Wednesday night and reaching the Great Lakes by the weekend. Additional, reinforcing blasts of cold are predicted to keep cycling south through Christmas and beyond.

Christmas Eve may feature temperatures 20 to 40 degrees below normal around Denver and Bismarck. By Christmas Eve, 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 is 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.

Given the differences in the models, we have most confidence in abnormally cold weather 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 which historically have the highest odds of snow cover Christmas Day.

However, some places farther south which 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.

--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 Alaska, building a bulging high pressure zone known as a ridge. The middle of the U.S. 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 meteorologist Ryan Maue. "The trajectory of the upper level vortex anomalies is straight out of the Arctic."

While temperatures are forecast to be much colder than normal, models are not predicting record cold. Maue characterized the upcoming outbreak of Arctic air as a "seasonable polar vortex episode." He said it does not look anywhere near as intense as the punishing January 2014 polar vortex outbreak that broke scores of records.

Author Information: Jason Samenow is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist.

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