'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 unleash the cold — first arriving in Montana and the Dakotas Wednesday night and reaching the Great Lakes by the weekend. In addition, 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 Day, such cold could envelop Minneapolis and Chicago. Subzero nighttime lows are a good bet.' - Washington Post