A stratospheric warming event is forecast by the European ensemble in about 12-13 days. The model has produced this result for the past 2-3 days therefore forecast confidence is increasing. The American ensemble is also forecasting this event s of the past day or so. When the stratosphere warms the weather atmosphere beneath (i.e. troposphere) contracts and cools. The colder and increasingly dense air in the troposphere creates high pressure at the surface forcing arctic air generation.
At the surface both the ECM ENS (Fig. 1) and GFS ENS (Fig. 2) forecast temperature departures from normal of 15-25F in south-central and east Canada just before and on Christmas Day. Temperature departures from normal of this cold magnitude represent arctic air. The arctic air may be colder than forecast due to the deep snow beneath this cold air mass.
While the core of the extreme cold is along and just north of the U.S./Canada border the expansive snow cover into the northern U.S. will allow the cold to extend to the Midwest U.S. and into the Northeast States for the Christmas period.
High temperatures on Christmas day are in the single numbers in Minneapolis, middle teens in Chicago and upper 20’s in Cincinnati. At night, Chicago could drop to zero Christmas night while Minneapolis drops to -10.
At the same time Burlington, VT reaches only 20 during Christmas Day with single nos. at night. Hartford and Boston are near 30 and likely snowing.
Fig. 1-2: As a result of stratospheric warming arctic air mass generation develops in south to east Canada where temperature departures from normal are -15F to -25F. The GFS ensemble projects temperature departures from normal near -30F in 13 days.