AccuWeather Global Weather Center -- 24 February 2014 -- AccuWeather.com report last week's thaw was a mere tease for the Midwest and Northeast with the polar vortex set to make an encore performance this week.
The stretch of mild weather across the Northeast came to an end with the weekend after temperatures rose into the 60s in Washington, D.C., and into the 50s in Philadelphia, New York City and Boston.
Highs on Monday will be 10 to 20 degrees colder throughout the Northeast and mid-Atlantic following the passage of a band of rain and nuisance snow on Sunday night.
Colder air has already erased the mild spell across the Midwest, and Sunday should prove to be one of the "warmest" days of this week.
Waves of even colder air will pour down from the depths of the Arctic to the northern Rockies, Midwest and Northeast as this week progresses and the polar vortex plunges southward.
Each cold blast will dip into the southern Plains and South, leading to some cooling. However, the core of the polar vortex will have a firm grip on the northern tier of the United States.
"The polar vortex is essentially a mass of very cold air that usually hangs out above the Arctic Circle and is contained by strong winds," stated AccuWeather.com Senior Meteorologist Alex Sosnowski.
Under the grip of the polar vortex, the Upper Midwest, Great Lakes and St. Lawrence Valley will endure several days in the teens and single digits. This includes Minneapolis, Chicago, Detroit, Montreal and Buffalo, N.Y.
Overnight lows well below zero, even approaching 30 below zero near the Canadian border, will dominate the Upper Midwest.
Temperatures will even be held below zero during the daylight hours on one or more days in Winnipeg, Canada, Fargo, N.D., and St. Cloud, Minn.