Bone-chilling cold that paralyzed a chunk of the USA this week and killed at least 18 people eased Friday as the Arctic air mass retreated ahead of a warmer-than-normal weekend in areas of the Midwest and Northeast.
Bryan Jackson, a National Weather Service (NWS) meteorologist, said that the core of the vortex was pulling north into eastern Canada, though residual icy air was still pushing over to the US Northeast.
Scientists call these "sudden stratospheric warming" events, when a continent-sized chunk of rapidly sinking air (usually over Eurasia) quickly heats up and disrupts the polar circulation, sometimes completely disintegrating it. Statistically, that's when the cold air floodgates can really open as the jet stream scrambles to contain the chaotic swirls of polar air that descend southward in the aftermath.
Temperatures yesterday morning ranged from below zero Fahrenheit to the teens in parts of New York, Pennsylvania, Massachusetts and New Jersey.
"It's got to be an nearly 50 degree difference, it feels like spring", said one commuter heading home from Chicago's downtown financial district, wearing only a sweatshirt.
Changing air currents caused it to slip down through Canada and into the US Midwest this week.
MI and many states in the Midwest have been in a cold weather pattern over the last six days that has gotten progressively worse: we have had icy rain, sleet, snow, blowing and drifting, zero visibility, temperatures below zero, and wind chills as low as -30.
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In Wisconsin, Governor Tony Evers has declared an emergency and requested help from the National Guard, according to the New York Times; In northern In, the University of Notre Dame said it would close its campus this evening through Thursday afternoon. "I'm not used to this".
In Chicago, the temperature early Thursday went down to -21 F (-29 C) - close to breaking the record for the lowest ever recorded, -27 F (-33 C). The central Plains will be in around 14C to 16C, almost seven or eight degrees above normal, the weather service said.
At least 18 deaths have been linked to the deep freeze since Saturday, and the number was expected to climb as authorities identified more victims.
More than 40 cold-temperature records were broken on Thursday, the coldest morning since the polar vortex moved in late on Tuesday.
The death toll rose from a previous 12 after at least nine more people in Chicago were reported to have died from cold-related injuries from 2019 polar vertex, according to Dr Stathis Poulakidas at the city's John H Stroger Jr Hospital.
But on Friday, Amtrak train services that were halted since Wednesday in Chicago's hub will resume, as will U.S. Postal service halted or limited in six Midwest states.