The agency forecast wind chill temperatures Thursday would still range between -20 to -50 degrees Fahrenheit (-29 to -46 Celsius) over parts of the Upper Midwest.
Bryan Jackson, a National Weather Service meteorologist, said the core of the vortex was pulling north into eastern Canada, though residual icy air was still pushing over to the US Northeast.
The term "Polar Vortex" has been widely used to describe the dangerously cold temperatures that have been plunging into the US from Canada.
"It has got to be an nearly 50 degree difference, it feels like spring", said a commuter heading home from Chicago's downtown financial district, wearing just a sweatshirt.More news: Gladys Knight’s Super Bowl national anthem performance tops two minutes
"In some areas of the Midwest, the coming wild temperature swing will be the most extreme warm-up from record winter cold ever seen in such a short-time period", Mr Jeff Masters, meteorologist at Weather Underground, told AFP. The Arctic air mass that veered off its normal polar route broke more than 340 records in communities across a cluster of states, according to the Midwestern Regional Climate Center. "Now the high pressure is over Pennsylvania and New York", Jackson said in a phone interview.
Rachel Liao (29), a student in NY, said she wished classes had been cancelled due to the cold. "I'm not used to this".
In the Upper Midwest, including Illinois, Indiana and OH, temperatures will reach well above zero F (minus 18 Celsius) on Friday, with highs making it into the teens and low 20s.
Even so, parts of Minnesota, Wisconsin, Michigan and Iowa were still experiencing significantly lower temperatures, Mr Jackson said.More news: Anthem Demo: BioWare Explains Issues And What's Being Fixed
The central Plains will be in the low 60s, almost 20¬° to 25° above normal, the weather service said. In many areas, temperatures have dropped to below 0 degrees Fahrenheit, and the weather has caused service disruptions, power outages, school cancellations, air travel disruptions, and even several deaths. Frigid temperatures are swallowing up many states thanks to a mass of arctic air making its way much farther south than it normally does, and NASA caught a glimpse of it using one of its trusty satellites.
The death toll rose after at least nine more people in Chicago were reported to have died from cold-related injuries, according to Stathis Poulakidas, a doctor at the city's John H Stroger Jr Hospital.
Amtrak restored rail service to and from Chicago on Friday, and the U.S. Postal Services resumed mail delivery in six Midwest states after suspending it for days.More news: CHP Set to "Tackle" DUI's on Super Bowl Sunday
More than 2,300 flights were cancelled across the USA on Thursday, according to the flight tracking website FlightAware.