Millions of Americans braced Tuesday for a dangerous polar vortex which began to settle over a large swath of the United States, threatening to set new records as schools and businesses closed and authorities warned of frostbite.

Jen Brackman and Aaron Brackman don large jackets and goggles while on a walk on the Stone Arch Bridge on January 29, 2019 in Minneapolis, Minnesota. The polar vortex is here -- tens of millions of people in the US braced on January 29, 2019, for a deep arctic chill, which authorities say could be life-threatening. (Photo: VCG)

Temperatures in almost a dozen states stretching over 1,200 miles from the Dakotas to Ohio were forecast to be the coldest in a generation, if not on record.

The National Weather Service (NWS) forecast temperatures between -10 to -40 degrees Fahrenheit (-23 to -40 Celsius) by Wednesday, with wind chill making it seem as cold as -65 degrees Fahrenheit in one area of Minnesota.

The culprit was a lobe of arctic air broken away from the polar vortex that usually encircles the North Pole. 

"A record cold air mass will continue into the central and northern US, spreading to the east coast on Tuesday," the NWS said. 

"Wind chills of -30 to -60 (Fahrenheit) can be expected over portions of the northern Plains and Great Lakes region."

Michigan and Wisconsin declared statewide emergencies in advance of the frigid temperatures. 

Americans were heeding warnings to stay home if possible on Wednesday, when temperatures were expected to be at their coldest. 

Scores of schools, businesses and government agencies announced closures in multiple states.

"People exposed to extreme cold are susceptible to frostbite in a matter of minutes," warned the NWS. "Areas most prone to frostbite are uncovered skin and the extremities, such as hands and feet."

Evan Roberts is covered in frost while jogging across the Stone Arch Bridge Tuesday, Jan. 29, 2019, In Minneapolis. Extreme cold and record-breaking temperatures are crawling into parts of the Midwest after a powerful snowstorm pounded the region, and forecasters warn that the frigid weather could be life-threatening. (File photo: VCG)

More than 70 warming centers were opened in Chicago, America's third city, where temperatures were forecast to potentially rival or overcome the all-time record of -27 degrees Fahrenheit.

Wind chill would make the air feel closer to -40 to -50 degrees in the windy city, officials said. 

Meanwhile the northeast and parts of the south were contending with a snow storm, which had previously dumped up to a foot of snow (30 centimeters) in the Midwest over the weekend. 

"Not only is brutal cold invading the central parts of the country, accumulating snow and freezing roadways all the way into the deep south could make travel dangerous," the NWS said. 

The snow was expected to make the arctic air feel even colder, the effects of which had already arrived in Canada. 

The frigid cold blanketing much of the US's northern neighbor, from Manitoba in the western Prairies region to Atlantic Canada, prompted a rare "hazardous" cold warning from Environment Canada.