“We need everyone to do your part and make sure you and your families are prepared,” said Illinois Governor JB Pritzker.

Companies told their workers to stay home. Schools were closed. Hundreds of flights were cancelled.

Temperatures in almost a dozen states stretching more than 1,930km (1,200 miles) from the Dakotas to Ohio were forecast to be the coldest in a generation, if not on record.

“One of the coldest arctic air mass intrusions in recent memory is surging south into the Upper Midwest before spreading across much of the eastern two-thirds of the country,” the National Weather Service said.

“Expect frigid temperatures, bitterly cold and life-threatening wind chills, likely leading to widespread record lows and low maximum temperatures from the Upper Midwest to the Great Lakes and Ohio Valley.”

The culprit? A swirl of arctic air that broke away from the polar vortex that usually encircles the North Pole.

The National Weather Service forecast temperatures between -23 to -40 degrees Celsius (-10 to -40 Fahrenheit by Wednesday across the Midwest, with wind chills making it seem even colder.

Chicago, America’s third largest city, was expected to be colder than parts of Antarctica.

Authorities in Illinois, Michigan and Wisconsin put emergency measures in place to handle the frigid weather.

Americans were asked to stay home if possible 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.

Lawrence Gottlieb of the University of Chicago Medical Centre said the threat was significant “when temps fall below zero, especially when there is a strong wind.”

Some 160 warming centres were opened in Chicago, where temperatures could potentially equal or exceed the all-time record of -27 degrees Fahrenheit. With the wind chill, it would feel like -40 or -50, officials said.

“They are life-threatening conditions and temperatures,” Chicago Mayor Rahm Emanuel told a news conference.

In Minneapolis, officials allowed residents to stay on public buses and trains for warmth.

In the northeastern and southern United States, snow was falling.

A cold emergency was declared in the US capital Washington, with additional services put on for the homeless.

In the southern city of Atlanta, some 300 flights were cancelled Tuesday, and more than 400 were cancelled in Chicago, a major regional hub for US airlines.

In Canada, the icy temperatures – stretching from Manitoba in the western Prairies region to the Atlantic seaboard – prompted a rare “hazardous” cold warning from the government.

Environment Canada reported record-breaking snowfall at the Ottawa airport, where some 50 flights were cancelled, with an accumulation of nearly a metre (more than three feet).

An additional 200 flights were cancelled at Toronto’s airport.

Scientists say climate change is causing more extreme weather, and one theory for polar vortex chills is that arctic air currents usually trapped around the North Pole are weakened and dislodged by a warming climate.

US President Donald Trump used the occasion to again voice scepticism about climate change, tweeting: “What the hell is going on with Global Waming? (sic) Please come back fast, we need you!”

But the National Oceanic and Atmospheric Administration, which operates NWS, tweeted, “Winter storms don’t prove that global warming isn’t happening,” with a link to a 2015 explanatory article.