Tens of millions of people in the United States are bracing for a potentially life-threatening deep arctic chill forecast to hit swaths of the country.

Companies have told their workers to stay home, schools are closed and hundreds of flights have been canceled.

The US Postal Service -- known for its commitment to bringing the mail whatever the weather -- has even reportedly suspended deliveries in Iowa due to the severe cold.

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

The cause is a swirl of arctic air that broke away from the polar vortex that usually encircles the North Pole.

Map of North America showing the temperature forecast for Wednesday, January 29, 2019. Photo: AFP

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

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, when temperatures are expected to be at their coldest. Scores of schools, businesses and government agencies announced closures in multiple states.

President Donald Trump used the occasion to again voice skepticism 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.