A “polar vortex” has dropped over a huge section of the US, sending temperatures plunging across the Midwest as the East Coast braced for the bitter Arctic blast.
Meteorologists predicted the lowest temperatures in two decades in some areas of the country, with the mercury at minus 37 Celsius in Milwaukee.
Icy roads and snow storms created travel chaos, with thousands of flights cancelled or delayed.
The northeastern US has endured heavy snow and deadly freezing conditions since the start of the year, and the extreme cold has claimed about a dozen lives nationwide.
“The coldest weather in years will be making its presence known from the Upper Midwest to the Mid-Atlantic region for the beginning of the work week,” the National Weather Service said on Monday.
Wind chill warnings were in effect from Montana as far south as Tennessee and Alabama, with temperatures feeling as cold as minus 45 Celsius, it said, describing the conditions as “life-threatening”.
On the East Coast, a brief lull in the deep freeze greeted commuters on Monday, but temperatures were expected to drop by 40 degrees overnight.
In Milwaukee, on the shores of Lake Michigan, officials warned residents to stay indoors and urged schools to shut down.
“Police are reaching out to homeless citizens in order to help them find the nearest shelter,” city spokeswoman Sarah DeRoo said.
All schools were closed in Minnesota. In Cook County in Illinois, which includes Chicago, warming centres were opened for anyone suffering, including those whose home heating did not work.
“This winter storm will be one for the record books, and we want to make sure everyone stays safe and warm until it passes,” Illinois state governor Pat Quinn said.
Exposed skin could suffer frostbite in as little as five minutes in such conditions, experts warn.
More than 3000 flights within, into or out of the US were cancelled on Monday, according to industry tracker FlightAware, including nearly 800 at Chicago O’Hare International, one of the country’s busiest airports.
Hundreds more flights were cancelled or delayed from other major hubs like New York’s John F Kennedy or Newark International.
In Wisconsin, Sunday’s National Football League playoff showdown between the Green Bay Packers and the San Francisco 49ers at the open-air Lambeau Field was one of the coldest NFL games in history.
The Packers helped the more than 70,000 football fans at the stadium battle the freeze by handing out free coffee, hot chocolate and hand warmers. Green Bay lost to San Francisco, 23-20.
Meteorologists said that a warming trend would mercifully begin at mid-week.