The coldest weather in two decades began to settle over the Midwest on Sunday, prompting officials to warn of “life-threatening” temperatures that sent thermometers plunging and Americans scurrying indoors.

Schools canceled Monday classes because of the subzero cold, with parts of Minnesota expecting overnight lows of 30 below zero. That didn’t count the wind chill, which makes the cold even more dangerous. The National Weather Service said in a statement that wind chills in some places could reach minus 65 — temperatures at which exposed skin will get frostbitten within 10 minutes.

“Right now, in the Twin Cities, in Minneapolis, it’s 11 below zero, and it’s pretty windy out there — the wind chill is 31 below zero,” National Weather Service meteorologist Shawn DeVinny said Sunday night. He added, “And we’re pretty much the warmest spot around” in Minnesota.

Wind chill warnings were in effect from Montana to Appalachia.

Farther south, overnight temperatures were expected to reach minus 7 in Kansas City, Mo.; minus 10 in Des Moines, Iowa; minus 8 in Chicago; minus 5 in Indianapolis; and minus 4 in St. Louis.

The ultrafrigid weather was blamed on a “polar vortex” that brought a mass of icy air down to the U.S. from its usual place over northern Canada.

That didn’t deter some football fans. In Green Bay, Wis., where the Packers faced the San Francisco 49ers in an NFL playoff game, fans braved temperatures so cold that beer froze, leading some tailgaters to hold their beverages over their grills to keep the drinks liquid.

Temperature at kickoff was 5 degrees — a far cry from the infamous “Ice Bowl” of 1967, when it was minus 13. But unlike that classic game, in which the Packers defeated the Dallas Cowboys, the home team fell to the 49ers on Sunday, 23-20.

Cold wasn’t the only concern across the region.

Much of Indiana was under a blizzard warning Sunday evening as a storm front swept along the Ohio River. Some areas received up to a foot of snow, including St. Louis.

Treacherous weather also imperiled air travel.

In New York, a Delta flight with 35 people on board skidded off an icy runway after landing at John F. Kennedy International Airport. No injuries were reported, but officials closed the airport for about two hours.

A few hours later, a private jet crashed on landing in Aspen, Colo., amid windy conditions, killing the co-pilot and injuring the other two pilots, one seriously. No passengers were aboard.