As the Midwest shivered through the region’s lowest temperatures in two decades, forecasters warned that life-threatening cold was heading eastward.

Icy, snow-covered roads and high winds made travel treacherous Sunday from the Dakotas and Michigan to Missouri as much of the nation braced for a dangerous cold wave that could break records Monday and Tuesday.

A whirlpool of frigid, dense air known as a “polar vortex” was expected to send temperatures plunging in more than half of the continental U.S., with wind chill warnings stretching from Montana to Alabama.

Chicago’s high on Monday won’t reach zero, and may just hit that on Tuesday, according to the National Weather Service. New York City’s low will fall to 8, Washington will see 5 and Dallas’ low will be 19. Orlando may drop to freezing at 32 degrees, and Miami will see nighttime temperatures in the 40s.

“The weather pattern across North America right now is set up to be very favorable for the southward transport of Arctic air,” said Bob Oravec, a National Weather Service meteorologist.

“It’s not going to be long-lived,” he added. “By the end of the week the temperatures definitely start to moderate across the whole of the country.”

It was 5 degrees at kickoff Sunday afternoon inside sold-out Lambeau Field for the NFL playoff game between the Green Bay Packers and the San Francisco 49ers, though a northwest wind made it feel like minus-10.

That apparently meant little to some of the players. The entire starting offensive line of the Packers went sleeveless under their green jerseys. A few 49ers went with the sleeveless look, too, including quarterback Colin Kaepernick.

But looking ahead, the forecast was extreme: 25 below zero in Fargo, N.D., minus-31 in International Falls, Minn., and minus-15 in Indianapolis and Chicago. Wind chills — what it feels like outside when winds are factored into the temperature — could drop into the minus-50s and -60s.

It hasn’t been this cold for almost two decades in many parts of the country. Frostbite in such frigid weather can afflict uncovered skin in a matter of minutes, experts warned, and hypothermia can set in quickly at 15 to 30 below zero.

Several Midwestern states received up to a foot of new snow Sunday. The National Weather Service said snowfall at O’Hare International Airport in Chicago totaled more than 11 inches by Sunday evening — the most since a Feb. 2, 2011, storm.

The St. Louis area had about a foot of snow, and northern Indiana had at least 8 inches. Officials closed several Illinois roads because of drifting snow and warned residents to stay inside.

Indiana Gov. Mike Pence ordered nearly 100 Indiana National Guard members to be ready to help rescue stranded motorists, move people to shelters and assist local emergency services workers in reaching people who need medical help.

Across the Great Lakes, significant lake-effect snow will continue downwind of the lakes.

Upstate New York is expected to pick up between 2 and 3 feet of new snow through Tuesday. This will be most common in areas downwind of Lake Erie and Lake Ontario.

Lorna West, a 43-year-old student and consultant from Columbus, Ohio, said she doesn’t believe people unaccustomed to such weather are ready for what’s coming. A Chicago native, she said thermal underwear, lots of layers and “Eskimo coats” with zipped hoods to block the wind were the norm growing up.

“And don’t go out if you don’t have to,” she said.

Travel problems started early Sunday. In New York City, a plane from Toronto landed at Kennedy International Airport and then slid into snow on a taxiway. No one was hurt, though the airport temporarily suspended operations because of icy runways.

About 1,300 flights were canceled Sunday at O’Hare and Midway international airports in Chicago, aviation officials said, and there also were cancellations at Logan International Airport in Boston, Tennessee’s Memphis and Nashville international airports, and Lambert international airport in St. Louis.

Nationwide, more than 7,300 flights were delayed and 3,514 were canceled, according to Flightaware.com, a website that tracks air travel.

School was called off Monday for the entire state of Minnesota, as well as cities and districts in Wisconsin, Indiana, Iowa and Illinois, including Chicago.

Southern states braced for possible record-low temperatures, too, with single-digit highs expected Tuesday in parts of Georgia and Alabama.

Temperatures are expected to dip into the 30s in parts of Central Florida on Tuesday. But Florida Citrus Mutual spokesman Andrew Meadows said it must be at 28 degrees or lower for four hours straight for fruit to freeze badly.

In western Kentucky, which could see 1 to 3 inches of snow, Smithland farmer David Nickell moved extra hay to the field and his animals out of the wind. He had also stocked up on batteries and gasoline, and loaded up the pantry and freezer. The 2009 ice storm that paralyzed the state and knocked out power to hundreds of thousands of people was fresh in his mind.

“We are hoping this isn’t going to be more than a few days of cold weather, but we did learn with the ice storm that you can wake up in the 19th century and you need to be able to not only survive, but be comfortable and continue with your basic day-to-day functions,” Nickell said.

In Clintonville, Ohio, Janine Dunmyre found her local grocery store stripped of staples including milk, eggs and juice — as well as some less-essential supplies.

“The chips aisle was decimated,” she said. “Like everyone is planning to sit around for two days with snack food.”