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The coldest air in almost 20 years swept over the Central U.S. toward the East Coast as the new week began, threatening to topple temperature records as icy, snow-covered roads and high winds made travel treacherous across a large swathe of the country.

A whirlpool of frigid, dense air known as a “polar vortex” was expected to suppress temperatures in more than half of the continental U.S. starting into Monday and Tuesday, with wind-chill warnings stretching from Montana to Alabama.

Mike Musher, a meteorologist with the U.S. Weather Prediction Center in College Park, Md., said 90 percent of the contiguous U.S. will be at or below the freezing mark Monday.

Monday may be the coldest day of the 21st century for the contiguous U.S., beating Jan. 16, 2009, said Matt Rogers, president of Commodity Weather Group in Bethesda, Maryland.

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“It definitely looks like one heck of a ‘polar pig’ shot,” Kyle Cooper, director of research with IAF Advisors in Houston, said in a Jan. 3 telephone interview, using a term for a bulge of cold air out of Canada.

“Models show it as quite intense and dropping down pretty far south,” Cooper said.

This week’s cold will have a greater impact because most people are back to work after the Christmas and New Year’s holidays, said Michael Schlacter of Weather 2000 in New York.

Temperatures sank to minus 20 and colder in northern Minnesota and Grand Forks, N.D.

The forecast is 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 high winds are factored into the temperature — could drop into the minus 50s and 60s.

Northeastern Montana was warned of wind chills up to 59 below zero.

“It’s just a dangerous cold,” said National Weather Service meteorologist Butch Dye in Missouri.

In Indianapolis, Mayor Greg Ballard upgraded the city’s travel emergency level to “red,” making it illegal for anyone to drive except for emergency personnel, emergency purposes or seeking shelter.

The last time the city issued a travel warning was during the epic 1978 blizzard.

New York City’s low will fall to 8, Washington will see 5 and Dallas’ low will be 19.

Southern states are bracing for possible record temperatures, too, with single-digit highs expected Tuesday in Georgia and Alabama.

Temperatures are expected to dip into the 30s in parts of Florida on Tuesday.

Hard-freeze warnings and watches, which are alerts for farmers, stretch from Texas to Central Florida.

Freezing temperatures spur energy demand as people turn up thermostats to heat homes and businesses.

“Everybody’s energy bills will be going up,” said Musher, the meteorologist. “I am sure there are going to be multiple records broken across a large part of the country.”

Several Midwestern states received up to a foot of new snow Sunday. The St. Louis area had about a foot of snow and northern Indiana had at least 8 inches.

The National Weather Service said snowfall at O’Hare International Airport in Chicago totaled more than 11 inches as of Sunday night — the most since a Feb. 2, 2011, storm.

As of Sunday afternoon, 1,217 flights had been canceled across the U.S., according to FlightAware, a Houston-based tracking service.

Officials closed several Illinois roadways because of drifting snow and warned residents to stay inside. Roads in the Midwest were particularly dangerous, and officials in Missouri said it was too cold for rock salt to be very effective.

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.

In Michigan, residents jammed stores to stock up on supplies.

“I made my husband go grocery shopping last night,” said Kim Tarnopol, 46, of the Detroit suburb of Huntington Woods.

Tarnopol was picking up cold medicine Sunday for her daughter at a CVS in nearby Berkley, Mich.

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

Chicago Public School officials reversed course late in the day Sunday, canceling classes for Monday ahead of the expected bitterly cold temperatures.

The change in plans came amid criticism from the Chicago Teachers Union, though officials say they made the decision after they “evaluated the situation again.”

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