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Winter is here: Nation shivers in an Arctic blanket

The polar vortex normally sits over the North Pole, but it's now moving south, bringing dangerously cold air all the way to Savannah, Ga. NBC's Al Roker reports.

The U.S. has basically gone into hibernation —  thanks to the coldest Arctic outbreak in at least 20 years, which promises to keep wind chills Tuesday as low as 40 below in parts of the country.

As many as 187 million Americans — more than half the nation's population — live in regions submerged under the rare "polar vortex" so broad that every state except Hawaii can expect some freezing temperatures.

In the Upper Midwest, the only unknown will be how big the numbers with minus signs before them will be — a day after temperatures already hit 20 to 30 degrees below zero over much of the region Tuesday.

"These are some temperatures that we haven't seen in decades," said Jen Carfagna, a forecaster for The Weather Channel.

The phenomenon called a polar vortex is essentially an arctic cyclone. It usually sits near the North Pole, but this week, like the birds, it has traveled south for the winter.

Hard freeze warnings for Tuesday extended all the way south to the Gulf Coast from Texas to Florida, while the entire northeast quarter of the country is under a wind chill warning — with some perceived temperatures forecast to hit 50-below in North Dakota and Minnesota, according to the National Weather Service.

Human flesh freezes in less than five minutes at that temperature, the weather service warned.

In northern Minnesota on Monday, the towns of Embarrass, Babbitt and Brimson all reached minus-40 — and that was the real air temperature, not the wind chill.

Gov. Mark Dayton ordered schools across the state closed — the first time that has happened in 17 years.

NBC's Anne Thompson reports from John F. Kennedy International Airport, where JetBlue says it won't fly again in New York or Boston until Tuesday afternoon.

And travel was so snarled by the crippling weather that JetBlue Airways took the extraordinary step of grounding flights at four of the nation's busiest airports Monday — the three major New York airports and Boston's Logan International.

"We regret the impact to our customers," said the airline, which said it hoped to be operational by 3 p.m. ET Tuesday.

By 10:30 p.m. ET, more than 4,500 flights into and out of U.S. airports had been canceled Monday — more than 1,600 of them at Chicago's O'Hare International Airport alone. Almost 300 had been canceled at Cleveland-Hopkins International Airport.

"It's tough on all of us. You can't stay out here real long," said George Sipus of Indianapolis Power and Light — which was still working late Monday night to restore power to 22,000 customers — told NBC station WTHR of Indianapolis.

Trash and garbage collectors are also at significant risk, said Josh Boxx, owner of Boxx Sanitation in Eau Claire, Wis., which shut down Monday because conditions were just too dangerous.

"It's a huge safety concern for our workers who have to be out in this cold, and it's hard on our equipment,” Boxx told NBC station WEAU.

The conditions were deadly in Christian County, Ill., where an elderly man was found dead from the cold just a few feet from safety, the sheriff's office said. 

The man tried to walk home after his car broke down about a mile away; his body was found in his driveway Monday morning. His identity wasn't immediately released.

As the snowy Midwest digs out from the latest storm, wind chills hit 50 degrees below zero in some places. NBC's Kevin Tibbles reports.

By Wednesday, the big chill should be on its way out, and by Thursday, highs in much of the Great Lakes and the Northeast will climb all the way into the 20s and the 30s, forecasters said.

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But in the meantime, misery was everywhere:

  • The 77-year-old former mayor of Shawnee, Kan., Tom Soetaert, was treated for hypothermia after he fell while taking out the trash Monday and lay in his garage for almost an hour before receiving help from delivery man for The Kansas City Star.

Soetaert's body temperature was 92 degrees when rescue crews arrived.

"It's a good feeling to know that I helped someone," the carrier, Jeff Stockwell, told NBC station KSHB of Kansas City, Mo. "I'm just really glad that I followed my instincts to check out the situation."

  • Indiana Gov. Mike Pence declared a state of disaster emergency for 29 counties, and Indianapolis Mayor Greg Ballard declared a level-red travel emergency, making it illegal for anyone to drive except in emergencies or to get to shelter. It was the first time that has happened since 1978.
  • In Lexington, Ky., temperatures fell 55 degrees in less than 24 hours, with wind chills at minus-20 Monday. A car lost control on a patch of black ice Monday morning on Interstate 75 in Laurel County, causing a two-vehicle crash that killed the driver, NBC station WLEX reported.
  • More than 250 members of the Indiana National Guard reported for duty to help stranded dialysis patients get to their treatments, state Adjutant General Martin Umbarger told NBC station WTHR of Indianapolis.
  • It was so cold that in Clarksville, Iowa, that firefighters were tagging in and out like wrestlers as they battled a house fire, only to find their hoses were frozen. The house was totaled, NBC station KWWL of Waterloo reported.
  • It was so cold overnight that both engines froze on an Amtrak train from Detroit to Chicago, stranding passengers for nine hours just past Kalamazoo, Mich., until another train arrived to tug it the rest of the way home. 

"Not exactly in the best of spirits," Valerie King, a journalism student at Northwestern University, posted to Twitter from inside the train. Finally in Chicago, she snapped a photo of the train's outside, which looked freezer-burned:

Erin McClam of NBC News contributed to this report.