By Alexander Smith and M. Alex Johnson, NBC News
The East and South were braced for record-breaking, dangerously-low temperatures Tuesday as the "polar vortex" responsible for the coldest Arctic outbreak in at least 20 years forced millions to stay inside.
Temperatures in New York City fell to 11 degrees, Philadelphia 7 degrees, and Washington, D.C., 8 degrees at 4 a.m. ET Tuesday, according to The Weather Channel.
The wind chill factor made it feed even colder - minus 10 degrees in New York, minus 16 in Philadelphia , and minus 10 in Washington, D.C. All these figures were expected to drop by up to four degrees during the morning.
"For people stepping outside their front door in any of these cities this morning it will be a real shock," said Kevin Roth, lead meteorologist at The Weather Channel.
"It is going to be cold enough to take your breath away."
As many as 187 million Americans — more than half the nation's population — live in regions submerged under a rare Arctic weather system so broad that every state except Hawaii can expect some freezing temperatures.
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.
The worst of the system passed across the Midwest and Plains Monday, sending temperatures there as low as minus 30 degrees and wind chill as low as minus 60 degrees.
That section arrived in the East Tuesday, bringing below-zero conditions to areas as far south as Georgia.
Get the latest weather news and your local forecast
The chill was expected to ease slightly by Wednesday, returning to January norms by the end of the week.
"These are some temperatures that we haven't seen in decades," said Jen Carfagna, a forecaster for The Weather Channel.
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.
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 4 a.m. ET Tuesday, more than 1,700 flights scheduled into or out of the U.S. had been canceled. Some 600 of these came at Chicago's O'Hare International Airport. Thousands of flights have been canceled across the U.S. since the weekend.
"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.
In addition to the cold, up to five feet of snow could bury the Tug Hill plateau, between Syracuse and Watertown, N.Y., on Wednesday.
Roth said this area was prone to heavy snowfall, but it usually comes earlier in the year, around November.