Dangerously cold air descended on the Midwest and Plains on Sunday, as new snow brought whiteout conditions to some areas and freezing rain threatened to cancel more flights in the Northeast.
The arctic blast — expected to be the coldest in decades — is bringing below-zero temperatures to more than half of the continental U.S. through Monday and Tuesday.
The coldest air was hitting the Dakotas and Minnesota, which clocked temperatures of 20 degrees below zero on Sunday. At its lowest, the wind chill in Minnesota was a numbing minus 50 in Flag Island.
“It’s just a dangerous cold,” National Weather Service meteorologist Butch Dye in Missouri said Sunday morning.
And as they brace for the bitter cold, Midwesterners also must dig out of another nasty snowstorm.
"If you don’t have to go outside, don’t do it"
Five to 9 inches fell Sunday in the Chicago area, while a foot was dumped in the St. Louis area. Eight to 10 inches was expected to pile up in central Illinois, Indiana and Michigan throughout the day.
Forecasts also called for several inches in western Tennessee and 1 to 3 inches in Kentucky.
Across the country, at least 2,400 flights had been cancelled Sunday, according to tracking website FlightAware.
Flights were temporarily grounded for two hours at New York’s John F. Kennedy International Airport after a plane skidded off the runway and into a snow bank at 8 a.m. No one was injured.
Chicago’s O’Hare and Midway international airports cancelled about 1,200 flights as well.
Major airports in Detroit, Philadelphia, Newark, N.J., and Boston were also reporting significant weather-related delays.
JetBlue announced it is waiving its change and cancellation fees for customers travelling to or from 17 Northeast airports affected by the weather beginning last Thursday through Monday.
Grocery stores were also feeling the squeeze from the storm. In St. Louis, shops sold out of the essentials before the weather onslaught.
“The problem is the bread is sold out. We’re out of milk. We sold out of chips, chicken wings, some meats,” Issa Arar of Salama Supermarket said.
The frigid air blasting into the Plains was part of what the National Weather Service called “incredibly cold and possibly record-breaking temperatures” expected throughout the week, with the brutally cold air expected to spread to the Northeast and Gulf Coast Monday and Tuesday.
Forecasters are expecting bitterly cold temperatures in many places: 25 below zero in Fargo, N.D., minus 31 in International Falls, Minn., and 15 below in Indianapolis and Chicago. Wind chills may reach 50, 60 or even 70 below zero — temperatures much of the country has not seen in decades.
At temperatures of 15 to 30 below, exposed skin can get frostbitten in minutes and hypothermia can quickly set in.
“If you don’t have to go outside, don’t do it,” said Michael Palmer, lead meteorologist at The Weather Channel.
Dr. Brian Mahoney, medical director of emergency services at Hennepin County Medical Center in Minneapolis, advised people to protect themselves against the intense cold by wearing hand and face protection. Mittens were preferable to gloves and layers of dry clothing are best, he said.
"A person not properly dressed could die easily in those conditions," National Weather Service meteorologist Scott Truett in St. Louis told The Associated Press, describing the expected wind chill in Missouri at daybreak Monday.
Despite the bone-chilling weather, Sunday’s NFC wild-card game between the Green Bay Packers and the San Francisco 49ers kicked off with temperatures at 5 degrees in Wisconsin — slightly warmer than anticipated.
“We suited up, we brought all the snowboarding gear we use ... and added to it,” said 49ers fan Jeff Giardinelli of Fresno Calif., as he walked across a parking lot with a friend. “Without the wind, which isn't here yet, we're good. When it gets windy, we'll be ready for it.”
Regardless of the forecast, fans scooped up the remaining 40,000 tickets needed to sell out the game this week.
Minnesota said schools in the entire state would be closed Monday — the first such closing in 17 years. Milwaukee and Madison, Wis., had also cancelled school for Monday.
Already, parts of New England dropped into the negatives Saturday, with East Brighton, Vt., seeing 30 below zero just after midnight and Allagash, Maine, hitting minus 36. The cold will sweep through other parts of New England where residents are still digging out from a snowstorm Friday.
That one led to at least 13 deaths, mostly from traffic accidents, in several states.
A temperature of minus-9 degrees in Hartford, Conn., early Saturday broke the record for the date and was the lowest recorded in the county since February 2009, according to NBC Connecticut.
“It’s the mother-lode of cold air,” Weather Channel coordinating meteorologist Tom Moore said. “On the heels of what will be the coldest air of the season, will be dangerous, life-threatening winds.”
The South also will dip into temperatures rarely seen. By Monday morning, western and central Kentucky could be below zero — "definitely record-breaking," said weather service meteorologist Christine Wielgos in Paducah, Ky.
And in Atlanta, Tuesday's high is expected to hover in the mid-20s.
NBC News' Erik Ortiz and Elisha Fieldstadt, and The Associated Press contributed to this report.