By F. Brinley Bruton, Staff Writer, NBC News

Potentially record-setting cold temperatures were descending across the Midwest and Northeast on Sunday, disrupting travel and prompting forecasters to warn people to stay indoors.

Detroit, Saint Louis, Indianapolis and Chicago saw continued snowfall overnight, which along with the cold disrupted thousands of flights.  Across the country, at least 2,000 Sunday flights had been cancelled or delayed, according to tracking website FlightAware.   About 43 percent of those scheduled to fly in or out of Chicago's O'Hare Airport were cancelled.

The Dakotas and Minnesota bore the brunt of the coldest weather, clocking temperatures of 20 degrees below zero on Sunday morning.

"And it is only getting colder," Michael Palmer, lead meteorologist at The Weather Channel 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 into to the Northeast and Gulf Coast Monday and Tuesday.

The snow is also expected to reach as far south as Tennessee and Alabama and icy conditions should even be expected in the Deep South Sunday, according to NWS. Atlanta's high temperature on Tuesday is forecast to be in the mid-20s, while the city's January average high is usually in the mid-50s, according to

Forecasters are expecting 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," Palmer warned.

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 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 looked set to play out in bone-chilling temperatures in Wisconsin.

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 almost 20 years.  Milwaukee and Madison 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 digging out from a snowstorm.

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.