Bitter temperatures flirted with record lows on Sunday as "life-threatening" conditions put millions across the country in a deep freeze for Valentine's Day.
More than 38 million people from the Plains into the Midwest and East Coast were under winter-storm warnings or advisories overnight, according to The Weather Channel.
The coldest air mass of the winter brought the thermometer down to minus 6 degrees overnight in Minneapolis, while New Yorkers were urged to take "extreme precauticons" against wind chill.
"These temperatures can be life threatening -- especially for seniors, infants and people with medical conditions," New York City Mayor Bill de Blasio warned.
Forecasters warned Sunday morning will likely to be the coldest for many -- with New England feeling the deepest chill.
"Stay indoors if you can," Weather Channel lead meteorologist Michael Palmer said. "With these type of temperatures you can get frostbite within a matter of minutes."
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Snow, winter weather 2015-2016 season across the U.S.
Bitter temperatures to put Northeast under deep freeze on Valentine's Day
LUBBOCK, TEXAS - DECEMBER 27: A highway worker tries to shield himself while walking to his truck on December 27, 2015 in Lubbock, Texas. Coming on the heels of several strong tornadoes, some northern parts of Texas are experiencing blizzard conditions with wind gusts up to 50 mph and as much as 13 inches of snow forecast. (Photo by John Weast/Getty Images)
NEDERLAND, CO - DECEMBER 15: Steve Hauser plows snow in the Caribou Shopping Center in Nederland, Colorado on December 15, 2015. Snow is expected to continue throughout the day. (Photo by Helen H. Richardson/The Denver Post via Getty Images)
NEDERLAND, CO - DECEMBER 15: A yellow lab waits patiently in his owner's snowy old truck along East 1st Street in Nederland, Colorado on December 15, 2015. Snow is expected to continue throughout the day. The dog looked warm and didn't wait long in the car as his owner returned shortly after an errand in the town. (Photo by Helen H. Richardson/The Denver Post via Getty Images)
DENVER, CO - DEC. 15: Anna Renton, 7, right, relaxes while neighbor Graham Langner, 7, has a snow fight with his nanny, Ashely Kish.The Denver metro area received more snow than expected overnight, closing schools across the city with some areas measuring a foot of snow by the early morning. (Photo by Kathryn Scott Osler/The Denver Post via Getty Images)
LAKEWOOD, CO - DECEMBER 15: Crut Liles clears snow from his walkways outside his home in Lakewood, December, 15, 2015. Many schools in the area are closed due to the snow storm. (Photo by RJ Sangosti/The Denver Post via Getty Images)
DENVER, CO - DEC. 15: A CDOT crew runs their plow truck southbound on Colorado Blvd. near E. 13th Ave. in Denver. The Denver metro area received more snow than expected overnight, closing schools across the city with some areas measuring a foot of snow by the early morning. (Photo by Kathryn Scott Osler/The Denver Post via Getty Images)
Al Frelk walks his dog, Shiba, 10, in Lords Park in Elgin, Ill., on Saturday, Nov. 21, 2015. Asked about the snow, Frelk said, 'It's beautiful, but can be tough to drive in. Though Shiba has been waiting 9 months for this.' The first winter storm of the season dropped more than 10 inches of snow at numerous Chicagoland locations. (Stacey Wescott/Chicago Tribune/TNS via Getty Images)
DENVER, CO - NOVEMBER 17: Snow hit the Denver metro area having an impact on air travel at Denver International Airport. They were clearing out from the snow on Tuesday, November 17, 2015. Raul Hernandez shovels snow in the economy lot. Hernandez and his crew had worked all night to clear snow. (Photo by Cyrus McCrimmon/The Denver Post via Getty Images)
LAKEWOOD, CO - NOVEMBER 11: Brandon Nelli, a student at Colorado Christian University in Lakewood, shovels snow during a Veteran's Day snow storm the moved into the area overnight, November, 11, 2015. Lakewood got almost 3 inches in some areas form the storm. (Photo by RJ Sangosti/The Denver Post via Getty Images)
DENVER, CO - NOVEMBER 11: A blanket of white snow covers Fort Logan National Cemetery during a Veteran's Day snow storm the moved into the area overnight, November, 11, 2015. (Photo by RJ Sangosti/The Denver Post via Getty Images)
MONUMENT, CO. - November 17: Bob Swift clearing his driveway of a deep drift of snow in Monument, CO. November 17, 2015 Monument, CO (Photo By Joe Amon/The Denver Post via Getty Images)
An above-average amount of snow covers a small cabin near where the first snow survey of winter conducted by the California Department of Water Resources in Phillips, California December 30, 2015. REUTERS/Fred Greaves
Cameron Shonnard backflips a jump at Squaw Valley in Olympic Valley, California, December 5, 2015. An El Nino is forecasted for California, and regular precipitation has been welcomed after years of drought. REUTERS/Max Whittaker TPX IMAGES OF THE DAY
Fresh snow clings to trees near Big Bend, California, December 4, 2015. An El Nino is forecasted for California, and regular precipitation has been welcomed after years of drought. Picture taken December 4. REUTERS/Max Whittaker
The U.S. Capitol dome can be seen behind piles of snow removed from parking areas and walkways around the Capitol grounds in Washington January 26, 2016. The snowbound Washington area was resuming partial business on Tuesday as trains and buses restarted near-normal service, while federal offices remained closed following a massive blizzard that hammered the U.S. East Coast. REUTERS/Jonathan Ernst TPX IMAGES OF THE DAY
Floodwaters cover Green Avenue after a winter storm in Manasquan, New Jersey, January 24, 2016. A morning high tide surge of 2 feet followed snowfall of about 2 feet in the first major storm of the season. REUTERS/Dominick Reuter
A couple poses for a photo on a snow pile during a snow storm in Times Square in the Manhattan borough of New York, January 23, 2016. REUTERS/Carlo Allegri TPX IMAGES OF THE DAY
WASHINGTON, DISTRICT OF COLUMBIA, UNITED STATES - 2016/02/15: Snow is once again falling across Washington, D.C on Monday, 15 February 2016. The bitter cold was replaced by snow, sleet and rain Monday in the mid-Atlantic states and the South, but many residents were able to hunker down at home with federal offices and many businesses closed for Washington's Birthday. (Photo by Probal Rashid/LightRocket via Getty Images)
A barn painted with the U.S. flag is seen in the snow covered field in Kanawha, Iowa, United States, January 16, 2016. Iowa will be the first state to hold its primary, with both Democratic and Republican events being held February 1, 2016. Picture taken January 16, 2016. REUTERS/Jim Young
Police officers gather outside the Montgomery County Courthouse, where actor and comedian Bill Cosby will arrive for a preliminary hearing on sexual assault charges, in Norristown, Pennsylvania February 2, 2016. Cosby has been charged with the 2004 sexual assault of Andrea Constand, a former women's basketball team manager at Temple University in Philadelphia, Cosby's alma mater. REUTERS/Mark Makela REUTERS/Mark Makela
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He said parts of New England will see "bitterly cold" wind-chill factors of 40 below zero, with air-surface temperatures also below zero for much of the Northeast.
New York City could get wind-chill factor of 20 below zero while upstate New York could experience wind chill of 50 below zero, he said.
Several cities had already set record lows as of 4:45 a.m. ET: Boston and Providence both hit minus 9, while Hartford hit minus 10 and Worcester, Massachusetts hit minus 15, according to the National Weather Service.
As of 4:45 AM Records as follows: Boston -9F, Providence -9F, Worcester -15F, Hartford (Bradley) -10F
"No one should be left out in the cold. These are bitter, nasty conditions," Lt. Gabriel Laureano told NBC Connecticut.
The Philadelphia area also was bracing for record-setting cold, with a low of 2 degrees expected and a wind-chill advisory in effect for the area outside the Poconos. Trenton's forecast low of 0 degrees would break a record set in 1916 while the expected minus 12 in the Poconos would break a 1970 record, according to NBC Philadelphia.
Further south, nearly the entire metro-D.C. area was under wind-chill advisories, according to NBC Washington. It reported that winds were expected to drive "feels like" temperatures to near or below zero.
Temperatures could warm up "a bit" on Sunday afternoon thanks to a separate storm moving up through the southern part of the U.S., the Weather Channel's Palmer said.
That storm system will shove the arctic air out but also bring a band of snowfall up through Washington, New York and Boston later Monday and into Monday night, Palmer added.