A mass of swirling, frigid air sent temperatures plunging across the Midwest on Monday and was headed for the Washington area, where forecasters say thermometers could drop overnight by as much as 40 degrees.

The windchill temperature in Duluth, Minn., was 56 degrees below zero. Far to the south, in Huntsville, Ala., the thermometer read 14 degrees.

Schools in Chicago and throughout Minnesota — jurisdictions that pride themselves on hardiness and great outerwear — were closed for the day. The temperatures were so low — 23 degrees below zero in Minneapolis, 12 below zero in Chicago, 9 below zero in Indianapolis — that officials shuttered local parks and warned that exposed flesh could become frostbitten, tire seals might leak and rock salt would do little, if anything, to melt ice on streets and sidewalks.

Traffic was very light on Minneapolis roads and highways. But the flow of pedestrians was brisk inside the eight miles of climate-controlled skyways that connect the high-rise buildings of the city’s downtown core.

“If you’ve lived here all your life, you grin and bear it — or you take a vacation,” said Keith Rolfzen of south Minneapolis, who does bulk newspaper delivery Thursdays, Fridays and Saturdays. “On days like this, we Minneapolitans are very appreciative of our skyways.”

Residents around Minneapolis do not let the record cold temperatures, brought on by a mass of frigid air known as a polar vortex, discourage them from venturing outdoors. (Star Tribune)

Kyle Jensen, a trust operations specialist for Wells Fargo, said his two children wanted to test the cold Monday morning, but his wife was keeping them inside. Jensen said he was headed to work through the skyway system for one simple reason: “Bank’s open today.”

The National Weather Service in the Twin Cities tweeted Sunday that forecasters for the first time were using the designation “particularly dangerous designation” to denote a windchill warning. The phrase is more commonly used to describe severe tornados or hurricanes.

Jason Samenow, chief meteorologist for The Washington Post’s Capital Weather Gang blog, said the “polar vortex” responsible for the mass of cold air will bring some of the lowest temperatures in decades. The NWS was forecasting that by Tuesday morning, about 1.5 million square miles — or half of the lower 48 states, including Little Rock, Atlanta, Richmond and the District — will have windchills below zero.

At Chicago’s O’Hare International Airport, 1,200 flights — about half the total traffic — were canceled as of late Sunday afternoon because of a major snowstorm, according to the flight tracker FlightAware. Airports in St. Louis and Indianapolis also saw about half their scheduled departures and landings canceled late Sunday afternoon, the same site showed. There were many additional delays and cancellations reported Monday.

Around the Washington region, activity focused on bracing for the cold, with governments publicizing hypothermia hotline numbers, including the District’s (202-399-7093). A windchill advisory was issued for the entire area through Tuesday evening, starting at 6 p.m. in the western suburbs and midnight elsewhere, the Capital Weather Gang reported at 6:45 a.m.

Although the thermometers read about 40 degrees early Monday, meteorologists said temperatures would drop to zero or below zero overnight. Forecast windchills are from 5 degrees below zero to 15 degrees below zero, with lower readings possible in colder areas, such as Loudoun and Frederick counties.

“The severe cold is just another way of Mother Nature reminding us we’re not in charge,” said Jeff Platenberg, assistant superintendent for facilities in the Fairfax County school system, where school engineers and maintenance workers have been checking on heating units and taking precautions to keep buildings warm. “With these historically freezing lows, the best thing we can do is remind parents to make sure children are dressed warmly because they will be standing out at a bus stop.”

Platenberg said it’s possible that schools could see delays or a cancellation Tuesday, especially if there are widespread power outages. Loudoun County cancelled classes Monday, and Frederick County delayed the start of school for two hours.

“In a county like Fairfax, there’s such a broad and diverse amount of terrain,” said Platenberg, who gets in his car at 3 a.m. to check road conditions when inclement weather is expected. “You’ve got hills and dales that can be pretty treacherous, including low spots near water.”

Fairfax school board member Ryan McElveen (At Large) said the extreme cold weather can wreak havoc on the school system in unexpected ways. “It’s not just the roads and the facilities,” McElveen said. “It’s whether our buses can start and whether there are freezing pipes and things like that.”

In the District, 46,000 students in the traditional public school system are slated to return to class Tuesday from winter break. Officials said they are checking school buildings to make sure heating systems are running and will consider closures, if necessary. “We are closely monitoring the situation,” spokeswoman Melissa Salmanowitz said.

Commuters were urged to make several last-minute checks of their vehicles, including the level of antifreeze in their car radiators.

Minnesota’s preparations qualified as unusual, although not unprecedented, in a state whose residents famously take winter’s cold and snow in stride. The last time a governor ordered the schools closed across the state was 1997.

“I’ve lived here for six years, and I’ve never heard of the cold closing anything,” said Kim Glynn, a doctor who was at the Mall of America in Bloomington with her husband and two children late Sunday morning.

Flaherty reported from Washington. T. Rees Shapiro and Emma Brown contributed to this report from Washington.