CHICAGO — Two of the most dreaded words in a Midwestern weather forecast — “polar vortex” — returned this week, promising life-threatening low temperatures that could shatter records and plunge much of the region into its deepest freeze in decades.

Gov. Tony Evers of Wisconsin declared an emergency and told the National Guard to be ready to help. The University of Notre Dame announced it was closing its northern Indiana campus from Tuesday evening until Thursday afternoon. And in Chicago, city leaders deployed buses as mobile warming centers and offered tips on how to thaw frozen pipes (hair dryers work well, they said, but don’t use an open flame).

“This is right up there with the best of the cold waves, and we’ve had some doozies over the years,” said Tom Skilling, the chief meteorologist at WGN-TV in Chicago, where he has worked for 40 years. Mr. Skilling predicted 72 hours of subzero wind chills and 48 hours of subzero temperatures so low that “we’re going to hear buildings and outdoor objects creaking.”

Forecasters expect Wednesday’s high temperature (yes, the high) to be minus 14 degrees Fahrenheit in Chicago and Minneapolis. If the forecast holds, that would be Chicago’s lowest high temperature for a single day since officials began keeping records. An expected low of minus 22 was expected to approach, though not surpass, the coldest temperature ever recorded in Chicago. Officials predicted that wind chill readings could plummet to minus 50 in Chicago and minus 60 in Minneapolis.

“This is what you would expect when you get into central and northern Canada,” said Brian Hurley, a National Weather Service meteorologist.

The vortex, a brutal mass of cold air within strong bands of circulating winds, has spread southward from its normal location near the North Pole in recent weeks, bringing arctic weather to the middle of the United States. Such weather events have become more common in recent years; scientists are not sure why, but some suspect a link to climate change.

[Read more about how it’s possible that such a cold snap can occur during a period of climate change.]

In Chicago, Mayor Rahm Emanuel made comparisons on Monday to the winter of 2013-14, when the city was plagued by frigid weather and referred to as “Chiberia,” the result of another polar vortex. This year’s vortex could bring even lower temperatures.

“All hands are on deck,” Mr. Emanuel said, adding that he was coordinating with state officials to assist homeless people who live alongside highways.

Across the region, social service agencies and local governments raced to warn older and disabled residents and homeless people about the approaching weather. Officials in South Bend, Ind., promised to open a warming center. In Peoria, Ill., landlords were warned to provide heat to tenants. And in Minnesota, groups mobilized to encourage homeless people to seek shelter.

“The approach to this really bitter cold is for providers to be very, very flexible and open and willing,” said Christine Michels, a senior program director for Catholic Charities of St. Paul and Minneapolis, which operates shelters.

Preparations for the bitter cold were being complicated on Monday by a quick punch of snow that blew over the Great Lakes, leaving several inches in major cities and snarling travel.

In Milwaukee, St. Paul and Minneapolis, public schools called off Monday classes. Gov. Gretchen Whitmer of Michigan sent most state workers home early. And in Chicago, where cars struggled to maneuver on side streets, even some dogs were wearing boots.

“We are getting a lot of snow in very little time,” Mayor Andy Schor of Lansing, Mich., said in a statement on Monday as he declared a snow emergency. “People need to stay off of the streets so that we can clear them properly.”

By late afternoon, as the snow pushed eastward, more than 1,600 flights had been canceled across the country, according to FlightAware, the majority of them bound to or from one of Chicago’s two airports.

Temperatures were expected to drop quickly on Monday night and then worsen. Mr. Hurley said the worst of the polar vortex was expected to extend from Northern Illinois and Wisconsin westward through Minnesota, Iowa and the eastern parts of the Dakotas, settling in late Tuesday and lasting into Thursday.

For many children and young adults — even hearty Midwesterners well acquainted with harsh winters — this week was expected to bring the coldest weather of their lifetimes. Mr. Hurley said temperatures this low had not been recorded in many Midwestern cities since 1994. With that in mind, schools and government agencies announced plans to close as officials urged people to stay indoors and work from home if possible.

Already, the Minneapolis school district has announced closings for Tuesday and Wednesday, and the University of Iowa has called off classes from Tuesday evening until Thursday afternoon. The United States Bankruptcy Court for the Northern District of Illinois canceled its Wednesday docket. And the Adler Planetarium in Chicago announced it would be closed on Wednesday and Thursday.

The polar vortex also prompted emergency preparations and school cancellations in the South, where temperatures were expected to be decidedly less polar, but where residents are less accustomed to dealing with the cold.

In Louisiana, where meteorologists expected one to two inches of snow, the Governor’s Office of Homeland Security and Emergency Preparedness said it had activated its crisis action team. Gov. Kay Ivey of Alabama declared a state of emergency.

Officials appeared the most unnerved in Georgia, which will host the Super Bowl on Sunday. Gov. Brian P. Kemp said state offices would be closed on Tuesday in 35 counties, including some in the Atlanta area, as parts of the state prepared for ice and up to two inches of snow.

But in Chicago, where Wednesday’s high temperature was expected to be 55 degrees lower than Atlanta’s, the warnings were far more dire.

“This isn’t going to be just irritating — this will be literally dangerous,” said Mr. Skilling, the meteorologist. “These terms are thrown around sometimes rather loosely, but this truly does qualify as a dangerous level of chill.”

Alan Blinder contributed reporting from Atlanta.

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