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.”

[Read some cold hard facts about the polar vortex: You could get frostbite in five minutes, and Des Moines will be colder than parts of Antarctica.]