CHICAGO (NYTIMES) - 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.

Govenor 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 (Jan 29) evening until Thursday (Jan 31) afternoon.

And in Chicago, city leaders deployed buses as mobile warming centers and offered tips on how to thaw frozen pipes.

"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.

Skilling predicted 72 hours of sub-zero wind chills and 48 hours of sub-zero 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 -25.5 deg C 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.

Officials predicted that wind chill readings could plummet to -45.5 deg C in Chicago and -51 deg C in Minneapolis.

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, Indiana, promised to open a warming centre. In Peoria, Illinois, landlords were warned to provide heat to tenants.

And in Minnesota, groups mobilised to encourage homeless people to seek shelter.

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.

The polar vortex also prompted emergency preparations and school cancellations in the South.

Officials appeared the most unnerved in Georgia, which will host the Super Bowl on Sunday. Govenor Brian P. Kemp said state offices would be closed on Tuesday (Jan 29) in 35 counties, including some in the Atlanta area, as parts of the state prepared for ice and up to 5cm of snow.