
"You're talking about frostbite and hypothermia issues very quickly, like in a matter of minutes, maybe seconds," said Brian Hurley, a meteorologist with the Weather Prediction Center.
Among the universities closed into Thursday are the University of South Dakota, Iowa State University and the University of Minnesota. Minneapolis Public Schools officials also canceled classes through Wednesday, when the region is expected to experience frigidly low temperatures not seen in a quarter century. Hundreds of Michigan schools were closed Tuesday, including in Detroit, while Chicago Public Schools canceled Wednesday classes because of the anticipated cold snap.
Subzero temperatures hit some states Tuesday, but Wednesday is expected to be the worst. Wind chills in northern Illinois could fall to negative 55 degrees (negative 48 degrees Celsius), which the National Weather Service called "possibly life-threatening."
Minnesota temperatures could hit minus 30 degrees (negative 34 degrees Celsius) with a wind chill of negative 60 (negative 51 degrees Celsius).
The potentially record-breaking low-temperature forecast in Milwaukee is negative 28 degrees (negative 33 degrees Celsius), with a wind chill as low as negative 50 (negative 45 degrees Celsius). The current record of minus 26 degrees (negative 32 degrees Celsius) was set in 1996.
One of those polar vortex pieces is responsible for the subzero temperatures across the Midwest this week.
The Chicago Zoological Society said it was closing the Brookfield Zoo on Wednesday and Thursday — marking only the fourth time the zoo has closed during its 85-year history — to ensure the safety of its employees and animals. At O'Hare International Airport in Chicago, the high-temperature Wednesday is expected to be negative 14 degrees (negative 25 degrees Celsius), which would break a record set on Jan. 18, 1994.
Homeless shelters were also preparing for the onslaught of cold. The Milwaukee Rescue Mission's call volume was "unusually high," but officials said there should still be enough beds for those who need them.
In Minneapolis, charitable groups that operate warming places and shelters were expanding hours and capacity "as they do whenever dangerous extreme temperature events occur," said Hennepin County Emergency Management Director Eric Waage. He said ambulance crews were handling all outside response incidents as being potentially life-threatening.
On Monday, snowplow drivers had trouble keeping up with the snowfall in Minnesota and Wisconsin, where some areas got as much as 15 inches (38 centimeters) of snow.
Rare snowfall was also forecast for some southern states.