A deep freeze set in across the Midwest on Sunday with low temperatures forecast in the single digits and a few below zero, turning the season's first major snow into ice that made some roads treacherous to travel.

After snow on Friday and Saturday across much of the region, temperatures plunged behind the front. The National Weather Service forecast 20 degrees or lower across six states from North Dakota to Illinois. Fargo, North Dakota, could dip to 11 degrees by early Sunday. Madison, Wisconsin, could see 7 degrees and Des Moines 12.

Parts of Iowa, northwest Illinois and southern Wisconsin could reach below zero, the weather service said.

Chicago's O'Hare International Airport had recorded 7 inches of snow by midday Saturday, which forced the cancellation of hundreds of flights. But operations were expected to return to normal on Sunday with only a few cancellations.

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In Minneapolis, where dozens of people huddled around fires to stay warm at an encampment outside a police station to protest the fatal shooting of a black man by officers there last week, the temperature was expected to reach 18 degrees early Sunday.

Chicago could see a low of 9 degrees early Sunday and around 20 degrees at noon when the Chicago Bears take on the Denver Broncos in an NFL game at Soldier Field.

The first snowfall of the season brought amounts ranging from a few inches to 20 from South Dakota through Michigan earlier in the weekend.

In the southern Wisconsin town of Janesville, between 10 and 20 inches of snow had fallen by late Saturday afternoon, the weather service said.

Southside True Value Hardware manager Matt Krienke said business had been good in the days leading up to the storm, but that it had become "very, very, very, very slick."

"People who don't need to drive don't need to be out," he said.

Callahan reported from Indianapolis. Associated Press writer Jeff Baenen in Minneapolis contributed to this.