But the warmup was expected to take a jarring jump in the coming hours and days — sending the battered region into a temperature roller coaster. The thaw gave cities and towns a chance to begin assessing deaths, injuries and damage to infrastructure that the deep freeze had caused. But the fast-rising temperatures were also causing a new set of risks to contend with, including the potential for flooding, abundant potholes and clogged stormwater drainage systems.
In Wilmington, Illinois, along the Kankakee River, the Police Department warned of flash flooding “that may occur without any warning” as ice moved downstream. In Niagara Falls, New York, crews raced to remove a large tree blocking a creek, a potential flood risk during the coming thaw. And at Oakland University in suburban Detroit, a library was closed Friday because of a water main break that flooded the building.
The difference in temperature that the Chicago region could experience by Monday: 73 degrees, from Thursday morning, when the city saw a low of minus 21 degrees, to Monday, when it may be 52.
“It’s fairly rare to see this much of a turnaround in temperature in this short of time,” said Todd Kluber, a meteorologist with the National Weather Service.
Chicagoans will go from a deep freeze Wednesday and Thursday to a mild warmup Friday, as air from the Pacific begins to move into the region. Then there is more weather whiplash: on Saturday, fog and freezing drizzle. On Sunday, more drizzle. On Monday, rain.
In Illinois river cities, officials eyed the warmer forecast nervously. Deputy Chief Don J. Gasparini Jr. of the Winnebago County Sheriff’s Office said he had been monitoring the Rock River in northern Illinois for the possibility of ice jams and floods.
“With the ice, it’s very unpredictable,” he said. “Because we don’t know when it’s going to jam up and how long the jam will last or how big it will be.”
Gasparini said the rain predicted by meteorologists would also be unwelcome, with the ground frozen and snow piles abundant. “The rain is not going to have any place to go,” he said.
In Newaygo County, Michigan, where a 21-mile ice jam on the Muskegon River had flooded yards and closed a bridge, the warm-up was welcome. Abby Watkins, the director of the county emergency service agency, said she expected the thaw to help clear out the ice and lower water levels. But it was not all good news: With giant ice hunks moving down the river, she said there was the possibility of more localized flooding.
“That ice then can still clog as it’s trying to move downstream,” Watkins said. “It’s kind of like playing a game of ‘Tetris.’ These large chunks of ice are trying to find a path.”
The cold air mass that has now moved east is uncommon, only seen about every five or 10 years, Kluber said. The Midwest’s deep freeze was brought by the polar vortex, a mass of cold air that is normally contained above the North Pole but in recent weeks broke apart, sending a block of icy air toward the United States.
The cold spell caused significant destruction. The deaths of at least 25 people were being linked to the weather system. Among the deceased: A 72-year-old woman found unresponsive in her garage Friday morning in Germantown Hills, Illinois.
Dr. Stathis Poulakidas, a burn surgeon at Chicago’s John H. Stroger Jr. Hospital of Cook County, said nearly 50 patients had sought treatment for frostbite in recent days, about half of them homeless.
Poulakidas said he expected about half those patients to require amputations, ranging from a single finger or toe to a whole arm or foot. Even with temperatures starting to increase, he said the full consequences of the polar vortex would not be known for months.
“When you get frostbite,” he said, “you don’t stop getting frostbite. It kind of evolves over a period of days to weeks to months.”
Other damage to infrastructure was just becoming clear.
In Escanaba, in Michigan’s Upper Peninsula, a water main had broken during the worst of the cold, sending water spewing onto a road, cutting water pressure in the city, and forcing workers to try to contain it behind a berm of snow in subzero temperatures. They had to chop through ice to find shut-off valves.
“It was bubbling out of the ground at a 2-foot height,” said Wendy Taavola, the city’s assistant public works director. “There was a pretty good size lake on one of our avenues.”
Still, by Friday, many cities appeared back to normal. In Chicago, people bustled along downtown sidewalks, and restaurants and shops were open.
“It’s back to the status quo,” said Angie Day, 34, a fundraiser for a nonprofit, as she climbed the stairs to a post office in the Uptown neighborhood Friday morning. She had spent the last two days working from home, but Friday was heading into the office.
The streets in the neighborhood were salted, the sidewalks spotlessly cleared of ice and snow. It was sort of like the whole thing hadn’t happened.
“I think Chicago does a really good job of dealing with cold,” Day said. “We just put on some good winter gear and keep going.”
This article originally appeared in The New York Times.