CHICAGO (AP) -- As dozens of moms and nannies pushing strollers descended on Chicago's Lincoln Park to enjoy a rare sunny, mild day, shrinking mounds of snow and growing puddles signaled that one of the cruelest winters in memory is about to get miserable in a whole different way.
"Now we are going to start the flood season," said Don Gutzmer, a contractor who was at the park's zoo for a meeting. "If all the snow melts at once, the ground can't absorb it."
Weeks of subfreezing weather are giving way, at least briefly, to temperatures in the 40s and 50s, putting many Midwestern cities on guard for flooding, roof collapses and clogged storm drains. Some areas expected a double whammy: warm, springlike air combined with heavy rains that could compound the problem and turn the big melt into a muddy, damaging mess.
In Chicago, the National Weather Service issued an advisory Wednesday warning that ice and deep snow could clog the city's drainage system. Street crews raced to clear catch basins of debris.
Officials in suburban Will County prepared to siphon warm water from a nuclear power plant's cooling pond into the Kankakee River in hopes of melting ice that can jam the channel and push floodwaters over the banks.
At the same time, emergency management authorities warned people in low-lying areas to be ready to move to higher ground, even going door-to-door to ensure families were aware of the danger. And landscaping companies' phones rang off the hook with calls from homeowners seeking teams to scoop snow onto dump trucks and haul it away before their basements flood.
"They're calling me to say, 'With this rain coming, where is that water and the snow going to go when it melts?'" said Jodey Schmiedekamp of Countryside Industries in suburban Chicago.
In Indiana, the weather service cautioned that melting snow piled as high as 18 inches will not be able to flow normally into rivers and streams because those channels are frozen. Between the snowmelt and the rain, some flooding would be unavoidable.
"A lot of bad things could happen tomorrow," said Marc Dahmer, a weather service meteorologist in Indianapolis.
Parts of Michigan have gotten so much snow that authorities fret about more roof collapses like the one that injured two women Wednesday in the Grand Rapids area, which has received 101 inches this season. Other collapses have been reported around the state since January.
If rain adds weight to the snowpack, it "can exacerbate the situation that's there," said John Maples, a weather service meteorologist in Grand Rapids.
The respite won't last long. Temperatures were expected to drop below freezing again within days, threatening to turn all that water back into ice and creating treacherous driving conditions.
While it lasts, the thaw may also reveal a struggle for survival that has played out all winter close to the frozen ground. As the ice and snow recedes around rivers, lakes and ponds, it could reveal dead fish, turtles, frogs, toads and crayfish that didn't make it.
"Winterkill begins with distressed fish gasping for air at holes in the ice and often ends with large numbers of dead fish that bloat as the water warms in early spring," said Gary Whelan, of the Michigan Department of Natural Resources.
After enduring so many snowstorms and painfully cold days, the people who emerged Wednesday in Chicago were delighted by sunshine and warmth that let them indulge in the simple pleasure of a walk or a jog.
"I should be in my office doing something, but I haven't been out there in three to four months," said Ning Du, 40, as she returned from a run along Lake Michigan in Chicago.
A block away, Caroline Vickrey and her friend Michelle Hoppe Villegas couldn't get past the change in people that seemed to reflect the change in the weather.
"Everybody is smiling and saying hello to each other," Vickrey said.
"My daughter was cheerful this morning (and) so pleasant," added Hoppe Villegas. "I was wondering what is going on here."
For many, the break from the cold arrived just in time to ease mounting cabin fever. But the weather service, which usually sticks to unemotional descriptions of high pressure systems and dew points, dashed hopes of a longer reprieve with an uncharacteristically punchy forecast.
"For anyone who may catch a touch of spring fever with the relative warmth of Thursday ... forgetaboutit," wrote meteorologist Gino Izzi. He predicted that the possible return of snow next week "is likely to leave many winter-weary souls ready to curl up into the fetal position and beg for mercy from old man winter!"
Associated Press writers Jeff Karoub in Detroit, Charles Wilson in Indianapolis and Ron Todt in Philadelphia contributed to this report.
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