Chicago - As a small army of moms and nannies pushing baby strollers descended on Chicago's Lincoln Park to take advantage of a sunny, relatively 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 different way.

"Now we are going to start the flood season," said Don Gutzmer, a mechanical 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 and Northeastern 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 snow may not melt fast enough to keep the city's drainage system open. Street crews raced to clear catch basins of debris.

"Many storm drains may be blocked by deep snow," the advisory said.

Officials in suburban Will County prepared to siphon warm water from a nuclear power plant's cooling pond into the Kankakee River in the hopes of melting ice that can jam the channel and push floodwaters over the banks.

At the same time, emergency management authorities warned people living in low-lying areas to be ready to move to higher ground, even going door to door to be sure families were aware of the danger. And landscaping companies' phones rang off the hook from homeowners worried about flooded basements.

"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 warned 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 seen so much snow that authorities worried about more roof collapses like the one that injured two women Wednesday in the Grand Rapids area, which has received 101 inches. 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. The rain and melted snow could turn to ice when temperatures drop again within days, and the Midwest might see the kind of treacherous conditions that already triggered dozens of crashes and interstate closures in Pennsylvania.

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, 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.

But after enduring so many snowstorms and painfully cold days, the people who emerged Wednesday in Chicago were more focused on the sunshine and warmth that allowed them to do something as simple as go for 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."