Heavy rains that flooded Chicago neighborhoods in the U.S., rendered freeways impassable and wreaked havoc on NASCAR street races downtown on Sunday are serving as stark reminders of urban centers' vulnerability during extreme weather events.

Warmer air over metropolitan areas combined with many square miles of impermeable concrete add up to intensifying storms that generate billions of gallons of run-off rainfall with nowhere to go, stressing cities' sewer systems, experts say. In major cities, extreme weather causes water and debris to flow into homes, businesses, and underground train systems.

After a historic deluge in April in Fort Lauderdale, Florida, residents had to wade through knee- and chest-high water, and navigate the streets on canoes and kayaks. In Texas, heavy rains across the drought-stricken Dallas-Fort Worth area last August led to hundreds of high-water calls.

With projections forecasting more frequent and powerful storms as climate change continues, cities like Chicago will have to look for new ways to mitigate flooding, said Max Berkelhammer, an associate professor in earth and environmental sciences at the University of Illinois-Chicago.

"That's really where the challenges are," Berkelhammer said. "You can build a lot of infrastructure but in a city like Chicago, in a storm like yesterday, you have to find a place for (the run-off) to go."

Sunday's flooding was caused by a storm system that stalled over the northeastern corner of Illinois. Instead of moving east over Lake Michigan, the storm pinwheeled around Chicago, dumping as much as 23 centimeters of rain in some areas over the course of Sunday afternoon.

Ed Staudacher, assistant director of maintenance and operations for the Metropolitan Water Reclamation District of Greater Chicago, said so much run-off flowed into the city's sewer system that it filled one of the city's three reservoirs with almost 5 billion gallons of water.

The Chicago River rose 2 meters during the storm, forcing workers to close a series of locks and reverse the river's flow from west to east into Lake Michigan to prevent more flooding, he said. The mayor issued a statement asking people to refrain from taking showers and washing dishes to prevent even more run-off from filling the sewers.

"I've lived through quite a few storms," Staudacher said. "(But) it's still nerve-wracking because you're watching gauges climb. It's a very stressful day but we have the systems in place to handle this."

Portions of some interstates crossing the city were closed due to flooding. Low-lying streets, especially under overpasses, became ponds too deep to cross with a car. NASCAR cancelled a downtown Xfinity Series race that was slated for Sunday morning - and delayed the start of a Cup Series race set to follow that afternoon.

Berkelhammer, the environmental scientist from the University of Illinois-Chicago, said modeling suggests storms will grow more intense as the climate warms even more in the coming years. That means trouble for many of America's biggest cities.

Metropolitan areas naturally produce more heat, which rises into the atmosphere and can lead to heavier rainfall. Many impervious surfaces in cities - concrete, asphalt, roofs, buildings - prevent rain from seeping into the earth. That all translates to more severe flooding, he said.

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