Heavy rains that flooded Chicago neighborhoods, rendered freeways impassable and wreaked havoc on NASCAR street races downtown 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.

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       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.