Chicago experienced a rare, but not unprecedented, flash flood Tuesday night in what some forecasters described as a "one-in-500-year event."

More than a month's worth of rain fell on Chicago in 90 minutes on Tuesday evening, according to the National Weather Service (NWS). "There were numerous reports of flooded viaducts, several reports of water rescues, and a few reports of flooded basements," the NWS said.