As of 12:30 p.m., heavy rain had already started to fall in some parts. A flash flood warning was in effect for DuPage and Cook County, the NWS said, with reports of "totally submerged viaducts and underpasses in the city of Chicago."

The National Weather Service issued a flash flood warning for Chicago, urging 600,000 residents to take precautions. More rain is expected overnight, with storms anticipated Saturday afternoon. Residents were seen wading through knee-deep water amid the severe weather disruptions.

A flood watch is in effect as a threat of severe storms is expected by the afternoon for the Chicago area. Torrential downpours could bring rain rates of one to two inches per hour. Some cells may follow one another,