In an advisory, the NWS warned that portions of the three states are under flash flood warnings due to expected thunderstorms that will bring heavy rain.

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.