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."

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