The ongoing storms, combined with the heavy rainfall the area experienced Saturday morning, are likely to cause flash flooding, especially in low-lying areas or places with poor drainage, the weather service said.
While rain is expected to stop about 2 a.m., high water may last through sunrise, the weather service said.
An earlier tornado watch was issued for parts of northwestern Illinois, Iowa and Missouri, in effect until midnight, according to the weather service. Illinois counties included in the tornado watch include Carroll, Henderson, Henry, Jo Daviess, Mercer, Rock Island, Stephenson, Warren and Whiteside.
About 11:30 a.m. Saturday, thunderstorms with heavy rainfall moved out of Will and Cook counties areas, leaving behind rainfall amounts of two to four inches and widespread flooding, the weather service said.
Dry conditions were expected to continue until late afternoon, when storms will redevelop and spread through Northern Illinois and Northwest Indiana Saturday night, according to the warning.
Saturday morning the city cancelled the Taste of Chicago because of heavy rainfall and flooding, according to a statement from the City of Chicago Office of Emergency Management. The festival is expected to reopen Sunday at 10 a.m.