The current record for the heaviest 24-hour rainfall total in Illinois is 16.94 inches at the NWS cooperative observer gauge in Aurora on July 17-18, 1996. An entire Water Survey report was written on the storm that produced this record.
The previous record 24-hour rainfall was 16.54 inches set on June 14-15, 1957, near Millstadt in St. Clair County (near East St. Louis), according to a Water Survey report. While the 1996 event occurred over 24 hours, the 1957 storm lasted only 12 hours – essentially producing the same amount of rain in less than half the time.
But here is the next interesting fact. Before the 1957 storm, the previous record 24-hour rainfall was 10.48 inches on October 1, 1954 set at Aurora, IL.
So Aurora held the statewide record 24-hour rainfall for about three years until it lost out in 1957 only to regained the crown in 1996, where the record remains to this day. I’m not sure if they deserve the golden rain gauge or the golden sump pump award for this honor.