What is the Chicago area’s heaviest 24-hour rainfall total, both officially or unofficially?

Dear Tom,

What is the Chicago area’s heaviest 24-hour rainfall total, both officially or unofficially?

Thanks,
Darren Maiman
Glen Ellyn

Dear Darren,

The heaviest 24-hour rainfall for the Chicago area occurred in Aurora on July 17-18, 1996 when waves of “training” thunderstorms brought 16.91 inches of rain, a record not only for the Chicago area, but also for the entire state of Illinois. Officially, Chicago’s heaviest 24-hour rainfall occurred during the massive flood event of August 13-14, 1987 when 9.35 inches inundated the O’Hare area with flood waters turning the airport into an inaccessible island. In terms of the city’s heaviest calendar-day rainfall, the soggy honor goes to July 23, 2011 which logged 6.86 inches of rain, the bulk of a three-day rain episode that produced 8.59 inches of rain.