Dear Tom,
What is the greatest precipitation event, either rain or snow, ever to occur in Chicago?

Mali Urban, Chicago

Dear Mali,
In the period of Chicago’s official weather records, which began on November 1, 1870, and has continued to the present time, Chicago’s greatest precipitation event occurred on August 13-14, 1987… about 35 years ago. As recorded at O’Hare International Airport, 9.35 inches of rain fell; 2.86 inches on August 13 and 6.49 inches on the 14th.

The O’Hare area and areas northwestward into Arlington Heights registered the greatest rainfall totals. Considerably lighter rain totals were recorded elsewhere in the Chicago area, although rains were still very substantial. Midway Airport on the southwest part of the area recorded 2.48 inches during the same two-day period. The second heaviest rainfall total in Chicago weather history was 8.41 inches, also recorded at O’Hare. It fell in the two-day period of July 22-23, 2011. The greatest rain total to occur within the metro area was a state-record 16.97″ observed just west of Aurora on July 17-18, 1996, part of an area from near Rockford, through Joliet that was inundated by a foot or more of rain.  All of Chicago’s heaviest precipitation events (water content of precipitation) have occurred as two-day events, starting on day one and continuing through midnight into day two.