What is the largest rainfall that Chicago has ever gotten?

Dear Tom,
What is the largest rainfall that Chicago has ever gotten?
James Archer, Chicago
Dear James,
Chicago’s very heaviest rainfall totals have all occurred as two-day events — that is, events that started on day one and continued through midnight into day two. (Incidentally, those events have all been associated with rain; the water content of the city’s largest snowfall events fall far short of the totals associated with rain.)
In precipitation records dating from 1870, the greatest two-day rainfall occurred on Aug. 13-14, 1987, at O’Hare International Airport: 9.35 inches. It was an event that affected primarily the northwest portion of the Chicago area. Areas from the airport northwestward toward Arlington Heights received the most rainfall. Midway Airport received only 2.48 inches. The second-heaviest rain, 8.41 inches, fell on July 22-23, 2011, also at O’Hare.

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