The torrential rainfall that pounded parts of Chicago on July 25 was so extreme, it typically occurs only once every 1,000 years, according to the National Weather Service.

Midday July 25, a corridor of extreme rainfall developed along a stalled atmospheric boundary on the south side of Chicago. As much as 5.61 inches of rain fell in Garfield Ridge in just two hours. 

Extreme rain was also observed in Bedford Park near the border with Clearing, where an unofficial gauge measured 5.98 inches in three hours. 

Such extreme rainfall rates only have a 0.1% chance of occurring in a given year, according to the NWS. This would have once been referred to as a 1-in-1,000 year rainfall event, but the NWS now avoids that phrasing since there is no guarantee it will be another 1,000 years before it happens again.

Other communities from far eastern DuPage County eastward through central Cook County recorded 3 to 5 inches of rain in three hours, including Darien, Burr Ridge, Bridgeview, West Lawn and West Englewood. Rainfall of this magnitude is only statistically expected in these areas once every 100 years.

This extremely rare rain event comes on the heels of the July 8 storm that dumped 5 inches of rain in less than three hours on a different part of central Chicago -- an event only expected once every 500 years, according to the NWS.

"As of July 25, 2025, the 2020s decade has had five extreme rainfall events in central Cook County," National Weather Service Senior Service Staff Hydrologist Scott Lincoln said. This is a marked increase compared to two extreme rainfall events in both the 2010s and 2000s, and one extreme rainfall event in each the 1990s, 1980s, 1970s and 1960s.

As global climate change warms Chicago area temperatures, it is leading to heavier rainfall events. For every one degree of warming, the atmosphere holds 4% more moisture.

"Multiple lines of evidence suggest that the threshold for 'extreme rainfall' in the Chicago area is changing," Lincoln said. "Analysis of daily rainfall in Chicago indicated that 'extreme' 1-day rainfall early in Chicago's history (1871-1930) was approximately 6 inches, while this value increased to just over 8 inches in more recent times (1961-2020)."