Extreme rainfall events in Chicago, including two observed this summer, are becoming more common as the climate warms.

Very small, localized thunderstorms brought intense rains to parts of Chicago on both July 8 and July 25. While data analysis of the more recent July rainfall event is ongoing, National Weather Service Senior Service Hydrologist Scott Lincoln tells CBS News Chicago that the 5 inches of rain that fell in less than three hours July 8 only has a 0.2% chance of happening in a given year. 

This would once have been referred to as a 1-in-500 year rainfall event, but the NWS now avoids that phrasing since there is no guarantee it will be another 500 years before it happens again.

Late in the evening on July 8, a very localized, torrential thunderstorm sat over portions of central Cook County west of the Loop on the Eisenhower Expressway. Parts of the Near West Side, North Lawndale, the United Center, West Garfield Park and East Garfield Park experienced extreme rainfall, with one unofficial rain gauge tallying 5.39 inches in just two hours.

"Comparing this event to our record of extreme rainfall events impacting the Chicago area since 1950 indicates that this is among the smallest recorded extreme rainfall events over that period, covering only an approximately three square mile area," Lincoln said.

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. The heaviest rain events in the Great Lakes region have gotten 45% heavier since 1958, according to Climate Central. This is impacting Chicago's water infrastructure and flooding basements more often.

"As of July 8, 2025, the 2020s decade has had four extreme rainfall events in central Cook County," Lincoln said. "Compared to the the 1950s, 2000s, and 2010s which each had two known extreme rainfall events within that same area." 

Lincoln noted rain gauge observations were not as widespread prior to the 1990s, potentially affecting historical data collection.

"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)." 

More detailed studies of rainfall frequency indicate a heavy rain event that has a 1% chance of occurring in a given year — once called a 1-in-100 year event — has gotten heavier. A NOAA publication indicated a 100-year event was 5.6 inches of rain in the late 1950s. An Illinois State Water Survey publication shows it has increased to 8.6 inches as of the late 2010s.