An Analysis of Extreme Rainfall Events in Chicago and Vicinity Since 1950

Flash flooding has long been one of the most impactful and costly weather hazards impacting the Chicago metropolitan area. One of the most expensive weather-related disasters in Cook County history occurred on 2 July 2023. This event was part of a cluster which occurred over a 1- year period from fall 2022 through fall 2023. These events brought increased attention to the threat of flooding in the immediate Chicago area. To provide proper climatological context for such events, a detailed study of extreme rainfall was conducted for the 1950-2023 period. All known records of extreme rainfall events were collected and summarized, including rain gauge observations, flood damage, surface weather observations and radar data. A threshold was set to determine what should be classified as an extreme event - at least 7.5 inches for peak storm total rainfall, or sub-daily rainfall intensities exceeding the 1% annual exceedance probability according to NOAA Atlas 14. Daily rainfall records for the official Chicago climate site were also analyzed for trends. Research indicates that at least 13 extreme rainfall events impacted Chicago and the immediate vicinity since 1950. The character of these events varies, with some events occurring over a one- to two-day period, and some events occurring over just a few hours. Weather patterns associated with extreme rainfall events in the study area include a wide range of situations. Flood impacts and damage costs also vary widely between events. Over the entire study period, the frequency of extreme rainfall events is approximately 1-2 per decade, but the frequency of events has increased, and the interval of time between events has decreased, in recent years. The distribution of 1-day rain events in the Chicago climate record also shows significant increases in recent decades when compared to the late 1800s and early 1900s, especially with higher rainfall values. These trends in extreme rainfall are generally consistent with other research and have potentially significant implications for flash flood impacts and stormwater infrastructure design in the Chicago area.