1. Cook County’s flash flood-prone exposure
- CoreLogic estimates that Cook County, Illinois is home to more than 1.4 million single-family residences (SFRs). Approximately 26% (~370,000) of which, with a combined reconstruction value of $120.8 billion, are at moderate to extreme risk of flash flooding, according to CoreLogic’s Flash Flood Risk Score (FFRS). Please note that this does not indicate that all 370,000 moderate- to high-risk SFRs flooded during the September 11-12 storm, nor that losses will total $120 billion. Many may not flood during more severe future rainfall events. However, the statistics above highlight that there is a high amount of vulnerable exposure at-risk within the county. If heavy precipitation events and flash flooding become a more common occurrence, the likelihood of catastrophic-level losses could increase.
- Figure 2 depicts the breadth of FFRS across Lincoln Park and Edgewater, neighborhoods that had reported incidents of flooded basements. The FFRS uses the local hydrology, meteorology, and flood-related environmental conditions of the region to determine a relative hazard-based risk score.
- Additionally, CoreLogic estimates that just over 50% of SRFs in Cook County have finished basements, being the predominant basement finishing type in this region (compared to slab or crawlspace).
- Flash flood risk is important to consider in addition to riverine flooding. Between 2005-2018, property damage from flash flooding has totaled $76.5 billion according to the National Weather Service (NWS) and National Climatic Data Center (NCDC). Flash flood losses are often uninsured when it occurs outside the FEMA Special Flood Hazard Areas.
2. Heavy rainfall across northern Illinois produced flash floods in across the Chicago area
- Heavy thunderstorms caused widespread flash flooding across northern Illinois starting Saturday, September 10 and continuing through Sunday, September 11. Over that time, the system moved eastward over the Chicago metropolitan area. Areas just north of Chicago saw short-term rainfall rates of 6 to 8 inches per hour, rates typical of tropical cyclones (Figure 1).
- In early August of this year, Rockford, Illinois received 6.15 inches of rain over two days (August 7-8). On September 11, the single-day rainfall total record in Rockford was broken when 4.34 inches of rain was recorded (previous record was 4.22 inches). September 11 now ranks third in September single-day rainfall totals, and twelfth all-time in single-day rainfall totals for the area.
- The National Weather Service (NWS) enacted flood warnings across northern Illinois on Sunday, September 11. By mid-day flash flood warnings were in place in Chicago and the neighboring cities to the southwest. Localized flash floods were still observed into Monday, September 12.
3. Flooded basements and overwhelmed wastewater systems
- There is a high probability of flash flooding in metropolises like Chicago (other examples include Seoul, South Korea and St. Louis, Missouri earlier this summer) when rainfall rates greatly exceed a city’s stormwater management capabilities.
- Reports of flooded vehicles on low-lying roads and in viaducts were recorded across the Chicago metropolitan area. Some vehicle operators required emergency rescue from local fire departments. Additionally, isolated incidents of flooded basements were reported in Portage Park, Edgewater, Lincoln Square and Albany Park neighborhoods of Chicago., 
- A secondary hazard associated with urban flash flooding is overwhelming municipal stormwater management systems. When this occurs, the enormous volume of flood water is strong enough force wastewater back into the basements of homes and businesses. The City of Chicago had previously installed “Rainblockers” meant to prevent back-flow by restricting the amount of stormwater from entering the system. For the most part this past weekend, the system was successful, but a few incidents of sewage and wastewater back-flow into homes via toilets, dish washers and laundry units were reported. Unfortunately, when the “Rainblockers” are effective, more water pools on streets and sidewalks, infiltrating basements and flooding elsewhere.
 National Weather Service, Chicago, Illinois, September 11, 2022: Heavy Rain and Flash Flooding. (NWS 2022). https://www.weather.gov/lot/2022sep11. Accessed September 12 2022.
 Reuters, Flash floods hit Chicago metro area, stranding cars. (Reuters 2022). https://www.reuters.com/world/us/flash-floods-hit-chicago-metro-area-stranding-cars-2022-09-11/. Accessed September 12 2022.