Impacts of potential future climate change on the expected frequency of extreme rainfall events in Cook, DuPage, Lake, and Will counties in northeastern Illinois

Numerous studies have attempted to identify the implications of climate change with respect to hydrologic extremes (e.g., IPCC, 2007; CCSP, 2008; Milly et al., 2008; USGCRP, 2017). These studies project future climate conditions with more frequent extreme precipitation events in many regions around the world, including parts of the United States. The U.S. Global Change Research Program (2017) indicates that “heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901 (high confidence).” There are important regional differences in trends, with the largest increases occurring in the northeastern United States (USGCRP, 2017), followed by the Midwest (Karl et al., 2009). USGCRP (2017) also states that “mesoscale convective systems (organized clusters of thunderstorms)–the main mechanism for warm season precipitation in the central part of the United States–have increased in occurrence and precipitation amounts since 1979 (medium confidence).” Climate model projections also indicate that northeastern Illinois, including the Chicago metropolitan area, will experience more frequent and more intense rainfall events in the future (Markus et al., 2012, 2016). These increases will lead to more intense and more frequent urban flooding events and to increased human, environmental, and economic risks. Thus, various planning and management measures need to be considered by urban communities responsible for administering ordinances governing the construction and maintenance of stormwater management systems and for floodplain management to address public safety concerns, property damage, and economic interruption from intense precipitation. Future climate projections based on general circulation models (GCM) are typically downscaled to finer temporal and spatial scales using statistical or dynamical downscaling models. However, watershed-scale climate data generated by climate models still do not provide precipitation data in a format useful for community engineers and planners to prepare, mitigate, and adapt to future conditions. In this study, a method is designed to analyze and express climate data in a format that can be readily used to assess future extreme precipitation events in models commonly used for sizing stormwater infrastructure and identifying flooding potential. This report presents a newly designed framework to determine future condition rainfall frequency maps for 24- and 48-hour duration rainfall events and for a range of recurrence intervals (also called return periods). This framework directly supports climate adaptation and mitigation by providing an understandable method for community engineers and planners to demonstrate the impact of climate change at the local level and develop specific adaptation strategies that will reduce future risk.