Average precipitation exceeds 48 inches a year in the south, compared to less than 32 inches in the north. Snowfall distribution is just the opposite, with averages of 36 inches a year in the north and less than 10 inches in the extreme south. Winter snowfall is heaviest in the Chicago area, enhanced by lake-effect snows from Lake Michigan. Even so, Illinois does not rely on the snowpack for significant contributions to water supply.
Variability in precipitation also extends over time. There have been major multi-year droughts in the 1930s and 1950s and major prolonged wet periods during the 1970s and 1980s. May and June are typically the wettest months, and January and February are the driest. Each year, Illinois has rainstorms producing 40 or more flash-floods each with 4 to 8 inches of rainfall in a few hours in localized areas.
Precipitation
Illinois:
Mean annual and seasonal precipitation maps
Mean annual snowfall maps
Monthly precipitation (downloadable file) - select type:
Precipitation table (Text)
Station Inventory (Text)
Precipitation, Temperature and Station Inventory together (Excel)
Rainfall trends in Northeast Illinois
Cook County Precipitation Network:
Annual 17-year average map
Seasonal and Monthly 17-year average maps
Monthly precipitation averages (Text)
Daily data for multiple stations
Latest CCPN Annual Report
Imperial Valley Water Authority (Mason & Tazwell Counties):
Raingage readings: 2001, 2002, 2003, 2004, 2005, 2006, 2007
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Illinois State Water Survey, Contact Information University of Illinois at Urbana-Champaign |
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