Rain inundated the south-central and northern parts of Illinois in July, causing flooding, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.
Multiple, intense rainfall events ran from the northwest to the southeast along and north of the Interstate 80 corridor and along the Interstate 64 corridor in July. This included incredibly intense rain in the St. Louis metro area on July 26.
Parts of Cahokia Heights and Belleville caught 7 to 8 inches of rain in less than eight hours, causing widespread and—in some places–catastrophic flooding. Rain totals in the Belleville area exceeded the estimated 0.2% annual exceedance, roughly a 1-in-500-year event.
In that same week, parts of Lake and Cook counties in northeast Illinois experienced their own heavy rain event, causing damaging flooding in Lake Bluff. The Chicago National Weather Service office reports that 4 to 6 inches of rain fell in less than 6 hours, which corresponds with a 0.5% annual exceedance probability or a 1-in-200-year event.
Despite the occurrence of flooding, these locations were not the wettest part of the state. Farther east, parts of Clay, Richland and Lawrence counties had between 12 and 14 inches of rain in July, which is 7 to 9 inches above normal.
In contrast, much of central Illinois from Monmouth to Danville remained somewhat to very dry. This area and parts of Alexander and Pulaski counties in far southern Illinois were 1 to 3 inches drier than normal in July. In response, the U.S. Drought Monitor introduced and expanded the severe to moderate drought classifications in east-central and far southern Illinois.
The preliminary statewide average total July precipitation was 5.69 inches, 1.63 inches above the 1991–2020 average.
Temperatures
For the second consecutive year, much of Illinois experienced a cooler or near normal July following a very warm June. However, eastern and southern Illinois were on the warm side.
Overall, July average temperatures ranged from the low 70s in far northern Illinois to around 80 degrees in southern Illinois. July was 1 to 3 degrees cooler than normal for most of northern Illinois and 1 to 3 degrees warmer than normal in southern Illinois.
The preliminary statewide average July temperature was 75.5 degrees, 0.1 degrees above the 1991–2020 average.
Outlooks
The latest outlook from the Climate Prediction Center leans to warmer than normal temperatures throughout the state for August. The August precipitation outlook also leans to a drier than normal month, but with less certainty.
Meanwhile, outlooks for September through November show warmer and drier than normal weather in Illinois.