CHAMPAIGN, Ill. — Illinois temperatures in May varied from periods of below average to periods of far above average, breaking records across the state, and rainfall was slightly below normal in May statewide, according to Illinois State Climatologist Trent Ford at the Illinois State Water Survey at the University of Illinois.
Following a cool April and first week in May, temperatures shot up May 9-15, with temperatures persistently 8 to 20 degrees above normal. The heat was accompanied by summer-like humidity, and dewpoint temperatures regularly reached into the upper 60s and low 70s.
Rockford and Chicago recorded their earliest 70-degree nighttime low temperature on record. Three stations broke their all-time May high temperature records, including a 96-degree high on May 11 in Stockton in Jo Daviess County.
The heat itself was remarkable, but so was how quickly it arrived following prolonged cooler weather in April and in the first week of May.
A rapid change from cool to hot weather, especially early in the warm season, is associated with an elevated risk of heat-related health effects. At least three heat-related deaths were reported in Chicago in the week of May 9.
Temperatures moderated in the third and fourth weeks of May, but the month still ended with temperatures above normal across the state.
May average temperatures ranged from the low 60s in northern Illinois to the high 60s in southern Illinois, between 1 and 5 degrees above normal. The preliminary statewide average May temperature was 64.7 degrees, 1.4 degrees above the 1991-2020 average.
Total May precipitation ranged from just under 2 inches in southeast Illinois to over 7 inches on the north side of the St. Louis Metro East.
The southwest and northeast corners of the state were 1 to 3 inches wetter than normal, while the northwest, southeast and much of central Illinois were 1 to 3 inches drier than normal.
The wettest point in the state in May was Granite City in Madison County, with a total of just under 8 inches of rain. In contrast, Fairfield in Wayne County received just 2.05 inches total in May.
The lack of widespread, heavy rain helped move spring fieldwork along very quickly following serious delays in April.
Meanwhile, most parts of the state received enough rain to keep soil moisture adequate to surplus to stave off drought.
The preliminary statewide average total May precipitation was 4.37 inches, 0.40 inches below the 1991-2020 average.
The latest outlooks from the Climate Prediction Center lean toward a colder than normal start to summer with equal chances of above and below normal precipitation in June.
Meanwhile, outlooks for the entire climatological summer, June through August, continue to lean toward warmer than normal conditions across the entire central United States.
Summer precipitation outlooks are also showing an equal chance of wetter and drier than normal conditions in Illinois, while drier than normal weather is expected farther west in Iowa, Nebraska and Kansas.