CHAMPAIGN — May was much colder and moderately wetter than average across Illinois, bringing an end to a wet spring season, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey.
The preliminary statewide average May temperature was 60.5 degrees, 2.2 degrees below the 30-year normal. Preliminary statewide average total precipitation for May was 5.41 inches, 0.81 inches wetter than the 30-year normal. Temperatures
Following a cool April, temperatures for the first half of May were mostly below average. Most stations in the northern two-thirds of the state observed daily minimum temperatures below freezing on May 8–13. Below freezing temperatures on May 10 spread as far south as Hardin County.
Although May freezes are not uncommon in Illinois, many tender perennials and flowering trees were at a more vulnerable state because of the warm start to this calendar year. A shift in the predominant winds brought warmer and more humid weather for the last two weeks of May.
May average temperatures ranged from the high 50s in northern Illinois to the low 60s in southern Illinois. May was colder than average in all but the very northeastern corner of the state.
Both April and May this year were colder than normal statewide, following three consecutive warmer than normal months to start the year. Although official numbers are not yet available, the 2020 climatological spring was very close to the 30-year normal. Precipitation
May started out dry, with most areas of the state receiving less than an inch of rain in the first two weeks. However, a series of storms moved across the Interstate 55 corridor around the middle of the month, leaving widespread totals of 2 to 4 inches between the St. Louis Metro East and Chicagoland.
These storms generated very heavy rainfall over the Chicagoland area and resulted in over 7 inches of rain in four days over a large area of the city and suburbs. Over 3.5 inches was recorded at Chicago’s O’Hare airport on May 14, the largest single-day May precipitation total ever recorded at that station.
Heavy rainfall across northern and central Illinois led to flash flooding in developed areas and inundated fields and standing water in rural parts of the state. Gauges in both the Illinois and Des Plaines Rivers topped flood stage, including a likely new record crest on the Illinois River at Morris.
Overall, May total precipitation ranged from over 9 inches in northeast Illinois to just under 4 inches in southeast and far western Illinois. Most of the state received between 100 and 150 percent of normal May precipitation, while the Chicagoland area received over 200 percent of normal.
Only southeast Illinois, from Washington to Massac Counties, was drier than normal but still received around 90 percent of normal May precipitation.
May ended a wetter than average climatological spring, with most areas of the state receiving between 100 and 125 percent of normal precipitation. Although Illinois had an even split between drier and wetter than normal springs over the past decade, the wet springs have been very wet.
Although only trace snowfall occurred in May, most areas in northern Illinois experienced more snowfall than average in the spring. Spring snowfall totals ranged from less than an inch in most of central Illinois to over six inches along the Interstate 80 corridor. Outlooks
Short-term, 8- to 14-day outlooks from the NOAA Climate Prediction Center show elevated odds of both above normal precipitation and above normal temperatures across the state.
Longer-term outlooks for climatological summer show continued elevated odds of wetter than normal conditions but equal chances of above normal, normal, and below normal temperatures.