May 2025 Overview
A colder, drier May for Illinois with decreases in streamflows, soil moisture, and water tables.
Temperatures averaged 61.9°F 1.3° below the 1991-2020 normal for May and the 55th coldest on record.
Precipitation statewide in May was 3.37 inches, 1.40 inches below the long-term statewide average and the 45th driest May on record statewide.
Mean streamflow statewide was below the median for the month.
Mean provisional streamflow aggregated statewide was below the long-term median flow for May, about 90% of median (Figure 1). Monthly mean discharge values in May ranged from below normal to normal in northern Illinois, and normal to above normal in southern Illinois.
Water surface levels at the end of May were below the full pool or target level at 6 of 20 reporting reservoirs. At the end of May, Lake Shelbyville was 0.6 feet above the summer target level, Carlyle Lake level was 0.4 feet above the summer target level, and Rend Lake was 4.4 feet above the spillway level.
Lake Michigan’s mean level was below its long-term mean for the month.
Shallow groundwater levels were 2.41 feet below the long-term average at the end of May. Levels averaged 0.40 feet below April and 1.70 feet below last year.
Soil moisture average 0.27 water fraction by volume (wfv) at the end of May, 25% lower than the beginning of the month. Levels declined in all regions but were steepest in northern Illinois. Smaller declines were seen through the top 20 inches while there was little change at 39 and 59 inch depths.
Weather/Climate Information (Trent Ford)
The following description of temperatures, precipitation, severe weather, and drought comes from data compiled by networks that report to the National Oceanic and Atmospheric Administration (NOAA). These data are provisional and may change slightly over time.
May in Illinois was colder and drier than normal statewide.
Temperatures averaged 61.9°F 1.3° below the 1991-2020 normal for May and the 55th coldest on record (Table 1a). May average temperatures ranged from the mid-50s in northern Illinois to the high 60s in southern Illinois (Figure 2). Several stations saw their first 90+ degree temperatures last month, including a daily record breaking 94 degrees at Chicago’s O’Hare Airport on May 15. Meanwhile, the state was largely spared a late spring freeze in May, with only a handful of stations experiencing temperatures below 40 degrees.
Precipitation statewide in May was 3.37 inches, 1.40 inches below the long-term statewide average and the 45th driest May on record statewide (Table 1a). While last month’s precipitation was typically variable across Illinois, May was overall drier in most places. Specifically, total May precipitation ranged from around 2 inches in the Chicagoland area to nearly 8 inches far southern Illinois. Most areas south of Interstate 70 were near to slightly wetter than normal, while most of northern and central Illinois were 1 to 3 inches drier than normal (Figure 3).
Severe Weather: Despite somewhat lackluster May precipitation totals, last month was very active on the severe weather front. The NOAA Storm Prediction Center listed 23 tornado reports, 92 severe wind reports, and 61 severe hail reports in Illinois in May. Among these include an EF-4 tornado in Williamson County, and 2-inch hail in Morgan and Sangamon Counties. Illinois set a new statewide tornado record in 2024, but we are ahead of where we were this time last year on statewide tornadoes, with 105 reports total. Entering June, Illinois was tied with Missouri with the highest number of tornado reports to date in 2025.
Spring: May capped off a very warm climatological spring season. March temperatures were 4 to 8 degrees above normal and April temperatures were 1 to 3 degrees above normal. Despite the milder May temperatures, the spring season was still a top 10 warmest on record in many places in the Midwest, including in St. Louis, Peoria, and Paducah. In recent years, early spring warmth has pushed spring phenology ahead of normal, increasing the risk of frost or freeze damage to tender perennials and horticultural crops. However, our colder winter this year helped extend dormancy a bit longer, resulting in a timely spring phenology and (hopefully) good fruit and berry crops this year. Overall, the spring average temperature in Illinois was 54.5°F, 2.1° above normal and the 11th warmest spring on record statewide (Table 1b).
Spring precipitation ranged from around 6 inches in far northern Illinois to over 20 inches in southern Illinois. The northern half of the state was 1 to 2 inches drier than normal, while southern Illinois was 2 to 8 inches wetter than normal. Last season was the 4th wettest on record in Salem and the 5th wettest spring in Centralia. Overall, though, average spring precipitation in Illinois was 11.45 inches, 0.49 inches below normal, making last season the 52nd wettest spring on record statewide (Table 1b).
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 19 stations across the state which collects hourly weather and soil information.
Winds averaged 6.6 mph in May, 1.5 mph lower than April and 0.6 mph lower than the network’s long-term average. ICN Bondville (Champaign County) had an average monthly wind speed of 10.3 mph, the network’s highest. The station also recorded the month’s highest wind gust, measuring 55.3 mph on May 16.
Temperatures averaged 62.4° across the network, 7.5° higher than April and 0.8° lower than the long-term average. Station highs were in the 80s and 90s with lows in the 30s and 40s. The highest temperature of the moth was 97.1°, recorded at ICN Big Bend (Whiteside County) on May 15. The lowest was 33.0°, reported by ICN DeKalb (DeKalb County) on May 9.
Soil temperatures rose 9 to 10 degrees to averages in the mid-60s. Under bare soils, temperatures ranged from 45.3 to 101.3° at 2 inches and 47.1 to 100.5° at 4 inches. Temperatures under sod ranged from 52.2 to 86.3° at 4 inches and 52.4 to 76.2° at 8 inches.
May was drier for most ICN stations. The network averaged 3.19 inches for the month, 2.94 inches less than April and 1.19 inches below the long-term average. ICN Snicarte (Mason County) had the lowest total of the month with 1.66 inches.
ICN Dixon Springs (Pope County) once again had the month’s highest total, reporting 5.71 inches. The station has received 33.55 inches so far in 2025, 12.17 inches more than normal for the January – May and only 14.63 inches below the annual long-term average.
Soil moisture at 2 inches decreased 25% over May. Levels declined in all regions with the greatest falls at the northern stations where soil moisture was 44% lower at the end of the month. Several northern stations were reporting near record low levels.
Similar trends occurred through the top 20 inches. Moisture levels showed little change at 39 and 59 inches.
Surface Water Information (Bill Saylor)
Provisional monthly mean flows for this month for 26 streamgaging stations located throughout Illinois are shown in Table 2, compared to statistics of past record of monthly mean flows at those stations for the same month. Both recent and long-term data are retrieved from USGS online data services following the end of the month. Years of record values in Table 2 represent the number of past monthly values included in the Table 2 statistics; at some stations, the available record may not be continuous. Additional source data may be available from USGS.
The statewide percent of historical mean flow and percent of historical median flow are calculated by dividing the sum of the average flows this month at stations in Table 2 by the sum of the historical mean and median flows calculated for the month, respectively, at the same stations. This method is intended to weight individual observations proportionately in the aggregate comparison. (The Illinois River and Rock River stations are excluded from the statewide calculation because other rivers listed in Table 2 contribute to their flow.)
Mean provisional flow aggregated statewide, using the available monthly mean data shown this month in Table 2, was below the median value for May (approximately 90 percent of the median) and below the mean for May (approximately 65 percent of the mean). Monthly mean discharge values in May ranged from below normal to normal in northern Illinois, and normal to above normal in southern Illinois.
Water-Supply Lakes and Major Reservoirs. Table 3 lists selected reservoirs in Illinois, their normal pool or target water surface elevation, and data related to observed variations in water surface elevations. Reservoir levels are obtained from a network of cooperating reservoir operators who report water levels each month. Current reservoir levels reported in Table 3 are representative of the end of the reported month and are presented as the difference in feet from seasonal target level or from full pool, as applies. Years of record represent the number of past reports for the same month used to calculate the average of the month-end values presented in Table 3. For some reservoirs, this average does not include additional period of record prior to a substantial change in reservoir operation. Most reservoirs in Table 3 serve as public water supplies, with the exceptions noted in the last column.
Compared to end-of-April water levels at 18 reservoirs for which levels were reported last month and this month, reported end-of-May water levels were lower at 12 reservoirs, higher at 5 reservoirs, and about the same as at the end of April at 1 reservoir. For the 20 reservoirs with measurements reported for the end of May, water levels were below normal target pool or spillway level at 6 reservoirs, above normal target pool or spillway level at 10 reservoirs, and at about the target level or spillway level at 4 reservoirs.
Major Reservoirs. Compared to water levels at the end of April, at the end of May the water level at Lake Shelbyville was 0.1 foot lower, Carlyle Lake was 2.0 feet lower, and Rend Lake was 0.9 feet lower. At the end of May, Lake Shelbyville was 0.6 feet above the summer target level, Carlyle Lake level was 0.4 feet above the summer target level, and Rend Lake was 4.4 feet above the spillway level.
Great Lakes. Current month mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The May 2025 mean level for Lake Michigan was 578.6 feet. The monthly mean level one year ago (May 2024) was 579.3 feet. The long-term average lake level for May is 579.1 feet, based on 1918-2023 data. In this period of record, the lowest mean level for Lake Michigan for May occurred in 1964 at 576.6 feet, and the highest mean level for May occurred in 2020 at 581.9 feet. The month-end level of Lake Michigan was 578.7 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables declined 0.40 feet in May with 19 of the 24 reporting wells decreased over the month.
The declines in water tables followed lower precipitation and soil moisture in May. Most wells were lower at the end of the month with the Fermi Lab well (DuPage County) falling 2.64 feet, month’s highest decline.
However, several wells did rise in May. Most notably the Perry well (Pike County) increased 11.90 feet to an end of month level of 7.80 feet. This was the well’s highest level since 2023.
Wells averaged 1.70 feet below May 2024, 2.57 feet below the 15-year average, and 2.41 feet below the long-term record (Table 4).
Data sources for the IWCS include the following:
CPC - Climate Prediction Center
ISWS - Illinois State Water Survey
MRCC - Midwestern Regional Climate Center
NCEI - National Centers for Environmental Information
NWS – National Weather Service
SPC – Storm Prediction Center
USACE – U.S. Army Corps of Engineers
USDM – US Drought Monitor
USGS – US Geological Survey
WARM – Water and Atmospheric Resources Monitoring Program