Overview
May was dry for most of Illinois.
Temperatures averaged 62.4°F, 0.8° below the 1991-2020 normal for May and the 63rd coldest on record (Figure 1).
Precipitation statewide in May was 2.84 inches, 1.93 inches below the 1991-2020 normal and the 32nd driest May on record statewide (Figure 1).
Soil moisture from 2 to 20 inches declined across Illinois in May. Moisture levels at 2 inches were 35% lower at the end of the month. Declines were observed in all regions with the highest in the north. Similar declines were observed through 20 inches while moisture levels remained high at the 39- and 59-inch depths.
Mean streamflow statewide was above the median for the month.
Mean provisional streamflow aggregated statewide was above the long-term median flow for May, about 125% of median (Figure 1). Monthly mean discharge values in May ranged mostly from below normal to above normal, varying considerably across Illinois (Figure 1).
Water surface levels at the end of May were below the full pool or target level at 6 of 18 reporting reservoirs. At the end of May, Lake Shelbyville was 2.2 feet above the summer target level, Carlyle Lake was 2.0 feet above the summer target level, and Rend Lake was 1.6 feet above the spillway level.
Lake Michigan’s mean level was slightly above its long-term mean for the month.
Shallow groundwater levels were 1.51 feet below the long-term average at the end of May. Levels averaged 1.10 feet below April and 0.12 feet above last year (Figure 1).
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 slightly cooler and much drier than normal statewide.
Temperatures averaged 62.4°F, 0.8° below the 1991-2020 normal for May and the 63rd coldest on record (Table 1a). May average temperatures ranged from the high 50s in northern Illinois to the high 60s in southern Illinois. Most of the state was slightly to somewhat cooler than normal last month, while the northwest corner of the state was around 1 degree warmer than normal (Figure 2). Hot and dry afternoons in the final week of the month broke 4 daily high maximum temperature records and 11 daily high minimum temperature records. Meanwhile, the cooler start to May broke 1 daily low maximum temperature record and 6 daily low minimum temperature records.
Precipitation statewide in May was 2.84 inches, 1.93 inches below the 1991-2020 normal and the 32nd driest May on record statewide (Table 1a). Last month was also the sixth consecutive May that was drier than normal statewide. Total May precipitation ranged from around 1 inch along the Wisconsin border to over 10 inches in south-central Illinois. The Interstate 70 corridor was near to wetter than normal, while the rest of the state was 1 to 4 inches drier than normal (Figure 3).
Spring: May capped off a very warm climatological spring season. The first month of the season was the 4th warmest March on record statewide, followed by the warmest April on record in Illinois. 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 the third warmest spring on record statewide in Illinois. We had the 3rd warmest spring on record in Springfield, Carbondale, and Champaign-Urbana, the 5th warmest in Peoria and Quincy, 6th warmest record in Chicago, the 7th warmest in Rockford and Moline.
May dryness was outweighed by exceptionally wet weather in March and April, resulting in a wetter than normal spring season for the northern two-thirds of the state. Spring total precipitation ranged from around 10 inches in eastern Illinois to nearly 20 inches in south-central Illinois.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 20 stations across the state which collects hourly weather and soil information.
Winds were lower in May, averaging 6.7 mph. Speeds were 2.6 mph lower than April and 0.5 mph below the network’s long-term average. ICN Bondville (Champaign County) continues with the highest monthly average with 10.5 mph for May. The highest recorded wind gust was 52.2 mph, measured at ICN Snicarte (Mason County) on May 18.
Air temperatures rose 4.3°, on average, from April to May average of 63.8°. Temperatures were 0.2° above the long-term average. Station highs were from the mid-80s to low 90s with lows from the 20s to 40s. The month’s highest temperature was 91.3°, recorded at ICN Big Bend (Whiteside County) on May 27. The lowest was 27.2°, measured at ICN St Charles (Kane County) on May 2.
Soil temperatures rose 5 to 6° to averages in the mid-60s. Under sod temperatures were 1 – 2° above the long-term averages while the bare soil temperatures were 0 – 1° below. Under bare soils, temperatures ranged from 39.3 to 101.0° at 2 inches and 37.2 to 91.2° at 4 inches. Temperatures under sod ranged from 49.5 to 89.5° at 4 inches and 52.1 to 80.7° at 8 inches.
The month was drier at most ICN stations. ICN Freeport (Stephenson County) recorded only 0.94 inches for the month, 8.99 inches less than April, 2.86 inches below the station’s long-term average, and the month’s lowest total. Only three stations – ICN Belleville (St Clair County), ICN Brownstown (Fayette County), and ICN Olney (Richland County) – had May totals higher than normal. The highest total for the month was 6.50 inches, recorded at Olney.
Soil moisture declined across the network in the top 20 inches. Levels at 2inches fell 35% to a network average of 0.22 water fraction by volume on May 31. The northern stations had the largest drops with soils ending the month near wilting point. Southern stations experienced a lower but still significant decline, ending the month 16% lower.
The declines were seen through the first 20 inches in all regions. Soil moisture remained high at the 39- and 59-inch depths.
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 above the median value for May (approximately 125 percent of the median) and below the mean for May (approximately 90 percent of the mean). Monthly mean discharge values in May ranged mostly from below normal to above normal, varying considerably across Illinois. The monthly mean streamflow of the Little Wabash River below Clay City was much above normal.
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 9 reservoirs, higher at 5 reservoirs, and about the same as at the end of April at 4 reservoirs. For the 18 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 11 reservoirs, and at about the target level or spillway level at 1 reservoir.
Major Reservoirs. Compared to water levels at the end of April, at the end of May the water level at Rend Lake was 0.1 foot higher, Lake Shelbyville was 1.3 feet higher, and Carlyle Lake was at about the same level as at the end of April. At the end of May, Lake Shelbyville was 2.2 feet above the summer target level, Carlyle Lake was 2.0 feet above the summer target level, and Rend Lake was 1.6 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 2026 mean level for Lake Michigan was 579.2 feet. The monthly mean level one year ago (May 2025) was 578.6 feet. The long-term average lake level for May is 579.1 feet, based on 1918-2025 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 579.3 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Drier weather and decreasing soil moisture led to lower water tables in the state. Twenty-two of the 24 reporting wells were lower at the end of May.
Most declines were less than 2 feet. However, the Fermi Lab (DuPage County) well fell 3.80 feet, the month’s highest decline. The area had a dry month with weather stations in nearby Aurora reporting rainfall three inches or more below normal for the month.
Despite the decreases, most wells reported totals higher than one year ago. The Fermi Lab well was 0.80 feet higher than May 2025. The Springfield (Sangamon County) well ended the month 2.52 feet lower than April and 1.03 feet higher than May 2026.
Wells averaged 0.12 feet above May 2025, 1.71 feet below the 15-year average, and 1.51 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