May 2024 Overview
May was warmer than normal for Illinois. The mean streamflow was above the median for the month. Shallow groundwater levels were slightly below the long-term average.
Temperatures averaged 67.1°F, 3.9° above the 1991-2020 normal for May and the 13th warmest May on record statewide (Figure 1). May average temperatures ranged from the high 50s in northern Illinois to the low 70s in southern Illinois, between 2 and 4 degrees above normal
Precipitation statewide in May was 4.45 inches, 0.32 inches below the long-term statewide average and the 51st wettest May on record statewide (Table 1a). May precipitation was variable across Illinois, with extremely wet conditions in far southern Illinois and near to slightly drier than normal conditions in central and northern Illinois.
Soil moisture at 2” averaged 0.31 water fraction by volume (wfv) at the end of May, 20% lower than the beginning of the month. Declines continued through 20” but moisture levels remained steady at 39” and greater.
The mean provisional streamflow aggregated statewide was above the long-term median flow for May, about 165% of median (Figure 1). Monthly mean discharge values in May ranged mostly from normal to above normal for the month.
Water surface levels at the end of May were below the full pool or target level at 3 of 20 reporting reservoirs. At the end of May, Lake Shelbyville was 2.2 feet above the June 1 target level, Carlyle Lake level was 0.9 feet above the June 1 target level, and Rend Lake was 3.2 feet above the spillway level.
Lake Michigan’s mean level was slightly above its long-term mean for the month.
Shallow groundwater levels were 0.75’ below the long-term average at the end of May (Figure 1). Levels averaged 0.41’ below April and 0.22’ above last year.
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 much warmer and slightly drier than average statewide.
Temperatures averaged 67.1°F 3.9° above the 1991-2020 normal for May and the 13th warmest on record (Table 1a). May average temperatures ranged from the high 50s in northern Illinois to the low 70s in southern Illinois, between 2 and 4 degrees above normal (Figure 2). Several stations saw their first 90-degree temperatures last month, including 91 degrees in Monmouth and at Chicago’s Midway Airport. Meanwhile, the state was largely spared a late spring freeze in May, with only a handful of stations experiencing temperatures below 40 degrees. Overall, the warmest place in the state last month was Du Quoin, with an average temperature of 72.5 degrees, and the coldest place in the state was Mundelein with an average temperature of 59.2 degrees. Last month was the 7th warmest May on record in the St. Louis area and in Quincy, the 10th warmest May on record in Carbondale, and the 11th warmest May on record in Peoria.
Precipitation statewide in May was 4.45 inches, 0.32 inches below the long-term statewide average and the 51st wettest May on record statewide (Table 1a). May precipitation was variable across Illinois, with extremely wet conditions in far southern Illinois and near to slightly drier than normal conditions in central and northern Illinois. Specifically, total May precipitation ranged from around 3.5 inches in west-central Illinois to nearly 10 inches in parts of southeast Illinois. Most areas south of Interstate 64 were 1 to 4 inches wetter than normal last month, while areas farther north were near 1-2 inches drier than normal (Figure 3).
Severe Weather: Unfortunately, much of last month’s precipitation came from thunderstorms that also brought severe weather to Illinois. The NOAA Storm Prediction Center listed 24 tornado reports, 153 severe wind reports, and 56 severe hail reports in Illinois in May. Among these include an EF-3 tornado that tore through parts of Jackson and Williamson Counties, causing significant damage around the Lake of Egypt area.
Spring: May wrapped up a very warm climatological spring season (March – May). Each of the three spring months had average temperatures between 2 and 6 degrees above average. The spring 2024 statewide average temperature was 56.2 degrees, 3.8 degrees above normal, and the 3rd warmest spring on record statewide (Table 1b). It was a top-five warmest spring on record throughout virtually the entire state, including the 3rd warmest on record in Chicago and the 2nd warmest on record in Peoria.
The statewide average temperature in Illinois from January to May in 2024 was the 2nd highest on record, only eclipsed by the incredibly warm start to 2012. Six of the top ten warmest January-May periods on record in Illinois have occurred since 2000, and all but one of the top ten have occurred since 1990, illustrating the large impact of climate change on winter and spring temperatures in Illinois.
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 6.2 mph in May, 1.0 mph lower than the month’s long-term average. ICN Monmouth (Warren County) had the windiest month with an average of 10.9 mph. The highest measured wind gust was 58.5 mph, recorded at ICN Belleville (St Clair County) on May 26.
Temperatures rose to an average of 67°, 11° warmer than April and 3° higher than the long-term average. Station highs were in the high 80s and low 90s with lows in the 40s and 50s. The highest temperature was 92°, recorded at ICN Dekalb (DeKalb County) on May 21. ICN St Charles (Kane County) had the month’s lowest temperature, measuring 42° on May 12.
Soil temperatures rose 10 – 15° to highs in the high 60s to low 70s. Under bare soils, temperatures at 2 inches ranged from 45 to 101° and from 5132 to 93° at 4 inches. Temperature under sod ranged from 57 to 88° at 4 inches and 56 to 82° at 8 inches.
Precipitation averaged 5.05” for May, 0.66” lower than April and 0.67” higher than the long-term average. The highest totals were in southern Illinois where ICN Carbondale (Jackson County) and ICN Dixon Springs (Pope County) recorded 8.72 and 11.80”, respectively. Most other stations had May totals 80 – 130% of their long-term averages.
Soil moisture declined overall in May as most stations dried out from a wet April. Moisture levels at 2” were 20% lower, on average, at the end of the month. Declines were seen through 20” while soil moisture remained steady and high at depths of 39” and greater.
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 records 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 165 percent of the median) and above the mean for May (approximately 120 percent of the mean). Monthly mean discharge values in May ranged mostly from normal to above normal for the month.
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 the additional period of record before 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 20 reservoirs for which levels were reported last month and this month, reported end-of-May water levels were lower at 10 reservoirs, higher at 7 reservoirs, and about the same as at the end of April at 3 reservoirs. For the 20 reservoirs with measurements reported for the end of May, water levels were below normal target pool or spillway level at 3 reservoirs, above normal target pool or spillway level at 13 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 2.0 feet higher, Carlyle Lake was 1.4 feet lower, and Rend Lake was 0.9 feet higher. At the end of May, Lake Shelbyville was 2.2 feet above the June 1 target level, Carlyle Lake level was 0.9 feet above the June 1 target level, and Rend Lake was 3.2 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 2024 mean level for Lake Michigan was 579.3 feet. The monthly mean level one year ago (May 2023) was 579.7 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 579.5 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District
Groundwater Information (Jennie Atkins)
Water tables declined 0.41’, on average, in May.
Thirteen out of 25 monitored wells ended the month lower than in April. Most declines were 2’ or less. However, 2 wells – Fermi (DuPage County) and Springfield (Sangamon County) – had declines of more than 3’.
Twelve wells showed increases for the month. The highest occurred at the Sparta (Randolph County) well which rose 3.16’ in May. The area received more than 7” of rain during the month.
Wells averaged 0.22’ above May 2023, 1.16’ below the 15-year average, and 0.75’ below the long-term record.
Data sources for the IWCS include the following:
CPC - Climate Prediction Center, https://www.cpc.ncep.noaa.gov/index.php
ISWS - Illinois State Water Survey, https://www.isws.illinois.edu
MRCC - Midwestern Regional Climate Center, https://mrcc.purdue.edu/
NCEI - National Centers for Environmental Information, https://www.ncei.noaa.gov
NWS - National Weather Service, https://www.nws.noaa.gov
SPC - Storm Prediction Center, https://www.spc.noaa.gov
USACE - U.S. Army Corps of Engineers, https://www.lre.usace.army.mil
USDM - U.S. Drought Monitor, https://droughtmonitor.unl.edu
USGS - U.S. Geological Survey, https://waterdata.usgs.gov/il/nwis
WARM - Water and Atmospheric Resources Monitoring Program, https://warm.isws.illinois.edu/warm/