May 2023 Overview
May temperatures were above and precipitation was below the long-term average in Illinois. Mean streamflow statewide was below the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures averaged 64.2 degrees in May, 0.9 degrees above normal (Figure 1). May average temperatures ranged from the high 50s in northern Illinois to the high 60s in southern Illinois, between 1 and 3 degrees above normal.
Precipitation statewide in May was 2.40 inches, 2.37 inches below normal (Figure 1). May total precipitation ranged from nearly 10 inches in isolated parts of central Illinois to less than 1 inch in northeastern Illinois.
Soil moisture at 2 inches declined 32 percent to an average of 0.20 water fraction by volume (wfv). Similar declines were seen at 4 and 8 inches, while levels remained high at depths of 39 inches and greater.
Mean provisional streamflow aggregated statewide was below the long-term median flow for May, about 80 percent of median (Figure 1). Monthly mean discharge values in May ranged mostly from below normal to normal.
Water surface levels at the end of May were below the full pool or target level at 7 of 22 reporting reservoirs. At the end of May, Lake Shelbyville was 0.3 feet above the seasonal target level, Carlyle Lake was at the seasonal target level, and Rend Lake was 3.3 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 0.53 feet below the long-term average at the end of May. Levels averaged 0.74 feet below April levels and 0.16 feet below those of last year.
Weather/Climate Information
The following description of temperatures, precipitation, drought, and spring conditions 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 warmer and much drier than normal, wrapping up a spring season that was also warmer and drier than normal.
Temperatures averaged 64.2 degrees in May, 0.9 degrees above normal, making it the 38th warmest May on record statewide. May average temperatures ranged from the high 50s in northern Illinois to the high 60s in southern Illinois, between 1 and 3 degrees above normal (Table 1a, Figure 2). The cooler start to the month and dry air over the state allowed nighttime low temperatures to regularly dip into the 40s. Parts of central and northern Illinois even saw the low 30s on a few nights in early May, including 30 degrees in Normal and Marseilles. The warmest point in the state in May was Cahokia at 67.6 degrees, and the coolest was Waukegan at 56.9 degrees.
Altogether, 24 daily high maximum temperature records and 2 daily high minimum records were broken in February. Four daily low maximum temperature records and 2 daily low minimum records were broken.
Precipitation statewide in May was 2.40 inches, 2.37 inches below normal and the 21st driest May on record statewide. Unfortunately, last month did not bring May showers for most of the state, continuing a dry pattern from April. May total precipitation ranged from nearly 10 inches in isolated parts of central Illinois to less than 1 inch in northeastern Illinois. Small parts of the state picked up 2 to 4 inches above normal precipitation in May, but most of the state ended the month between 1 and 4 inches below normal (Table 1a, Figure 3). May was the 4th driest on record in Chicago, with only 0.71 inches in the city.
Drought: The combination of drier than normal conditions in April and May and warmer than normal weather throughout much of the past three weeks has taken a significant toll on our soil moisture and streamflow. Soils throughout the state were somewhat to much drier than normal down to 8 to 10 inches and drying quickly. Streamflow in many streams in northern and central Illinois is well below normal for the start of summer. Impacts so far have been isolated to drying lawns and stressed shrubs and young trees; however, there is a concern of increased or worsening impacts if dry and hot weather continues in June.
We have received dozens of condition and impact reports over the past week in Illinois, which have helped tremendously with drought monitoring. Although most of these impacts so far have been on the mild side, there is increased concern because they are occurring early in the growing season. With the heat of summer, continued dryness through June would likely worsen impacts to ecology, agriculture, and water resources. Please continue to report conditions and impacts using the National Drought Mitigation Center’s CMOR system: go.illinois.edu/cmor.
Spring: May wrapped up a warmer and drier spring that brought us lots of pleasant and sunny weather. The statewide average temperature in climatological spring (March–May) was 52.8 degrees, 0.4 degrees above normal and the 32nd warmest spring on record (Table 1b). Meanwhile, the wetter conditions in March could not make up for large deficits in April and May, and spring ended with a statewide total of 9.27 inches of rain, 2.67 inches below normal, and the 34th driest spring on record.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 19 stations across the state that collect hourly weather and soil information.
Winds averaged 5.8 mph across the network in May, 2.2 mph lower than in April and 1.4 mph below the historical average. ICN Bondville (Champaign County) was the windiest station again this month with a monthly average of 9.5 mph. The highest measured wind gust was 64.7 mph, recorded at ICN Monmouth (Warren County) on May 7.
Temperatures rose 11 degrees from April to an average of 65 degrees, 1 degree higher than the historical average. Station highs ranged from the high 80s to low 90s with lows mainly in the 30s. The month’s highest temperature was 92 degrees recorded at ICN Champaign (Champaign County) on May 31. The lowest was 32 degrees measured at ICN Dekalb (DeKalb County) on May 3.
Soil temperatures rose 10–14 degrees in May to the mid- to high 60s. Under bare soils, temperatures ranged from 34 to 107 degrees at 2 inches and 36 to 94 degrees at 4 inches. Temperatures under sod ranged from 43 to 89 degrees at 4 inches and 47 to 81 degrees at 8 inches.
Precipitation averaged 2.91 inches in May, a 0.45-inch increase from in April, but 1.47 inches lower than the historical average. Stations in all regions had a dry month. Only 3 out of the 19 ICN stations had totals that were equal to or above their historical average. ICN St. Charles (Kane County) recorded 1.16 inches, 30 percent of normal.
However, ICN Rend Lake (Jefferson County) received 5.55 inches, thanks to heavy isolated showers. Sixty-five percent of its monthly total was received on two days: May 7 and 13.
Dry weather caused soil moisture to fall across the state in May. Moisture levels at 2 inches fell an average of 32 percent across the network. At stations in northern and central Illinois, levels are near or at wilting points. The largest drops occurred at northern stations, which averaged a 46 percent decrease. Levels at the east-central stations only fell 15 percent, but they were driest in the network at the beginning of the month.
Moisture levels declined 27 and 22 percent at 4 and 8 inches, respectively. Soil moisture at 39 inches and greater remained high with little change in May.
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 the past record of monthly mean flows at those stations for the same month. Both recent and long-term data are retrieved from United States Geological Survey (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 80 percent of the median) and below the mean for May (approximately 60 percent of the mean). Monthly mean discharge values in May ranged mostly from below normal to normal. The monthly mean flows of the Pecatonica River at Freeport and the South Fork Sangamon River near Rochester were above normal for May.
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 an 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 22 reservoirs for which levels were reported last month and this month, reported end-of-May water levels were lower at 14 reservoirs, higher at 6 reservoirs, and about the same as at the end of April at 2 reservoirs. For the 22 reservoirs with measurements reported for the end of May, water levels were below normal target pool or spillway level at 7 reservoirs, above normal target pool or spillway level at 9 reservoirs, and at about full pool level at 6 reservoirs.
Note: The April 2023 reported pumpage from Lake Vermilion (Danville), omitted from last month’s Illinois Water and Climate Summary, was 176.5 million gallons.
Major Reservoirs. Compared to water levels at the end of April, at the end of May the water level at Lake Shelbyville was 3.7 feet higher, Carlyle Lake was 0.9 feet higher, and Rend Lake was 1.3 feet lower. At the end of May, Lake Shelbyville was 0.3 feet above the seasonal target level, Carlyle Lake was at the seasonal target level, and Rend Lake was 3.3 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 2023 mean level for Lake Michigan was 579.7 feet. The monthly mean level one year ago (May 2022) was 579.9 feet. The long-term average lake level for May is 579.1 feet, based on 1918–2022 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 579.7 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Dry weather caused water tables to fall at 23 of the 25 monitored wells in May, ending the month with a state average that was 0.74 feet below April levels. (Table 4)
The largest decreases occurred in northern Illinois, which had been recovering from lower levels last year. The Freeport well (Stephenson County) continued its decline, ending the month 3.07 feet lower than in April. Water levels at the site had fallen 6.60 feet in April and May but are 4.09 feet higher than in May 2022. Declines were also measured at other northern wells including Fermi Lab (DuPage County), which was 2.58 feet lower than in April and Mt. Morris (Ogle County) which was 3.23 feet lower.
Only two wells showed an increase in May; Brownstown (Fayette County) rose 0.29 feet and Snicarte (Mason County) rose 9.51 feet from in April.
Wells averaged 0.16 feet below May 2022 levels, 1.02 feet below the 15-year average, and 0.53 feet 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/
