Overview
August temperatures were equal to and precipitation was above the long-term average in Illinois. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures averaged 73.6 degrees in August, right at the 30-year normal (Figure 1). August average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois.
Precipitation statewide in August was 4.01 inches, 0.45 inches above the 30-year normal (Figure 1). Total August rainfall ranged from less than 2 inches in parts of northwest Illinois to over 10 inches in southeast Illinois.
Soil moisture, on average, showed little change at 2 and 4 inches with levels averaging 1 percent higher at the end of August. Levels at 8 and 20 inches rose 8 and 4 percent, respectively. However, significant differences occurred regionally with wetter soils in central and southern Illinois and drier soils in the north.
Mean provisional streamflow aggregated statewide was above the long-term median flow for August, about 150 percent of median (Figure 1). Monthly mean discharge values in August ranged mostly from normal to above normal for the month.
Water surface levels at the end of August were below the full pool or target level at 12 of 19 reporting reservoirs. At the end of August, Lake Shelbyville was 0.1 foot below the seasonal target level, Carlyle Lake was 0.5 feet above the seasonal target level, and Rend Lake was 0.6 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 1.48 feet below the long-term average at the end of August. Levels averaged 0.81 feet below July levels and 1.30 feet below last year’s levels.
Weather/Climate Information
The following description of temperatures, precipitation, drought, and summer 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.
August in Illinois was wetter than normal with near normal temperatures statewide. The climatological summer was slightly drier than normal with near normal temperatures.
Temperatures averaged 73.6 degrees in August, right at the 30-year normal and tied for the 61st warmest on record statewide. August average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois (Figure 2, Table 1a). Most of the state reached the mid- to upper-90s late in the month, and Chicago’s O’Hare airport recorded 100 degrees on August 24, the first triple-digit temperature in Chicago in 11 years. The intense heat was followed by a fleeting taste of fall air, and nighttime temperatures dipped into the high 40s in many places, including 48 degrees in Marseilles. The warmest point in the state last month was Cahokia at 77.4 degrees, and the coolest point was Stockton at 69.7 degrees.
Altogether, 61 daily high maximum temperature records and 98 daily high minimum records were broken in August. Cooler days and nights in August broke 19 daily low maximum temperature records and 5 daily low minimum temperature records statewide. Three August high maximum temperature records were broken, and 7 August high minimum temperature records were broken last month, including an 80-degree low temperature in Du Quoin on August 21. The 79-degree low temperature in Stockton on August 24 set their all-time high minimum temperature record, which was last broken in 2019.
Precipitation statewide in August was 4.01 inches, 0.45 inches above the 30-year normal and the 46th wettest on record statewide. A more active storm track last month brought wetter weather to central and southern Illinois, helping to further relieve earlier drought conditions. Total August rainfall ranged from less than 2 inches in parts of northwest Illinois to over 10 inches in southeast Illinois (Figure 3, Table 1a). Most areas of the state south of Interstate 80 were 1 to 5 inches wetter than normal in August, while much of northern Illinois was 1 to 3 inches drier than normal.
The dryness last month was most intense in northwest Illinois from the Quad Cities to the Wisconsin border. Freeport had its third driest August on record with only 0.80 inches, about 3 inches below normal. Meanwhile, Fairfield in southern Illinois had its fourth wettest August on record with 8.11 inches.
Drought: Drought conditions have improved across much of the state since mid-summer. The August 29 U.S. Drought Monitor map has 16 percent of the state in at least moderate drought compared to over 50 percent on August 1. The wetter weather in August helped improve crop and pasture conditions across the state, stabilize declining streams and pond levels, and promote ecological health in natural lands. Despite the recovery, drought likely and significantly impacted crop yield potential this year, and its impact on tree health–especially in urban areas–will not be well known until next year. However, rain in July and August kept 2023 from joining the most severe drought years of 2012 and 1988. One exception to the wider drought improvement is in northwest Illinois, where drought conditions expanded in August. Crop impacts have been reported in this part of the state through August, and soils remain somewhat to very dry from the Quad Cities up to Rockford.
Summer: August wrapped up a drier than normal summer that had near-normal temperatures statewide. The statewide summer average temperature in Illinois was 73.5 degrees, 0.3 degrees below the 30-year normal and tied for the 63rd coolest summer on record (Table 1b). Statewide total summer precipitation was 10.64 inches, 1.63 inches below normal and the 48th driest summer on record.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information from 19 stations across the state.
Winds averaged 4.4 mph in August, 0.3 mph higher than in July and 0.1 mph lower than ICN’s long-term average. ICN Stelle (Ford County) was the windiest station of the month with a monthly average of 6.3 mph. The highest recorded wind gust was 49.0 mph, measured at ICN Snicarte (Mason County) on August 25.
Temperatures rose 1 degree from July to an average of 76 degrees. Station highs were in the 90s and low 100s. ICN Rend Lake (Jefferson County) had the month’s highest temperature, recording 101 degrees on August 25. Lows were in the 40s and 50s, with ICN Big Bend (Whiteside County) measuring 43 degrees on August 31, the network’s lowest temperature.
Soil temperatures fell 1 to 3 degrees in August to averages in the mid- to high 70s. Under bare soils, temperatures at 2 inches ranged from 57 to 112 degrees and from 58 to 103 degrees at 4 inches. Temperatures under sod ranged from 66 to 97 degrees at 4 inches and 67 to 89 degrees at 8 inches.
Precipitation was higher than normal in central and southern Illinois. ICN Brownstown (Fayette County) recorded 8.19 inches in August, 5.00 inches higher than normal for the month. More than 3 inches was received on just one day, August 26. ICN Springfield (Sangamon County) received 7.11 inches for the month, 3.43 inches (48 percent) on August 9.
Northern Illinois, however, had a much drier month. ICN Freeport (Stephenson County) recorded only 0.76 inches in August, 3.31 inches less than usual for the month. ICN Big Bend (Whiteside County), DeKalb (DeKalb County), and St. Charles (Kane County) all had lower than normal precipitation for the month.
Overall, ICN averaged 4.40 inches of rain in August, 0.39 inches lower than in July and 1.10 inches higher than the long-term network average.
Soil moisture followed precipitation conditions. Large increases in moisture levels were seen in the central and southern regions with drying across the north.
Two-inch soil moisture levels rose 29 percent in west central and 18 percent in southern Illinois. Levels in both regions rose throughout the soil column to depths of 39 inches. There were no significant changes at 59 inches.
Soil moisture in the north declined 48 percent at 2 inches and 42 percent at 4 inches. The drying continued through 20 inches. Moisture levels remained steady 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 the past record of monthly mean flows at those stations for the same month. Both recent and long-term data are retrieved from U.S. 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 above the median value for August (approximately 150 percent of the median) and slightly below the mean for August (approximately 95 percent of the mean). Monthly mean discharge values in August ranged mostly from normal to above normal. Monthly mean discharge values of the Kaskaskia River at Vandalia and the Embarras River at Ste. Marie were below 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 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-July water levels at 19 reservoirs for which levels were reported last month and this month, reported end-of-August water levels were lower at 10 reservoirs, higher at 6 reservoirs, and about the same as at the end of July at 3 reservoirs. For the 19 reservoirs with measurements reported for the end of August, water levels were below normal target pool or spillway level at 12 reservoirs, above normal target pool or spillway level at 6 reservoirs, and at about the full pool level at 1 reservoir. Carlinville supply continues to be from its second lake, and Sparta Reservoir was supplemented by pumping from the Kaskaskia River into the lake for water quality management.
Major Reservoirs. Compared to water levels at the end of July, at the end of August the water level at Lake Shelbyville was 0.2 feet lower, Carlyle Lake was 0.4 feet higher, and Rend Lake was 0.6 feet lower. At the end of August, Lake Shelbyville was 0.1 foot below the seasonal target level, Carlyle Lake was 0.5 feet above the seasonal target level, and Rend Lake was 0.6 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 August 2023 mean level for Lake Michigan was 579.7 feet. The monthly mean level one year ago (August 2022) was 580.0 feet. The long-term average lake level for August is 579.3 feet, based on 1918–2022 data. In this period of record, the lowest mean level for Lake Michigan for August occurred in 1964 at 576.7 feet, and the highest mean level for August occurred in 2020 at 582.1 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)
Water tables continued to decline at most Water and Atmospheric Resources Monitoring Program (WARM) wells in August, despite the increased rains for most of the state. Twenty of the 25 wells were lower at the end of the month as statewide levels averaged 0.81 feet lower than in July (Table 4).
The largest declines were in western Illinois where the Perry well (Pike County) was 4.15 feet lower, and the Belleville well (St. Clair County) was 2.56 feet lower at the end of August. The wells were 9.89 feet and 5.20 feet below August 2022 levels, respectively.
Improvements were seen at several locations, most notably at the Springfield well (Sangamon County), which rose 1.89 feet in August, the month’s highest increase.
Wells averaged 1.30 feet below August 2022 levels, 2.12 feet below the 15-year average, and 1.48 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://www.isws.illinois.edu/warm
