August 2024 Overview
August in Illinois was slightly warmer and much drier than average statewide.
Temperatures averaged 73.7°F, 0.1° above the 1991-2020 normal for August and the 60th warmest. August average temperatures ranged from the low 70s in northern and central Illinois to the high 70s in southern Illinois, around 1 degree above normal in most of northern and southern Illinois, and slightly below normal in central Illinois.
Precipitation statewide in August was 2.82 inches, 0.74 inches below the long-term statewide average, and the 46th driest August on record statewide. Total August rainfall ranged from less than 1 inch in parts of southwest and far southeast Illinois to over 5 inches in east-central Illinois.
Soil moisture at 2 inches averaged 0.26 water fraction by volume (wfv) at the end of August, 28% lower than the beginning of the month. Decreases were observed through 20 inches with levels steady at depths of 39 inches and greater
Mean streamflow statewide was above the median for the month.
Mean provisional streamflow aggregated statewide was above the long-term median flow for August, about 160% of median (Figure 1). Monthly mean discharge values ranged mostly from normal to above normal for August.
Water surface levels at the end of August were below the full pool or target level at 17 of 21 reporting reservoirs. At the end of August, Lake Shelbyville was 0.6 feet below the summer target level, Carlyle Lake level was 0.5 feet below the summer target level, and Rend Lake was 2.0 feet above the spillway level.
Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 0.62 feet below the long-term average at the end of August. Levels averaged 1.62 feet below July and 0.99 feet 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.
August in Illinois was slightly warmer and much drier than average statewide.
Temperatures averaged 73.7°F 0.1° above the 1991-2020 normal for August and the 60th warmest on record (Table 1a). August average temperatures ranged from the low 70s in northern and central Illinois to the high 70s in southern Illinois, around 1 degree above normal in most of northern and southern Illinois, and slightly below normal in central Illinois (Figure 2). Most parts of the state pushed into the mid to upper 90s in the final week of the month, and a few spots saw temperatures above 100 degrees, including in Quincy. The intense heat was preceded by a fleeting taste of fall air, and many places saw nighttime temperatures dip into the high 40s, including 45 degrees in Barrington and 46 in Normal. The warmest point in the state last month was Olmstead at 78.4 degrees, and the coolest point was Shabbona at 69.1 degrees.
Precipitation statewide in August was 2.82 inches, 0.74 inches below the long-term statewide average, and the 46th driest August on record statewide (Table 1a). Last month was also the first drier than normal August in Illinois since 2020. Total August rainfall ranged from less than 1 inch in parts of southwest and far southeast Illinois to over 5 inches in east-central Illinois. A narrow band of the Interstate 74 corridor from Moline to Danville was wetter than normal last month, while most of the rest of the state was between 1 and 3 inches drier than normal (Figure 3). The dryness last month was most intense in southern Illinois. August was the 4th driest on record in Carbondale with 0.69 inches, the 3rd driest on record in Vandalia with 0.52 inches, and the fifth driest on record in Edwardsville with 0.63 inches.
Summer: Climatological summer encompasses June, July, and August, and the season often brings more than its fair share of intense weather. This past season was on the mild side, temperature-wise. June started the season with warmer than normal conditions but was succeeded by a cooler than normal July. August wrapped up the summer with near normal temperatures. The summer 2024 statewide average temperature was 74.1 degrees, 0.3 degrees above normal, and the 49th warmest summer on record (Table 1b). The statewide average temperature in Illinois from January to August in 2024 was the 3rd highest on record, only eclipsed by 2012 and 1921. Nine of the top ten warmest January-August periods on record in Illinois have occurred since the 1980s, illustrating the large impact of climate change on winter and spring temperatures in Illinois.
Summer 2024 was slightly wetter than normal for the entire state but began with a dry June and some concerns of crop and plant stress from the dryness and heat. However, a very wet July across the state cleared all concerns of drought and flipped the switch in some places to flooding concerns. July was also one of the most intense severe weather months in memory in Illinois. August was more of a mixed bag, with drier conditions in northern and southern Illinois. In all, summer precipitation totals ranged from 6 inches in parts of southern Illinois to over 20 inches in parts of northwest and southwest Illinois. This past season was the 7th wettest summer on record in Salem with 19.02 inches, and the 4th wettest summer on record at Carlyle Reservoir with 18.96 inches. Overall, summer 2024 statewide total precipitation was 12.41 inches, 0.14 inches above normal and the 40th wettest summer on record (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 4.4 mph, 0.1 mph lower than the long-term average. ICN Monmouth (Warren County) was once again the windiest station with an August average of 7.6 mph. Monmouth also had the month’s highest recorded wind gust, measuring 49.7 mph on August 27.
Temperatures averaged 73° in August, equal to the network’s long-term average. Station highs were in the mid to high 90s with lows in the high 40s and low 50s. The month’s warmest temperature was 99°, measured at ICN Dixon Springs (Pope County). The lowest was 45°, recorded at ICN DeKalb (DeKalb County) on August 22.
Soil temperatures averaged from the mid to high 70s. Under bare soils, temperatures ranged from 57 to 107° at 2 inches and 59 to 104° at 4 inches. Temperatures under sod ranged from 67 to 90° at 4 inches and 65 to 96° at 8 inches.
August was drier for most ICN stations. Fifteen of the 19 stations had monthly totals below their monthly long-term average. ICN Carbondale (Jackson County) recorded only 0.63 inches in August, 21% of normal. However, four stations in central Illinois received higher than normal precipitation. ICN Champaign (Champaign County) had the month’s highest with 5.08 inches, 1.62 inches higher than its long-term average. More than two inches of the station’s total were recorded in one hour on August 30.
Soils were drier at the end of the month. Soil moisture levels at 2 inches were 28% lower, on average, at the end of August. Levels at the southern stations declined 44% as the region experienced a dry August after a wet July. Meanwhile in the north, soil moisture showed no overall change.
Similar patterns were observed through 20 inches. Soil moisture remained steady at depths of 36 inches 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 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 August (approximately 160 percent of the median) and was about equal to the mean flow for August. Monthly mean discharge values in August ranged mostly from normal to above normal for the month. The monthly mean flow of the Vermilion River at Pontiac remained slightly below normal for August.
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 applied. 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 18 reservoirs for which levels were reported last month and this month, reported end-of-August water levels were lower at 15 reservoirs, slightly higher at 2 reservoirs, and about the same as at the end of July at 1 reservoir. For the 21 reservoirs with measurements reported for the end of August, water levels were below normal target pool or spillway level at 17 reservoirs, and above normal target pool or spillway level at 4 reservoirs. The Carlinville water system continued to pump from its second lake. Salem Lake inflow was supplemented by pumping from Carlyle Lake beginning at the end of August. As a reminder, Lake Decatur water level is actively managed within a 0.7-ft summer target range.
Major Reservoirs. Compared to water levels at the end of July, at the end of August the water level at Lake Shelbyville was 1.9 feet lower, Carlyle Lake was 3.1 feet lower, and Rend Lake was 1.7 feet lower. At the end of August, Lake Shelbyville was 0.6 feet below the summer target level, Carlyle Lake level was 0.5 feet below the summer target level, and Rend Lake was 2.0 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 2024 mean level for Lake Michigan was 579.7 feet. The monthly mean level one year ago (August 2023) was 579.7 feet. The long-term average lake level for August is 579.3 feet, based on 1918-2023 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 579.6 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables decreased at all but one monitored well in August.
Wells ended the month 1.62 feet lower than in July. Only the Perry well (Pike County) showed no decline. The highest drop was at the Belleville well (St Clair County) which ended the month 3.27 feet lower. The ICN station at the site recorded 1.88 inches of rain in August, 58% of normal, and St Clair County was listed as abnormally dry in the August 27 drought map. However despite the decline, the Belleville well ended the month 0.89 feet above the 15-year average.
Wells averaged 0.99 feet above August 2023, 1.03 feet below the 15-year average, and 0.62 feet below the long-term record (Table 4).
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/