August 2022 Overview
August temperatures and precipitation were 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 73.7 degrees F, 0.1 degrees above the 1991–2020 normal for August (Figure 1). Monthly average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois.
Precipitation statewide in August was 3.86 inches, 0.30 inches above the long-term statewide average (Figure 1).
Soil moisture levels at 2 inches increased 11 percent in August to a state average of 0.26 water fraction by volume (wfv). Moisture levels at 4 inches and deeper remained steady.
Mean provisional streamflow aggregated statewide was above the long-term median flow for August, about 265 percent of median (Figure 1). Monthly mean discharge conditions relative to long-term records for August differed considerably by location.
Water surface levels at the end of August were below the full pool or target level at 10 of 21 reporting reservoirs. At the end of August, Lake Shelbyville was 0.3 feet below the seasonal target level, Carlyle Lake was 1.2 feet above the seasonal target level, and Rend Lake was 1.9 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 0.4 feet below the long-term average at the end of August (Figure 1). Levels averaged 0.9 feet below those in July and 0.4 feet below last year’s levels.
The following description of temperatures, modified growing degree days, 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 wetter than average statewide.
Temperatures averaged 73.7 degrees F, 0.1 degrees above the 1991–2020 normal for August (Figure 2, Table 1a). Monthly average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois. The warmest place in the state was Bean Ridge in Alexander County with an average August temperature of 78.1 degrees. The coolest place in the state was Shabbona in DeKalb County with an average August temperature of 69.1 degrees.
During the hot week at the beginning of the month, high temperatures regularly reached into the 90s across the state, including 96 degrees in Cahokia Heights and 95 degrees at Chicago Midway Airport. However, a cold front moved through around August 9 and brought much cooler, drier air into the region. Nighttime low temperatures in the second and third weeks of August dipped into the 50s statewide, including 50-degree lows in Woodford and Pike Counties.
Modified growing degree days (DD, base 50 degrees, from May 1) ranged from around 3000 DD in southern Illinois to 2500 DD in northern Illinois. DD accumulation this season has so far been nearly 100 DD above normal statewide.
Precipitation statewide in August was 3.86 inches, 0.30 inches above the long-term statewide average (Figure 3, Table 1a). August rainfall was variable, with parts of Jo Daviess and Stephenson Counties in northwest Illinois receiving almost 15 inches in the month and areas in Adams and Hancock Counties receiving less than 0.50 inches of rainfall in August.
Severe weather: Heavy rainfall early in the month brought two to three times the normal August rainfall to the Effingham-Olney and Freeport areas. In the August 7–8 event, between 5 and 11 inches of rain fell around Freeport in less than 48 hours, inundating roads, flooding homes, and causing flooding along the Pecatonica River. The Quad Cities National Weather Service office has a detailed summary of the event and the damage it caused: https://www.weather.gov/dvn/summary_080822. Just days earlier, a swath of east-southeast Illinois between Effingham and Olney saw 7 to 10 inches of rain in just 24 hours. Many fields from Effingham to Wayne County were flooded by both runoff and flooding from the Little Wabash and Embarras Rivers.
Drought: Multiple rounds of rainfall in late August brought much-needed drought relief to east-central Illinois, where June and July rainfall deficits had exceeded 7 inches. Meanwhile, drought removed in eastern Illinois migrated to western Illinois following an exceptionally dry month in some places. The U.S. Drought Monitor removed the moderate drought classification from most of Champaign County and all of Vermilion County but added moderate to severe drought in all or parts of Henderson, Warren, Hancock, and Adams Counties in western Illinois, reflecting the recent dryness.
Summer (June–August) was slightly warmer than normal across most of Illinois. Seasonal temperatures averaged 74.3 degrees, 0.9 degrees above the 1991–2020 normal (Table 1b), ranging from the low 70s in northern Illinois to the mid- to high 70s in southern Illinois. There was quite a bit of monthly variability. June was 1 to 3 degrees warmer than normal, while July and August temperatures were within 1 degree of normal statewide.
Summer precipitation averaged 11.83 inches statewide, 0.61 inches below average (Table 1b). Seasonal totals were highest in areas of extreme rainfall, namely northwest Illinois and along the Interstate 70 corridor from St. Louis to Effingham. Summer total precipitation ranged from around 25 inches in south-central Illinois to less than 6 inches in western Illinois. Summer precipitation departures followed a similar south-to-north pattern. Summer total precipitation was more than 12 inches above normal in south-central Illinois and 8 to 10 inches below normal in western Illinois.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information from 19 stations across the state.
Wind speeds averaged 4.3 mph in August, 0.2 mph lower than in July and the network’s long-term average. ICN Stelle (Ford County) had the month’s highest average at 6.3 mph. The highest recorded wind gust was 55.0 mph, measured at ICN Peoria (Tazewell County) on August 20.
Temperatures were slightly lower in August, averaging 73 degrees, equal to the long-term average but 3 degrees lower than the July average. Station highs were in the low to mid-90s with lows in the 50s. The network’s highest temperature was 96 degrees, recorded at ICN Dixon Springs (Pope County) on August 28. The lowest temperature was 50 degrees, measured at ICN DeKalb (DeKalb County) on August 12.
Soil temperatures were 1 to 3 degrees cooler than in July with averages in the mid- to high 70s. Under bare soils, temperatures ranged from 59 to 113 degrees at 2 inches and 63 to 101 degrees at 4 inches. Temperatures under sod ranged from 66 to 98 degrees at 4 inches and 69 to 89 degrees at 8 inches.
Precipitation averaged 4.44 inches for the month, 0.78 inches lower than in July and 1.32 inches higher than the long-term average. ICN Freeport (Stephenson County) recorded 12.32 inches, the month’s highest. Six other stations, in all regions of the state, recorded rainfall totals greater than 5 inches. The month’s driest station was ICN Perry (Pike County), which received 0.54 inches for August.
Soil moisture at 2 inches increased 11 percent on average. The largest increases were in east central Illinois where soil moisture levels increased 114 percent to an end-of-month average of 0.30 water fraction by volume (wfv). Moisture levels also increased at northern and west central stations. In southern Illinois, levels increased 27 percent to an average of 0.22 wfv. Despite this, however, soil moisture levels were above the wilting points in all regions at the end of August.
Soil moisture increased at depths from 4 through 20 inches with increases in all regions except the south. Moisture levels at 39 and 59 inches remained high and showed little change for the month.
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 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 265 percent of the median) and above the mean for August (approximately 165 percent of the mean). Monthly mean discharge values in August ranged from much below normal to much above normal, differing considerably by location.
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 the 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 21 reservoirs for which levels were reported last month and this month, reported end-of-August water levels were lower at 14 reservoirs and higher at 7 reservoirs. For the 21 reservoirs with measurements reported at the end of August, water levels were below normal target pool or spillway level at 10 reservoirs, above normal target pool or spillway level at 6 reservoirs, and at about full pool level at 5 reservoirs. The supply at Carlinville was pumped from their Lake 2, which was about 0.7 feet below full pool. Inflow to Salem Lake was supplemented by pumpage from Carlyle Lake during the last week of August.
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 higher, Carlyle Lake was 2.4 feet lower, and Rend Lake was 0.5 feet lower. At the end of August, Lake Shelbyville was 0.3 feet below the seasonal target level, Carlyle Lake was 1.2 feet above the seasonal target level, and Rend Lake was 1.9 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 2022 mean level for Lake Michigan was 580.0 feet. The monthly mean level one year ago (August 2021) was 580.8 feet. The long-term average lake level for August is 579.3 feet based on 1918–2021 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 580.0 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Eighteen out of 24 wells reported lower water table levels in August. Western Illinois saw the largest declines as the Good Hope well in McDonough County fell 3.7 feet from in July to an end-of-month level of 10.3 feet because of drier weather. Heavy rains in the north led to rising water tables. The Freeport well in Stephenson County rose 2.4 feet in August to 18.1 feet.
Overall, well levels averaged 0.9 feet below July levels.
Water tables were slightly lower than last year’s, averaging 0.4 feet below August 2021 levels and ranging from 5.4 feet below to 3.0 feet above.
Levels averaged 0.9 feet below the 15-year average and 0.4 feet below the period of 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