Temperatures and precipitation were above the long-term average in Illinois in August. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were near the long-term depths.
Air temperatures averaged 75.6°F, 2.0° above the 1991–2020 normal for August (Figure 1). Monthly average temperatures ranged from the low to mid-70s in northern Illinois to the low to mid-80s in southern Illinois.
Precipitation statewide in August was 3.72 inches, 0.18 inches above the long-term statewide average (Figure 1).
Soil moisture at 2 inches declined an average of 11% in August. The largest drops were in northern Illinois where levels ended the month at or near the wilting points. Declines were observed from 2 to 20 inches, while soil moisture remained high at deeper depths.
Mean provisional streamflow aggregated statewide was above the long-term median flow for August, about 260% of median (Figure 1). Monthly mean discharge values in August ranged from normal to much above normal for the month.
Water surface levels at the end of August were below the full pool or target level at 9 of 23 reporting reservoirs. At the end of August, Lake Shelbyville was 4.9 feet above the summer target level, Carlyle Lake level was at the summer target level, and Rend Lake was 2.6 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels statewide were near the long-term average in August with an average departure of 0.08 feet above the period of record (Figure 1). Levels averaged 1.09 feet below July 2021 and 0.35 feet below August 2020 levels.
Weather/Climate Information (Trent Ford)
August in Illinois was warmer and wetter than average statewide.
Temperatures averaged 75.6°F, 2.0° above the 1991–2020 normal for August (Table 1a, Figure 2). Monthly average temperatures ranged from the low to mid-70s in northern Illinois to the low to mid-80s in southern Illinois. These temperatures were 1 to 4° above normal across most of the state, with the larger departures in northern Illinois. Apart from the first few days of the month, August temperatures were largely above normal across the state (Figure 2). Most of the northern half of the state experienced 5 to 10 days last month with temperatures that were 10 to 15 degrees above normal. The highest temperature observation of the month, 99°F, was recorded in Carbondale (Jackson County) on August 30.
Unusually high nighttime low temperatures last month were partly caused by persistently high humidity. One useful measure of humidity is the dewpoint temperature, which is the air temperature at which the air is saturated. Higher dewpoint temperatures indicate higher humidity, and humans begin to feel uncomfortable as dewpoints exceed 65 degrees. Many places around the state last month experienced unusually high frequencies of high dewpoint temperatures, particularly at night or in the early morning. For example, nearly 60% of 3 a.m. dewpoint temperature observations at Champaign’s Willard Airport last month exceeded 65 degrees, compared to around 30% on average.
Modified growing degree days (DD, base 50°, from April 1) ranged from around 2700 DD in northern Illinois to nearly 3500 DD in southern Illinois. DD accumulation this season so far (from April 1) has been near to 100 DD below normal in southern Illinois, 100 to 200 DD above normal in central Illinois, and between 200 and 300 DD above normal in northern Illinois.
Precipitation statewide in August was 3.72 inches, 0.18 inches above the long-term statewide average (Table 1a). August precipitation totals varied quite a bit across the state, from less than an inch in parts of northwest and western Illinois to over 11 inches in north-central Illinois. August was 1 to 3 inches drier than normal throughout most of the northern two-thirds of the state and was between 1 and 5 inches wetter than normal in southern Illinois.
Severe weather reports: The NOAA Storm Prediction Center recorded 163 severe weather reports for August in Illinois: 17 for tornadoes, 7 for hail, and 139 for wind. (Multiple reports can be generated for a single event). Seven confirmed tornadoes touched down in northern Illinois on August 9 alone, including three EF-1 tornadoes. The five National Weather Service offices serving the state issued a combined 18 tornado warnings in August, the second most for August on record going back to 1986. Additionally, there were 93 severe thunderstorm warnings issued in Illinois in August.
Drought: Drought in northern Illinois persisted from spring and early summer into August. Wetter conditions in northeast Illinois in August helped slightly reduce the coverage of severe drought, although August ended with severe drought still throughout much of McHenry and Lake Counties. Meanwhile, moderate drought expanded across northwest Illinois in August, thanks to well below normal precipitation. The month ended with moderate drought in all or parts of most northern Illinois counties from Rock Island County to Lake County.
Summer (June–August) was overall warmer than normal across most of Illinois. Seasonal temperatures averaged 74.8°F, 1.0° above the 1991–2020 normal (Table 1b), ranging from the low- to mid-70s in northern Illinois to the low- to mid-80s in southern Illinois. Summer temperatures were within 1 degree of normal in southern and south-central Illinois and were 1 to 3 degrees above normal in northern and north-central Illinois.
Summer precipitation averaged 13.93 inches statewide, 1.66 inches above average (Table 1b). Seasonal totals were highest in southern and south-central Illinois and lower overall in northern and northwest Illinois. Summer total precipitation ranged from nearly 20 inches in south-central Illinois to less than 10 inches in northwest Illinois. Summer precipitation departures followed a similar south-to-north pattern. Summer total precipitation was 6 to 8 inches above normal in south-central Illinois, and 4 to 5 inches below normal in far northwest Illinois. August, in particular, was very dry across northwest Illinois, coinciding with the expansion of moderate drought in the U.S. Drought Monitor in this region.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information at 19 stations across the state.
Wind speeds averaged 4.2 mph in August, 0.1 mph greater than in July and 0.3 mph less than the network’s long-term average. ICN Monmouth was the windiest with a monthly average of 6.0 mph. The highest recorded wind gust was 51.0 mph, measured at ICN Big Bend on August 11.
Air temperatures rose 4° to an average of 78° for August, 5° below the long-term average. Station highs were in the low to mid-90s with monthly lows in the 40s and 50s. The month’s highest temperature was 96°, recorded at ICN Rend Lake on August 26. The lowest was 49°, reported by ICN DeKalb on August 2.
Soil temperature averages remained in the high 70s and low 80s, around 2° warmer than the long-term average. Under bare soil, temperatures ranged from 63 to 113° at the 2-inch depths and 65 to 102° at 4 inches. Temperatures under sod ranged from 68 to 94° at 4 inches and 69 to 89° at 8 inches.
Precipitation declined from July to a network average of 3.10 inches, near the long-term average of 3.12 inches. The northern stations remained drier, receiving 2.11 inches for the month. ICN Brownstown was the wettest station with 5.12 inches reported for the month.
Soil moisture at 2 inches declined an average of 11% in August. Moisture levels were lowest in northern Illinois, which ended the month with an average of 0.14 water fraction by volume (wfv), near the wilting points of most of the soils monitored. In comparison, southern stations had average moisture levels of 0.31 wfv at the end of the month, the wettest in the state.
Soil moisture levels declined from 2 to 20 inches but remained high at the 39- and 59-inch depths.
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 260 percent of the median) and above the mean for August (approximately 160 percent of the mean). Monthly mean discharge values in August ranged from normal to much above normal for the month.
Water-Supply Lakes and Major Reservoirs. Table 3 lists reservoirs in Illinois, their normal pool or target water surface elevation, and other data related to observed variations in water surface elevations. Reservoir levels are obtained from a network of cooperating reservoir operators who are contacted each month by ISWS staff for the current water levels. Reservoir levels are reported in terms of their difference from normal pool (or target level). The average of the month-end readings for the period of record is reported in terms of the difference from normal pool or target level (column 6 of Table 3), and the number of years of record for each reservoir also is given (column 7). Most reservoirs serve as public water supplies, with the exceptions noted in the last column.
Compared to end-of-July water levels at 24 reservoirs for which levels were reported last month and this month, reported end-of-August water levels were lower at 13 reservoirs, higher at 7 reservoirs, and about the same as at the end of July at 4 reservoirs. For the 24 reservoirs with measurements reported at the end of August, water levels were below the normal target pool or spillway level at 9 reservoirs, above the normal target pool or spillway level at 11 reservoirs, and at about the full pool level at 4 reservoirs.
Major Reservoirs. Compared to water levels at the end of July, at the end of August the water level at Lake Shelbyville was 3.2 feet lower, Carlyle Lake was 2.4 feet lower, and Rend Lake was 1.6 feet lower. At the end of August, Lake Shelbyville was 4.9 feet above the summer target level, the Carlyle Lake level was at the summer target level, and Rend Lake was 2.6 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 2021 mean level for Lake Michigan was 580.8 feet. The monthly mean level one year ago (August 2020) was 582.1 feet. The long-term average lake level for August is 579.3 feet, based on 1918–2020 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 580.7 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Shallow groundwater levels fell 1.09 feet on average from July (Table 4). Declines were observed at 23 of the 25 stations monitored. The Carbondale well had the largest drop with a decline of 2.85 feet from July. The Snicarte well level increased 0.51 feet in August, the largest increase of the month.
Overall, well levels were near the long-term average with a state average of 0.08 feet above the period of record. Readings ranged from 4.32 feet below to 3.68 feet above normal levels.
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, http://rivergages.com, 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