Temperatures were below and precipitation was above the long-term average in Illinois in July. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were slightly above the long-term depths.
Air temperatures statewide averaged 74.7°F in July, 0.7° below the long-term average (Figure 1). The southwest crop reporting district (CRD) was the warmest with an average of 77.2°F. The lowest regional average temperature was 73.0°F, reported by the northeast CRD. Departures from average ranged from 1.2° below average in the east-southeast CRD to 0.2° below average in the northwest CRD.
Precipitation statewide averaged 4.90 inches, 0.84 inches above the long-term average (Figure 1). The southeast CRD was the wettest with an average of 7.55 inches. The driest was the northeast CRD with an average of 2.46 inches. Departures from average ranged from 1.46 inches below average in the northeast CRD to 3.44 inches above average in the southeast CRD.
Soil moisture at 2 inches declined 19% on average in July to 0.29 water fraction by volume (wfv). Double-digit decreases also occurred at the 4- and 8-inch depths, while levels remained steady from 20 inches to 5 feet.
Mean provisional streamflow aggregated statewide was above the long-term median flow for July, about 370% of median (Figure 1). Monthly mean discharge values in July were normal for the month in northern Illinois and above normal to much above normal for the month elsewhere. The Illinois River and portions of the Mississippi River below the Illinois River crested above the local flood stages early in July.
Water surface levels at the end of July were below the full pool or target level at 5 of 21 reporting reservoirs. At the end of July, Lake Shelbyville was 8.1 feet above the summer target level, Carlyle Lake was 2.4 feet above the summer target level, and Rend Lake was 4.2 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels statewide were slightly above the long-term average this month with an average departure of 0.18 feet above the period of record (Figure 1). Levels averaged 0.38 feet below June 2021 and 0.86 feet below July 2020 levels.
Weather/Climate Information (Kevin Grady)
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.
July in Illinois was cooler and wetter than average across much of the state.
Temperatures averaged 74.7°F, 0.7° below the long-term statewide average for July (Table 1, Figure 2). Monthly average temperatures were in the low to mid-70s at most stations across the state. After a near average end to June, much of July was cooler than average throughout Illinois, with monthly temperature departures of around 1–2° below average common in central and southern Illinois. July temperatures in northern Illinois were closer to but primarily still slightly below average. Monthly minimum temperatures generally ranged from the low 50s to the low 60s. For many stations, these minimum readings came around the Fourth of July weekend, although the coldest reading of the month, 48°F, was recorded at a station near Paxton (Ford County) on July 12. A heat wave affected much of Illinois around the last week of July. Most stations reached their monthly maximum temperatures during this week, generally ranging from the upper 80s to the mid-90s. The warmest reading of the month, 99°F, was recorded at a station near Rock Island (Rock Island County) on July 28.
Modified growing degree days (DD, base 50°, from April 1) ranged from just over 1800 DD in northern Illinois to just under 2400 DD in far southern Illinois. This was slightly above the long-term average across most of the northern half of Illinois, with the highest departures up to around 150 DD above average in the northeastern corner of the state. Most of the southern half of Illinois was slightly below the long-term average by up to around 50 DD below average.
Precipitation averaged 4.90 inches in July, 0.84 inches above the long-term statewide average (Table 1). The wettest part of the state in July was southern Illinois, with precipitation totals south of I-70 commonly 6–9 inches. A station near Hutsonville (Crawford County) recorded the highest monthly total in the state of 11.37 inches. Monthly totals in southern Illinois were well above average for the month, often by at least 3–4 inches. These large totals were fueled by high humidity and southerly flow off the Gulf of Mexico throughout most of July in the area. Central Illinois also started July with above average precipitation. However, the area then dried out for the second half of the month, leading to closer to average monthly totals across most of central Illinois, generally in the range of 3–5 inches.
As it has been throughout most of 2021, northern Illinois was drier than average in July. Northeastern Illinois was especially dry, with monthly totals in the area commonly less than 3 inches, often an inch or more below average. Many stations in Lake County and surrounding areas received well below 2 inches for July, 2 or more inches below average at many stations. O’Hare Airport received 1.90 inches for the month, 1.81 inches below average.
Severe weather reports: The NOAA Storm Prediction Center recorded 82 severe weather reports for July in Illinois: 8 for tornadoes, 9 for hail, and 65 for wind. (Multiple reports can be generated for a single event.) This included a report of 3-inch hail near Fountain Green (Hancock County) on the evening of July 9. There were also many wind reports in the Chicago area during the early morning hours of July 29 as a line of severe thunderstorms moved through the region.
Drought: As July began, the United States Drought Monitor (USDM) depicted abnormally dry (D0) or drought (D1–D3) conditions across much of northern Illinois to the north of the I-88 corridor. This included an area of extreme drought (D3) in northeastern Illinois in northern Lake and McHenry Counties and northeastern Boone County. Most of northern Illinois has received below average precipitation throughout much of 2021. July continued this dry trend in the area, although there were no large areas of further deterioration during the month, thanks in part to slightly cooler than average temperatures. The only significant change to the USDM map in northern Illinois in July was the improvement of the D3 area to D2 (severe drought) on the July 20 map, as some mid-month rains were enough to improve conditions a bit in the area. However, soil moisture and streamflows continued to be well below average at the end of July in the northeastern corner of Illinois, leading to impacts such as trees dropping leaves and corn showing stress. The July 27 map was the 11th consecutive one to depict the area as D2 or worse, going back through mid-May. Finally, there were also some pockets of D0 conditions in southern Illinois at the start of July, but heavy precipitation throughout the month in the area quickly led to their removal from the USDM map.
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 declined to an average of 4.1 mph, 1.3 mph less than in June and 0.8 mph less than the network’s long-term average. ICN Stelle had the windiest month with a monthly average of 6.1 mph. The highest recorded wind gust was 55.1 mph, reported by ICN Perry on July 9.
Air temperatures averaged 74° in July, 0.7° lower than in June and 0.8° lower than the long-term average. Most ICN stations had monthly highs in the low 90s with lows in the 50s. The month’s highest temperature was 94°, recorded at ICN Rend Lake on July 6. The lowest was 52° at ICN Bondville on July 3.
Soil temperatures rose 1–4° on average in July to the high 70s and low 80s. Under bare soil, temperatures ranged from 61 to 113° at 2 inches and 63 to 105° at 4 inches. Temperatures under sod ranged from 68 to 96° at 4 inches and 68 to 90° at 8 inches.
Precipitation averaged 5.49 inches across the network, 0.09 inches less than in June but 2.13 inches above the long-term average. Northern ICN stations had a drier month than the rest of the state. ICN Freeport in northwest Illinois received 2.71 inches in July, 67% of the station’s long-term average.
The central and southern stations saw significantly more rain with 5 reporting more than 8 inches. The highest total was 9.80 inches at ICN Rend Lake.
Soil moisture at 2 inches declined 19% on average in July. Levels fell 30% at northern stations to an average of 0.19 water fraction by volume (wfv), which is near the wilting points for the soils monitored. In southern Illinois, soil moisture remained high with no overall change.
Moisture also declined at the 4- and 8-inch depths. Levels remained steady from 20 inches to 5 feet.
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 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 July (approximately 370 percent of the median) and above the mean for July (approximately 250 percent of the mean). Monthly mean discharge values in July were normal for the month in northern Illinois and above normal to much above normal for the month elsewhere.
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-June water levels at 20 reservoirs for which levels were reported last month and this month, reported end-of-July water levels were lower at 6 reservoirs, higher at 8 reservoirs, and about the same as at the end of June at 6 reservoirs. For the 21 reservoirs with measurements reported at the end of July, water levels were below normal target pool or spillway level at 5 reservoirs, above normal target pool or spillway level at 13 reservoirs, and at about full pool level at 3 reservoirs.
Major Reservoirs. Compared to water levels at the end of June, at the end of July the water level at Lake Shelbyville was 1.1 feet higher, Carlyle Lake was 0.6 feet higher, and Rend Lake was 2.1 feet higher. At the end of July, Lake Shelbyville was 8.1 feet above the summer target level, Carlyle Lake was 2.4 feet above the summer target level, and Rend Lake was 4.2 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 July 2021 mean level for Lake Michigan was 580.7 feet. The monthly mean level one year ago (July 2020) was 582.2 feet. The long-term average lake level for July is 579.4 feet, based on 1918–2020 data. In this period of record, the lowest mean level for Lake Michigan for July occurred in 1964 at 576.7 feet, and the highest mean level for July occurred in 2020 at 582.2 feet. The month-end level of Lake Michigan was 580.8 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Comparison to Period of Record Shallow groundwater levels in 26 observation wells were slightly above the long-term average for July. Levels were 0.18 feet above average and ranged from 4.04 feet below to 3.41 feet above normal levels (Table 4).
Comparison to June 2021 Shallow groundwater levels were slightly below those of the previous month. Levels averaged 0.38 feet below and ranged from 2.56 feet below to 2.40 feet above June 2021 levels.
Comparison to July 2020 Shallow groundwater levels in June were below levels from one year ago. Levels averaged 0.86 feet below and ranged from 6.31 feet below to 1.18 feet above July 2020 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