* Table 2 was updated September 13, 2023.
Overview
July temperatures were equal to and precipitation was above the long-term average in Illinois. Mean streamflow statewide was below the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures averaged 75.4 degrees in July, right at the 30-year normal (Figure 1). July average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, within 1 degree of normal virtually everywhere in the state.
Precipitation in July was 4.59 inches, 0.53 inches wetter than normal (Figure 1). July precipitation ranged from around 2 inches in western Illinois to over 10 inches in parts of southern Illinois.
Soil moisture at 2 inches decreased 15 percent on average and decreased 7 and 6 percent at 4 and 8 inches, respectively. Moisture increased slightly or remained steady at depths of 20 inches and greater.
Mean provisional streamflow aggregated statewide was below the long-term median flow for July, about 75 percent of the median (Figure 1). Monthly mean discharge values in July ranged mostly from below normal to normal.
Water surface levels at the end of July were below the full pool or target level at 9 of 17 reporting reservoirs. At the end of July, Lake Shelbyville was 0.1 foot above the seasonal target level, Carlyle Lake was 0.1 foot above the seasonal target level, and Rend Lake was 1.2 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 1.60 feet below the long-term average at the end of July (Figure 1). Levels averaged 0.75 feet below June levels and 1.39 feet below last year’s levels.
Weather/Climate Information
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.
July had near normal average temperatures and was much wetter than normal in Illinois.
Temperatures averaged 75.4 degrees in July, right at the 30-year normal and the 62nd warmest on record statewide. July average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, within 1 degree of normal virtually everywhere (Figure 2, Table 1). The welcomed lack of humidity in the first two-thirds of the month allowed nighttime low temperatures to regularly dip into the low 60s and even 50s in some parts of the state. July nighttime lows included 51 degrees in Aurora, 53 in Chenoa, and 54 in Ottawa. Daily high temperatures were closer to normal, but rarely exceeded 90 degrees in the northern two-thirds of the state through most of July. The coolest place in the state last month was Stockton in Jo Daviess County with an average temperature of 70.5 degrees, and the warmest place in the state in July was Cahokia with an average temperature of 79.8 degrees.
While much of July was refreshingly mild, the heat came on strong in the last few days of the month. Daily high temperatures on July 27 and 28 exceeded 90 degrees across the state and topped 100 degrees in parts of western Illinois. Combined with extremely high humidity, heat index values pushed well over 100 degrees and even exceeded 110 to 115 degrees in some parts of central Illinois. Champaign reached a heat index of 115 degrees and Decatur had an incredibly dangerous 120-degree heat index on the afternoon of July 27. The latter number is only the third day on record with a 120+ degree heat index in Decatur and the first day since 1999.
Precipitation statewide in July was 4.59 inches, 0.53 inches wetter than normal and the 36th wettest on record statewide. July precipitation ranged from around 2 inches in western Illinois to over 10 inches in parts of southern Illinois. The southern and northeastern parts of the state were 2 to 6 inches wetter than normal last month because of very heavy precipitation events. Meanwhile, much of central Illinois was within 1 inch of normal and western Illinois was somewhat drier than normal (Figure 3, Table 1). Last month was the wettest July on record at Chicago’s Midway airport, following what was the fourth driest April to June on record there.
Severe weather: The prolonged dryness in May and June in the Chicagoland area was indeed broken by a very intense rainfall event from a series of thunderstorms on July 1 and 2. Among the highest 24-hour rainfall totals included 8.12 inches in Garfield Park, 8.6 inches in Cicero, and 8.96 inches in Berwyn. Based on the Illinois State Water Survey’s Bulletin 75 estimates, the event exceeded the estimated 1 percent annual exceedance probability storm in a few places. The heavy rain inundated stormwater systems and caused widespread flooding across the city, with thousands reporting flooded basements in Chicago alone.
Never to be outdone, southern Illinois decided to get in on the heavy rain action and experienced an incredibly intense series of training storms on July 18 and 19. Parts of Williamson, Johnson, Pope, Alexander, and Pulaski counties picked up over 8 inches of rain in less than 24 hours, with rainfall rates exceeding 3 inches per hour. Flash flooding occurred across the region, including at the Massac County Youth Fair.
Drought: July was the first wetter than normal month since March. In fact, April to June this year was the sixth driest on record statewide, marking how far into drought the state dove before last month. Consistent rain in southern and northern Illinois helped reduce drought conditions, and rain since the end of July in central Illinois has helped improve conditions there. As of August 8, moderate-to-severe drought remained in parts of western and northern Illinois; however, soil moisture and streamflow conditions have improved quite a bit since the early part of July. The rain in July and the first week of August will help make 2023 a decent crop year, but more consistent rain is needed through August and September to both (1) avoid impacts from low flow conditions on our larger rivers, and (2) help improve groundwater levels as we move into winter.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) consists of 19 stations across the state that collect hourly weather and soil information.
Winds decreased in July to a network average of 4.1 mph, 0.9 mph lower than in June and 0.7 mph lower than the long-term average. ICN Bondville (Champaign County) had the windiest month again with a 5.9 mph average. The highest recorded wind gust was 46.3 mph, measured at ICN Belleville (St. Clair County) on July 29.
Temperatures rose 3 degrees from in June to an average of 75 degrees, equal to the network’s long-term average for the month. Station highs were in the 90s and 100s. ICN Perry (Pike County) had the highest temperature, reporting 101 degrees on July 28. Lows were in the 50s for most stations, with ICN DeKalb (DeKalb County) measuring 51 degrees on July 19, the lowest of the month.
Soil temperatures rose 3 to 5 degrees to averages from the high 70s to low 80s. Under bare soils, temperatures at 2 inches ranged from 58 to 113 degrees and from 60 to 103 degrees at 4 inches. Temperatures under sod ranged from 68 to 103 degrees at 4 inches and 68 to 94 degrees at 8 inches.
Precipitation increased for most stations. ICN St. Charles (Kane County) recorded 10.60 inches in July, 9.51 inches more than in June and 7.08 inches higher than its long-term average. ICN Dixon Springs (Pope County) also had a wet month with 9.36 inches, 7.70 inches more than in June and 5.15 inches above the long-term average.
Conversely, three stations had a drier month than in June. ICN Brownstown (Fayette County) received only 1.33 inches in July, 0.50 inches less than in June and 2.45 inches below the long-term average.
Overall, the network averaged 4.79 inches in July, 3.00 inches more than in June and 1.00 inch greater than the long-term average.
Soil moisture was higher at the beginning of July because of rains in the last week of June. Levels at 2 inches averaged 0.25 water fraction by volume (wfv) across the network on July 1 and declined 15 percent over the month. Most of the drying occurred in central Illinois, where levels fell more than 40 percent. At the northern stations, soil moisture rose 29 percent, and a 5 percent increase was seen in the south.
Similar patterns occurred through 8 inches. Soil moisture increased slightly or remained steady at depths of 20 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 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 below the median value for July (approximately 75 percent of the median) and below the mean for July (approximately 50 percent of the mean). Monthly mean discharge values in July ranged mostly from below normal to normal. Monthly mean discharge values were above normal in far southern Illinois.
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-June water levels at 14 reservoirs for which levels were reported last month and this month, reported end-of-July water levels were lower at 6 reservoirs and higher at 8 reservoirs. For the 17 reservoirs with measurements reported for the end of July, water levels were below normal target pool or spillway level at 9 reservoirs, above normal target pool or spillway level at 7 reservoirs, and at about the full pool level at 1 reservoir. Salem Lake inflow was supplemented by occasional pumping from Carlyle Lake in July.
Major Reservoirs. Compared to water levels at the end of June, at the end of July the water level at Lake Shelbyville was 0.1 foot higher, Carlyle Lake was 0.1 foot higher, and Rend Lake was 0.6 feet lower. At the end of July, Lake Shelbyville was 0.1 foot above the seasonal target level, the Carlyle Lake level was 0.1 foot above the seasonal target level, and Rend Lake was 1.2 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 2023 mean level for Lake Michigan was 579.7 feet. The monthly mean level one year ago (July 2022) was 580.1 feet. The long-term average lake level for July is 579.4 feet, based on 1918–2022 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 579.8 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 July, despite the increased rains. Twenty-two of the 25 wells were lower at the end of July as statewide levels averaged 0.75 feet lower than in June.
The average decline was less than that in June when water levels fell 1.75 feet. At 19 wells, monthly declines were less than in June, while three wells increased in July. This is not unexpected, even with the increased rain, since improvements in groundwater generally follow those in precipitation and soil moisture. Also, over half of the state was still in drought at the end of the month.
The largest drop was in western Illinois where the Perry well (Pike County) was 3.19 feet lower at the end of July than at the end of June. Meanwhile, the Galena well (Jo Daviess County) rose 1.88 feet, the month’s highest increase.
Wells averaged 1.39 feet below July 2022 levels, 2.18 feet below the 15-year average, and 1.60 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://warm.isws.illinois.edu/warm/
