July 2024 Overview
July was cooler and wetter than normal statewide.
Temperatures averaged 74.1 degrees in July, 1.3 degrees above below and 35th coolest July on record statewide (Figure 1). July average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, between 1 and 2 degrees below normal for most of the state
Precipitation statewide in July was 6.36 inches, 2.30 inches above average and the 7th wettest July on record statewide (Figure 1). Total July precipitation ranged from around 4 inches in parts of northeast Illinois to over 12 inches in south-central Illinois, between 1 and 7 inches above normal.
Soil moisture at 2 inches averaged 0.35 water fraction by volume (wfv) at the end of July, 53% higher than the beginning of the month. Increases 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 July, about 210% of median (Figure 1). Monthly mean discharge values ranged mostly from normal to much above normal for July.
Water surface levels at the end of July were below the full pool or target level at 7 of 17 reporting reservoirs. At the end of July, Lake Shelbyville was 1.3 feet above the summer target level, Carlyle Lake level was 2.6 feet above the summer target level, and Rend Lake was 3.6 feet above the spillway level.
Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 0.21’ above the long-term average at the end of July (Figure 1). Levels averaged 0.72’ above June and 1.80’ 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.
July was cooler and wetter than normal statewide.
Temperatures averaged 74.1 degrees in July, 1.3 degrees above below and 35th coolest July on record statewide (Table 1). July gave us a mixed bag of temperatures, but more days last month were cooler than normal. Temperatures in the third week of the month were consistently 3 to 10 degrees below normal. While last month also brought some intense heat, it was fleeting.
July average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, between 1 and 2 degrees below normal for most of the state (Figure 2). Persistent northwest flow over the big ridge out west helped keep weather cooler and less humid for much of the month. Impressively cool July nighttime lows included 47 degrees in Taylorville and in Joliet. The coolest place in the state last month was Stockton in Jo Daviess County with an average temperature of 69.8 degrees, and the warmest place in the state in July was Carbondale with an average temperature of 77.0 degrees.
Precipitation statewide in July was 6.36 inches, 2.30 inches above average (Table 1) and the 7th wettest July on record statewide.
Total July precipitation ranged from around 4 inches in parts of northeast Illinois to over 12 inches in south-central Illinois, between 1 and 7 inches above normal (Figure 3). Several spots in the St. Louis Metro East area picked up more than 13 inches of rain in July, including an incredible 16 inches in Mascoutah. In fact, last month was the wettest July on record in St. Clair County, likely surpassing the previous record by more than 1 inch.
Severe Weather: Illinois was starting to get a little dry after a drier than normal June. In fact, over half the state was considered abnormally dry in the U.S. Drought Monitor on July 2. The remnants of one of the strongest early season Atlantic hurricanes on record, Beryl, bent up through the Midwest, producing significant rainfall over a 48-hour period across parts of central and east-central Illinois. Parts of Champaign and Douglas counties picked up more than 4 inches in two days from Beryl, dramatically improving soil moisture and streamflow.
Less than a week later, an intense storm system moved across the northern half of the state, producing very heavy rainfall and a derecho that brought severe straight-line winds. Most of the state picked up at least 1 inch of rain over the 48-hour period between July 14 and 16, and multiple spots had over 6 inches, including just over 8 inches in Fulton County. Heavy rain in St. Clair and Washington counties caused significant flooding and forced the opening of a dam spillway near Nashville.
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 4.3 mph, 1.8 mph lower than June and 0.7 mph lower than the network’s long-term July average. ICN Monmouth (Warren County) again had the windiest month with an average of 7.0 mph. The highest recorded wind gust was 70.4 mph, measured at ICN St Charles (Kane County) on July 14.
Temperatures rose 3° from June to a July average of 78°, 3° warmer than the long-term average. Station highs were in the high 80s to mid 90s with lows in the 40s and 50s. The month’s highest temperature was 94°, recorded at ICN Fairfield (Wayne County) on July 15. The lowest was 46°, measured at ICN Bondville (Champaign County) on July 1.
Soil temperatures rose 1 to 3° in July to averages in the high 70s and low 80s. Under bare soils, temperatures ranged from 59 to 111° at 2 inches and 62 to 104° at 4 inches. Temperatures under sod ranged from 67 to 99° at 4 inches and 67 to 92° at 8 inches.
July was a wet month across the state. The network averaged 7.43 inches for the month, 3.64 inches above its long-term average. Sixteen stations reported totals higher than normal with ICN Belleville (St Clair County) recording 15.95 inches, the month’s highest total.
Soil moisture at 2 inches increased 53% in July to end of month network average of 0.35 water fraction by volume (wfv). The highest increases were in southern Illinois where moisture levels more than doubled. Northern stations, however, remained relatively steady with levels declining 3% overall.
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 July (approximately 210 percent of the median) and above the mean for July (approximately 140 percent of the mean). Monthly mean discharge values in July ranged mostly from normal to much above normal. Monthly mean discharge values were below normal for July in some central Illinois watersheds.
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 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 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 17 reservoirs for which levels were reported last month and this month, reported end-of-July water levels were lower at 2 reservoirs, higher at 12 reservoirs, and about the same as at the end of June at 3 reservoirs. For the 17 reservoirs with measurements reported for the end of July, water levels were below normal target pool or spillway level at 7 reservoirs, above normal target pool or spillway level at 9 reservoirs, and at about the target level or spillway level at 1 reservoir.
Major Reservoirs. Compared to water levels at the end of June, at the end of July the water level at Lake Shelbyville was 1.0 foot higher, Carlyle Lake was 2.6 feet higher, and Rend Lake was 1.1 feet higher. At the end of July, Lake Shelbyville was 1.3 feet above the summer target level, Carlyle Lake level was 2.6 feet above the summer target level, and Rend Lake was 3.6 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 July 2024 mean level for Lake Michigan was 579.8 feet. The monthly mean level one year ago (July 2023) was 579.7 feet. The long-term average lake level for July is 579.4 feet, based on 1918-2023 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 increased at most monitored wells in July
Wells ended the month 0.72’ higher with 75% of monitored wells reporting increases. The largest increase was at the Belleville well (St Clair County) which rose 3.41’ in July. The well is located at the ICN Belleville station. The station recorded over 15 inches for the month. The SWS #2 well, also located in St Clair County, rose 2.07’.
Levels decreased at six wells. The largest decline was at the Springfield well (Sangamon County) which was 2.04’ lower at the end of July.
Wells averaged 1.80’ above July 2023, 0.34’ below the 15-year average, and 0.21’ above 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/