September 2024 Overview
It was a warm and dry September in Illinois.
Temperatures averaged 68.6 degrees in September, 1.8 degrees above normal and 30th warmest on record statewide. September average temperatures ranged from the mid-60s in northern Illinois to the low 70s in southern Illinois.
Precipitation statewide in September was 2.78 inches, 0.57 inches below the 30-year normal and 41st driest on record statewide. Total September precipitation ranged from less than 1 inch in far northwest Illinois to over 8 inches in parts of southern Illinois.
Soil moisture at 2 inches averaged 0.29 water fraction by volume (wfv) at the end of September, 23% higher than at the beginning of the month. Increases were observed through the first 8 inches. Levels declined from 20 to 59 inches, ending the month 7 – 8% lower.
The mean streamflow statewide was slightly above the median for the month.
The mean provisional streamflow aggregated statewide was slightly above the long-term median flow for September, about 105% of the median (Figure 1). Monthly mean discharge values ranged mostly from below normal to above normal for September.
Water surface levels at the end of September were below the full pool or target level at 16 of 21 reporting reservoirs. At the end of September, Lake Shelbyville was 0.8 feet below the seasonal target level, Carlyle Lake level was 0.2 feet below the seasonal target level, and Rend Lake was 1.2 feet above the spillway level.
Lake Michigan’s mean level was slightly above its long-term mean for the month.
The mean streamflow statewide was slightly above the median for the month.
Shallow groundwater levels were 0.58 feet below the long-term average at the end of September. Levels averaged 0.52 feet below August and 1.88 feet 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.
September in Illinois was warmer and drier than normal statewide.
Temperatures averaged 68.6 degrees in September, 1.8 degrees above normal and 30th warmest on record statewide.
September average temperatures ranged from the mid-60s in northern Illinois to the low 70s in southern Illinois. Most of the state was between 1 and 5 degrees above normal, with the highest departures in the northern third of the state (Figure 1). High temperatures regularly reached the 90s in mid-September, including 96 degrees in Mt. Vernon and 95 in Peoria. Meanwhile, cooler air in early and later parts of the month brought nighttime low temperatures in the 30s across much of the state, including 36 in Elgin and 39 in Normal. The coolest point in the state was Stockton with an average temperature of 64.3 degrees and the warmest part of the state was Smithland Lock & Dam in Massac County with an average temperature of 72.9 degrees. Last month was the second warmest September on record in Chicago.
Precipitation statewide in September was 2.78 inches, 0.57 inches below the 30-year normal and 41st driest on record statewide.
September is not usually a very wet month across Illinois, and the first two-thirds of last month played to type with very dry conditions in the Midwest. However, September is also the heart of Atlantic hurricane season, and tropical weather broke up our dry streak, at least in southern Illinois, toward the end of the month. Total September precipitation ranged from less than 1 inch in far northwest Illinois to over 8 inches in parts of southern Illinois. Most areas of the state south of Interstate 64 were 1 to 5 inches wetter than normal for the month while much of central and northern Illinois were 1 to 3 inches drier than normal (Figure 2).
September was extremely dry in northwest Illinois. In fact, it was the second driest September on record just across the river in Dubuque Iowa, with only 0.08 inches total. It was also the second driest September on record on Illinois’ side of the river in Mount Carroll, with 0.23 inches total. Meanwhile, it was the fourth wettest September on record at the Carbondale Water Treatment Plant, with over 8 inches for the month.
Drought: The first three weeks of September were extremely dry across much of the Midwest. Figure 4 shows precipitation rankings from Sept. 1 to Sept. 20 across the region, with 1 indicating that period was the driest on record for that station. Much of the region had a top-five driest start to September on record, including the driest first 20 days of the month in Quincy, Danville, Moline, and Rockford. The dryness culminated in large expansion of moderate drought across Illinois in mid-September, including over 45% of the state in moderate to severe drought as of the Sept. 17 U.S. Drought Monitor.
The remnants of Hurricane Helene were pulled to the northwest and moved into the lower Ohio River Basin in the final week of the month. The tropical system brought widespread and, at times, heavy rain across southern Illinois. Three-day rainfall totals exceeded 6 inches in parts of southern Illinois; however, the previously dry soils helped absorb much of the rain and mitigate significant flooding in the region. The hurricane remnants provided widespread drought relief across southern Illinois, dramatically reducing drought extent and severity in the latest U.S. Drought Monitor. The heavy rain across the Ohio River Basin also helped improve streamflow along the Ohio and Mississippi Rivers. The rain temporarily assuaged low river level concerns and impacts to barge traffic; however, the Mississippi River at Memphis is forecasted to once again drop below low stage in the second week of October.
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 4.4 mph in September, equal to August’s average but 0.6 mph lower than the network’s long-term average. ICN Monmouth again had the windiest month with a monthly average of 7.6 mph. The highest wind gust was 40.6 mph, recorded at ICN Fairfield (Wayne County) on September 27.
Temperatures averaged 68° in September, a 5° decrease from August and 2° warmer than the long-term average. Station highs were in the low to mid 90s with lows in the high 30s to mid-40s. The month’s highest temperature was 94°, measured at ICN Fairfield (Wayne County) on September 21. The lowest was 38°, recorded at ICN St Charles (Kane County).
Soil temperatures fell 5 to 6° to averages in the low 70s. Under bare soils, temperatures ranged from 54 to 101° at 2 inches and 56 to 98° at 4 inches. Temperatures under sod ranged from 60 to 89° at 4 inches and 63 to 84° at 8 inches.
ICN averaged 3.13 inches of rain in September, 0.16 inches higher than the long-term average. The majority of this fell in the last ten days of the month. The network received 0.37 inches from September 1 – 20 and 2.76 inches from the 21 – 30.
The highest totals were at the southern stations where five stations had recorded more than five inches for the month. ICN Carbondale (Jackson County) received 7.77 inches, the month’s highest. More than six inches of the total were recorded on two days – September 23 (2.96 inches) and September 27 (3.10 inches).
However, September was much drier for the northern stations where ICN Freeport (Stephenson County) measured only 0.27 inches for the month.
Soil moisture levels at 2 inches rose 89% in southern Illinois, mainly due to the heavy rains at the end of the month. Increases were seen at 4 and 8 inches.
Moisture levels in southern Illinois declined overall in September. Levels at 2 inches were 20% lower at the end of the month than the beginning with similar declines seen from 4 to 20 inches.
There was no overall change in levels at the central stations.
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 slightly above the median value for September (approximately 105 percent of the median) and below the mean for September (approximately 50 percent of the mean). Monthly mean discharge values in September ranged mostly from below normal to above normal for the month. The monthly mean flow of the Illinois River at Valley City was much below normal for September.
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-August water levels at 20 reservoirs for which levels were reported last month and this month, reported end-of-September water levels were lower at 13 reservoirs, higher at 6 reservoirs, and about the same as at the end of August at 1 reservoir. For the 21 reservoirs with measurements reported for the end of September, water levels were below normal target pool or spillway level at 16 reservoirs, above normal target pool or spillway level at 3 reservoirs, and at about the target level or spillway level at 2 reservoirs.
Major Reservoirs. Compared to water levels at the end of August, at the end of September the water level at Lake Shelbyville was 0.2 feet lower, Carlyle Lake was 0.3 feet higher, and Rend Lake was 0.7 feet lower. At the end of September, Lake Shelbyville was 0.8 feet below the seasonal target level, Carlyle Lake level was 0.2 feet below 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 September 2024 mean level for Lake Michigan was 579.3 feet. The monthly mean level one year ago (September 2023) was 579.5 feet. The long-term average lake level for September is 579.2 feet, based on 1918-2023 data. In this period of record, the lowest mean level for Lake Michigan for September occurred in 1964 at 576.6 feet, and the highest mean level for September occurred in 1986 at 582.0 feet. The month-end level of Lake Michigan was 579.2 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables continued to decrease at most monitored wells in September.
Wells averaged 0.52 feet lower at the end of the month with 73% of wells reporting lower levels. The largest drops were in northern and west central Illinois in areas that received less than an inch of rain over the month. The Good Hope well (McDonough County) fell 2.91’, the month’s largest decline.
However, several southern wells saw significant increases due to heavy rain in the area at the end of the month. The Belleville well (St Clair County) rose 1.26’ and the Carbondale well (Jackson County) rose 2.49’, the month’s largest increase.
Wells averaged 1.88 feet above September 2023, 0.79 feet below the 15-year average, and 0.58 feet below 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/