April 2024 Overview
April was warmer and wetter than normal for Illinois. Mean streamflow was above the median for the month. Shallow groundwater levels were near the long-term average.
Temperatures averaged 55.0°F, 2.4° above the 1991-2020 normal for April and the 18th warmest April on record statewide (Figure 1). April average temperatures ranged from the high 40s in northern Illinois to the low 60s in southern Illinois, between 1 and 4 degrees above normal.
Precipitation statewide in April was 5.91 inches, 1.67 inches above normal and the 11th wettest April on record statewide (Figure 1). April total precipitation ranged from around 3 inches in northeast Illinois to over 10 inches in southwest Illinois.
Soil moisture at 2” averaged 0.41 water fraction by volume (wfv) at the end of April, rising 19% over the month.
Mean provisional streamflow aggregated statewide was above the long-term median flow for April, about 165% of median (Figure 1). Monthly mean discharge values in April ranged mostly from normal to much above normal.
Water surface levels at the end of April were below the full pool or target level at 3 of 22 reporting reservoirs. At the end of April, Lake Shelbyville was 0.2 feet above the May 1 target level, Carlyle Lake level was 2.2 feet above the May 1 target level, and Rend Lake was 2.3 feet above the spillway level.
Lake Michigan’s mean level was slightly above its long-term mean for the month.
Shallow groundwater levels were 0.07’ above the long-term average at the end of April (Figure 1). Levels averaged 0.97’ above March and 0.09’ last year.
Weather/Climate Information (Trent Ford)
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.
April in Illinois was much warmer and wetter than normal.
Temperatures averaged 55.0°F, 2.4° above the 1991-2020 normal for April and the 18th warmest April on record statewide (Table 1). April average temperatures ranged from the high 40s in northern Illinois to the low 60s in southern Illinois, between 1 and 4 degrees above normal (Figure 2). The prolonged period of warm weather in the middle of the month broke 14 daily high maximum temperature records and 22 daily high minimum temperature records in the state. Meanwhile, the brief shots of colder air in April broke nine daily low maximum temperature records and six daily low minimum temperature records.
Most places saw highs briefly reach into the mid-80s in the middle of the month, and a few stations around the St. Louis Metro East area flirted with 90 degrees. In contrast, most places in northern Illinois saw several nights with low temperatures dipping well into the 20s, including 21 degrees in Elgin on April 20th. Stockton was the coldest place in the state last month with an average temperature of 48.9 degrees, and Olmsted was the warmest place with an average temperature of 61.7 degrees.
Precipitation statewide in April was 5.91 inches, 1.67 inches above normal and the 11th wettest April on record statewide (Figure 3, Table 1). April total precipitation ranged from around 3 inches in northeast Illinois to over 10 inches in southwest Illinois (Figure 3). Most of the state between Interstates 88 and 64 was 1 to 5 inches wetter than normal last month, while far southern Illinois and the northern half of Chicagoland were within 1 inch of normal April rainfall. Highland in Madison County took the April precipitation crown with 9.58 inches total, the third wettest April on record there.
Snow: The first 80-degree day in many parts of Illinois is on par with any of the great weather gifts we have, like a Christmas morning snow or an October afternoon with winds below 20 mph. But if you’ve lived in Illinois for more than a year, you should be aware of the snow that inevitably follows that first taste of summer temperatures. Alas, a storm system snuck measurable snow into northern Illinois on April 2nd and 3rd. Totals ranged from just over a tenth of an inch as far south as Peoria to over 4 inches in eastern Jo Daviess County.
Thankfully, the latter half of the month came without measurable snowfall, leaving most of the state south of Interstate 74 snowless for April. The likely end-of-season snowfall in Illinois is only near to above normal in the northwest corner, thanks to a very heavy January snowstorm. Meanwhile, the northeast part of the state was 10 to 20 inches below normal snowfall in the 2023-24 snow season.
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 8.7 mph in April, 0.1 mph lower than March and equal to the long-term average. ICN Monmouth (Warren County) had the windiest month with a monthly average of 13.4 mph. ICN Bondville (Champaign County) had the highest recorded wind gust, 53.4 mph, measured on April 18.
Temperatures in April were warmer than normal, averaging 56° or 3° higher than the network’s long-term average. ICN Rend Lake (Jefferson County) had the month’s highest temperature, recording 90° on April 15. The lowest temperature was 26°, measured at ICN DeKalb (DeKalb County) on April 21.
Soil temperatures rose 14 to 16° in April to averages in the mid-50s. Under bare soils, temperatures at 2 inches ranged from 34 to 91° and from 35 to 81° at 4 inches. Temperature under sod ranged from 40 to 77° at 4 inches and 40 to 71° at 8 inches.
April was a wet month with an average of 5.71” for the network, 2.50” higher than March and 1.75” above the long-term average. Fourteen out of nineteen stations had totals greater than 5 inches. ICN Olney (Richland County) reported 8.15 inches, the month’s highest total.
Soil moisture increased for much of the network in April. Moisture levels at 2 inches rose 19%, on average, ending the month in southern and central Illinois above field capacity for most of the soils monitored. Only the northern stations saw decreases as the regional average ended April 16% lower.
Similar patterns occurred through 20 inches. Soil moisture remained high at 39 and 59 inches.
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 April (approximately 165 percent of the median) and above the mean for April (approximately 135 percent of the mean). Monthly mean discharge values in April ranged mostly from normal to much above normal.
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-March water levels at 22 reservoirs for which levels were reported last month and this month, reported end-of-April water levels were lower at 2 reservoirs, higher at 16 reservoirs, and about the same as at the end of March at 4 reservoirs. For the 22 reservoirs with measurements reported for the end of April, water levels were below normal target pool or spillway level at 3 reservoirs, above normal target pool or spillway level at 16 reservoirs, and at about the target level or spillway level at 3 reservoirs.
Major Reservoirs. Compared to water levels at the end of March, at the end of April the water level at Lake Shelbyville was 5.8 feet higher, Carlyle Lake was 3.9 feet higher, and Rend Lake was 0.9 feet higher. At the end of April, Lake Shelbyville was 0.2 feet above the May 1 target level, Carlyle Lake level was 2.2 feet above the May 1 target level, and Rend Lake was 2.3 feet above the spillway level. (The target operating levels at Lake Shelbyville and Carlyle Lake increase from April to May.)
Great Lakes. Current month mean and end-of-month values are provisional and are relative to International Great Lakes Datum 1985. The April 2024 mean level for Lake Michigan was 578.9 feet. The monthly mean level one year ago (April 2023) was 579.3 feet. The long-term average lake level for April is 578.8 feet, based on 1918-2023 data. In this period of record, the lowest mean level for Lake Michigan for April occurred in 1964 at 576.2 feet, and the highest mean level for April occurred in 2020 at 581.7 feet. The month-end level of Lake Michigan was 579.1 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables continued to improve in April. Sixty eight percent of the monitored wells rose over the month to an average increase of 0.97’ (Table 4).
The Mt Morris well (Ogle County) rose 3.77’, the month’s highest. Six other wells recorded gains of 2’ or greater. However, two wells – Fermi Lab (DuPage County) and DeKalb (DeKalb County) – ended the month more than a foot lower.
Wells averaged 0.09’ above April 2023, 0.31’ below the 15-year average, and 0.07’ above 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/