April 2023 Overview
April temperatures were above and precipitation was below 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 53.4 degrees F, 0.8 degrees above the 1991–2020 normal for April (Figure 1). Temperatures ranged from the high 40s in northern Illinois to the high 50s in southern Illinois, less than 2 degrees above normal statewide.
Precipitation statewide in April was 2.44 inches, 1.80 inches below normal (Figure 1). Total April precipitation ranged from nearly 8 inches in parts of southern Illinois to around 1 inch in parts of east-central Illinois.
Soil moisture at 2 inches declined 34 percent to an average of 0.27 water fraction by volume (wfv). Similar declines were seen at 4 and 8 inches while levels remained high at depths of 39 inches and greater.
Mean provisional streamflow aggregated statewide was below the long-term median flow for April, about 90 percent of median (Figure 1). Monthly mean discharge values in April ranged mostly from normal to above normal.
Water surface levels at the end of April were below the full pool or target level at 7 of 21 reporting reservoirs. At the end of April, Lake Shelbyville was 3.4 feet below the May 1 target level, Carlyle Lake was 0.8 feet below the May 1 target level, and Rend Lake was 4.6 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels were 0.08 feet below the long-term average at the end of April (Figure 1). Levels averaged 1.28 feet below those in March and 0.18 feet below last year’s levels.
April in Illinois was warmer and much drier than normal.
Precipitation statewide in April was 2.44 inches, 1.80 inches below normal and the 29th driest April on record statewide (Figure 3, Table 1). The first half of the month was very dry, and the week of April 8 to 13 was completely dry across the entire state. The last time the entire state measured exactly 0 inches of precipitation for a full seven days was November 11–17, 1999, and it looks like that week last month may be one of–if not the only–completely dry April week on record in Illinois. It is very rare to have such a dry spell in spring in Illinois. Combined with the unusually high temperatures and very low humidity, the dry weather in early April had farmers clamoring to get as much work done as possible.
Snow: The northern third of the state picked up between 0.10 and 2 inches of snow last month, slightly more than normal in northwest Illinois and near to slightly less than normal in northeast and north-central Illinois. The central and southern parts of the state were spared the snowfall, although trace amounts were reported as far south as Decatur in mid-April.
This past snowfall season, from October 1 to April 30, was the third least snowy on record in Springfield (6.3 inches) and the second least snowy on record in Champaign (7.0 inches).
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 averaged 8.0 mph in April, 0.7 mph lower than in March and the historical long-term average. ICN Monmouth (Warren County) was the windiest station with a monthly average of 12.4 mph. The highest recorded wind gust was 55.5 mph, measured at ICN Snicarte on April 15.
Temperatures averaged 54 degrees in April, 13 degrees warmer than in March and 1 degree higher than the long-term average. The month began with highs in the 80s for the central and southern stations. On April 4, ICN Perry (Pike County) recorded a daily high of 86 degrees, the month’s warmest. However, temperatures fell to freezing toward the end of the month as colder weather moved into the state. ICN Bondville (Champaign County) recorded a low temperature of 26 degrees on April 24, the lowest of the month.
Soil temperatures rose 10 to 12 degrees from March to averages in the mid-50s. Under bare soils, temperatures ranged from 32 to 85 degrees at 2 inches and 35 to 80 degrees at 4 inches. Temperatures under sod ranged from 39 to 75 degrees at 4 inches and 40 to 67 degrees at 8 inches.
April was drier at most ICN stations. Precipitation averaged 2.46 inches, 2.74 inches lower than in March and 1.50 inches lower than the long-term average. The central stations reported the lowest totals with ICN Stelle (Ford County) reporting only 0.80 inches, April’s lowest total. ICN Rend Lake (Jefferson County) received 4.78 inches, the month’s highest.
April began with damp soils due to a wet March. However, the month’s warmer, drier weather caused soil moisture to decline across ICN. Soil moisture at 2 inches decreased 34 percent, on average, in April to a network average of 0.27 water fraction by volume (wfv). Declines were higher at the central stations as levels fell 49 percent in west-central Illinois to 0.18 wfv, slightly above the wilting points of the soils monitored.
Similar decreases were seen at 4 and 8 inches, falling 27 and 19 percent on average, respectively, over the month. Levels remained high at depths of 39 inches and greater, showing no significant changes in April.
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 the 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 April (approximately 90 percent of the median) and below the mean for April (approximately 75 percent of the mean). Monthly mean discharge values in April ranged mostly from normal to above normal. The monthly mean flows of the Vermilion River at Pontiac and the LaMoine River at Ripley were below normal for April.
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 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-March water levels at 21 reservoirs for which levels were reported last month and this month, reported end-of-April water levels were lower at 17 reservoirs, higher at 3 reservoirs, and about the same as at the end of March at 1 reservoir. For the 21 reservoirs with measurements reported for the end of April, water levels were below normal target pool or spillway level at 7 reservoirs, above normal target pool or spillway level at 12 reservoirs, and at about full pool level at 2 reservoirs.
Major Reservoirs. Compared to water levels at the end of March, at the end of April the water level at Lake Shelbyville was 2.1 feet lower, Carlyle Lake was 3.6 feet lower, and Rend Lake was 0.8 feet lower. At the end of April, Lake Shelbyville was 3.4 feet below the May 1 target level, Carlyle Lake level was 0.8 feet below the May 1 target level, and Rend Lake was 4.6 feet above the spillway level. (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 2023 mean level for Lake Michigan was 579.3 feet. The monthly mean level one year ago (April 2022) was 579.8 feet. The long-term average lake level for April is 578.8 feet, based on 1918–2022 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 579.5 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water tables decreased at 21 of the 25 monitored wells in April, ending the month on average 1.28 feet below March levels (Table 4).
The Fermi Lab (DuPage County) well fell 2.74 feet in April, the month’s largest decrease. April is the second month in which water levels declined in the well from February’s 12-month high. The Fermi Lab well was 2.68 feet below its April 2022 levels and 1.15 feet below the 15-year average.
Statewide, wells averaged 0.18 feet below April 2022 levels. Most wells showed overall declines or small increases (< 0.5 feet) over the past year. However, two wells in northern Illinois have had well levels increase more than 7 feet. The Freeport (Stephenson County) well has risen 7.72 feet, and the Mt. Morris (Ogle County) well has risen 13.18 feet since April 2022.
Wells averaged 0.55 feet below the 15-year average and 0.08 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/