Temperatures and precipitation were above the long-term average in Illinois in June. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures statewide averaged 74.1°F in June, 1.9° above the long-term average (Figure 1). The southwest crop reporting district (CRD) was the warmest with an average of 76.0°F. The lowest regional average temperature was 72.7°F, reported by the northeast CRD. Departures from average ranged from 0.6° above average in the southeast CRD to 3.1° above average in the northwest and northeast CRDs.
Precipitation statewide averaged 5.04 inches, 0.39 inches above the long-term average (Figure 1). The east CRD was the wettest with an average of 6.58 inches. The driest was the southwest CRD with an average of 2.72 inches. Departures from average ranged from 1.69 inches below average in the southwest CRD to 1.93 inches above average in the east CRD.
Soil moisture at 2 inches rose 7% on average in June. Moisture levels rose in all regions except in southern Illinois. Increases were observed at depths from 4 to 20 inches. Levels remained high and steady at 3 and 5 feet.
Mean provisional streamflow aggregated statewide was above the long-term median flow for June, about 150% of median (Figure 1). Monthly mean discharge values ranged from normal to above normal for June in most areas of Illinois. The Illinois River and the Mississippi River below the confluence of the Illinois River reached local flood stages at some locations by the end of the month.
Water surface levels at the end of June were below the full pool or target level at 5 of 22 reporting reservoirs. At the end of June, Lake Shelbyville was 7.0 feet above the summer target level, Carlyle Lake was 1.8 feet above the summer target level, and Rend Lake was 2.1 feet above the spillway level. Lake Michigan’s mean level was above its long-term mean for the month.
Shallow groundwater levels statewide were slightly below the long-term average in June with an average departure of 0.17 feet below the period of record (Figure 1). Levels averaged 0.09 feet below May 2021 and 0.32 feet below June 2020 levels.
Weather/Climate Information (Kevin Grady)
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.
June in Illinois was warmer and wetter than average across most of the state.
Temperatures averaged 74.1°F, 1.9° above the long-term statewide average for June (Table 1, Figure 2). Monthly average temperatures were above average in the low to mid-70s across the state, with departures ranging from around 1° above average in southern Illinois to around 4° above average in northern Illinois. These above average temperatures were primarily due to a warm first half of June. After a couple of cooler days to start the month, the rest of the first half of June was above average across the state. Parts of northern Illinois had over 8° above average temperatures in the first half of June thanks in part to drier soils there, while southern Illinois was closer to but still slightly above average. Nearly all stations in Illinois reached the 90s at least once in the middle part of the month. The warmest reading of the month, 99°F, was recorded at the Rockford Airport (Winnebago County) on June 11, the Springfield Airport (Sangamon County) on June 18, and near Belleville (St. Clair County) on June 19.
Temperatures then cooled for the second half of June, when most of Illinois was closer to average. Many stations reached their monthly minimum temperatures around June 22, generally ranging from the mid-40s in northern Illinois to the mid-50s in southern Illinois. A few stations, primarily in northern Illinois, recorded their monthly minimum temperatures during the first couple of days of June instead. The coldest reading of the month, 43°F, was recorded at stations in Henry, DuPage, McHenry, and DeKalb Counties on June 1.
Modified growing degree days (DD, base 50°, from April 1) ranged from around 1200 DD in northern Illinois to over 1500 DD in far southern Illinois. This was slightly above the long-term average across most of the northern half of Illinois, with the highest departures up to around 150 DD above average in the northeastern corner of the state. Most of the southern half of Illinois was slightly below the long-term average by up to around 50 DD below average.
Precipitation averaged 5.04 inches in June, 0.39 inches above the long-term statewide average (Table 1). June started dry across most of Illinois, with precipitation totals generally less than 2 inches in most areas outside of southeastern Illinois between June 1 and 20. For much of Illinois, these totals were 1–2 inches below average for this period. A shift in the atmospheric pattern toward the end of the month brought several rounds of heavy rain to parts of central and northeastern Illinois. Between June 23 and 30, most areas between I-70 and I-80 received at least 3–4 inches of rain, with some places receiving even higher amounts locally. Many stations in Bloomington and parts of McLean and surrounding counties received 7–10 inches of rain between June 25 and June 28, leading to serious flooding problems in the area and temporarily closing I-55 and I-74 due to flooding. These late-month rounds of heavy precipitation led to June totals of at least 6 inches across much of central Illinois and the southern areas of Chicagoland, at least 2–3 inches above average. Monthly totals in McLean County were as high as 12–13 inches, while a station near Monee (Will County) had the highest monthly total in the state with 14.73 inches. After receiving 2.70 inches below average in May, O’Hare Airport received 6.78 inches of rain in June, 2.68 inches above average.
Areas of northwestern Illinois, especially those near the Wisconsin border, largely missed out on the heavy precipitation at the end of June. Monthly totals in the northwestern corner of Illinois were as low as 1–3 inches, 3–4 inches below average at some stations. Southern Illinois also did not receive the heavy precipitation and largely had June totals of less than 4 inches, 1–3 inches below average.
Severe weather reports: The NOAA Storm Prediction Center recorded 103 severe weather reports for June in Illinois: 11 for tornadoes, 8 for hail, and 84 for wind. (Multiple reports can be generated for a single event.) There were more total storm reports in June in Illinois than there were during the entire spring season (March–May), when there were 87. Many of these June reports came during the second half of the month, with most of them being recorded north of I-70. One of the more notable severe events was a tornado that moved across southern DuPage County during the late evening hours of June 20 from Naperville to near Willow Springs. As confirmed by the National Weather Service, this tornado had a peak intensity of EF-3, making it the strongest tornado in the Chicagoland area since 2015.
Drought: As June began, the United States Drought Monitor (USDM) depicted abnormally dry (D0) conditions or worse across parts of northern Illinois. This included an area of severe drought (D2) in the northeastern corner of Illinois, which received much below average precipitation in the first half of 2021. There were also some pockets of D0 conditions in eastern Illinois. As the below average precipitation continued during the first part of June in northern Illinois, conditions continued to deteriorate, especially in northwestern Illinois, and the USDM expanded their depiction of D0–D2 conditions westward. On the June 15 map, an area of extreme drought (D3) was introduced in northeastern Illinois, covering most of Lake and McHenry Counties, the eastern half of Boone County, and parts of counties to their south. Very dry soils, low streamflows, and increasingly concerning crop stress in the area all led to the introduction of the D3 classification. This was only the fourth D3 event in Illinois since 2000. This was also the first D3 event since the very dry year 2012, when most of Illinois experienced extreme D3 drought (or worse) during the latter part of the summer.
Parts of northeastern Illinois received some heavy precipitation toward the end of June, enough to cause improvements in conditions in many areas. The D3 area was subsequently reduced to only include places near the Wisconsin border, with much of the rest of northeastern Illinois improving at least one category on the Drought Monitor map by the end of the month. The heavy precipitation also helped eliminate the D0 conditions in east central Illinois. Northwestern Illinois did not receive as much rain, and so there were fewer changes to the map there. Southern Illinois was also below average for June, leading to some pockets of D0 conditions being introduced in southwestern and southeastern Illinois near the end of the month.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information from 19 stations across the state.
Wind speeds averaged 5.4 mph in June, a decline of 1.2 mph from in May and 0.5 mph less than the network’s long-term average. ICN Bondville had the highest average at 8.5 mph. The highest recorded wind gust was 50.0 mph, reported by ICN Monmouth on June 18.
Air temperatures rose 14° from in May to an average of 75°, 3° warmer than the long-term average. The coolest temperature was 43°, recorded on June 1 at ICN St. Charles. ICN Brownstown had the month’s highest temperature, measuring 99° on June 18.
Soil temperatures increased 11–14° in June to temperatures averaging in the mid- to high 70s. Under bare soil, temperatures ranged from 52 to 121° at 2 inches and 55 to 101° at 4 inches. Temperatures under sod ranged from 58 to 96° at 4 inches and 58 to 97° at 8 inches.
Precipitation averaged 5.58 inches for the month, 1.40 inches higher than in May and 1.99 inches greater than the long-term average. Fifty-seven percent of the rain, 3.18 inches, fell in the last week between June 24 and 30. ICN Stelle received 6.21 inches over the seven days, pushing its monthly total to 9.81 inches, the highest in June.
Not all stations had a wet month, however. ICN Freeport reported only 1.31 inches for June, 2.77 inches less than average.
Soil moisture at 2 inches decreased for most of the month. Levels fell 27% on average in the first three weeks of June. However, the heavy rain at the end of June caused soil moisture to rise in all regions. The largest increases were in east central Illinois where moisture levels doubled in the last week of June. Only southern Illinois did not see an overall increase for the month. As a result, the soil moisture increased slightly overall at the 2-inch level.
Similar patterns were seen in the 4- to 20-inch depths, and soil moisture levels increased overall at all depths in June. Levels remained high at 3- and 5-foot depths, showing little change for the month.
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 June (approximately 150 percent of the median) and slightly above the mean for June (approximately 110 percent of the mean). Monthly mean discharge values in June ranged mainly from normal to above normal for the month. Monthly mean streamflow of the Vermilion River at Pontiac was much above normal for June.
Water-Supply Lakes and Major Reservoirs. Table 3 lists reservoirs in Illinois, their normal pool or target water surface elevation, and other data related to observed variations in water surface elevations. Reservoir levels are obtained from a network of cooperating reservoir operators who are contacted each month by ISWS staff for the current water levels. Reservoir levels are reported in terms of their difference from normal pool (or target level). The average of the month-end readings for the period of record is reported in terms of the difference from normal pool or target level (column 6 of Table 3), and the number of years of record for each reservoir also is given (column 7). Most reservoirs serve as public water supplies, with the exceptions noted in the last column.
Compared to end-of-May water levels at 21 reservoirs for which levels were reported last month and this month, reported end-of-June water levels were lower at 8 reservoirs, higher at 11 reservoirs, and about the same as at the end of May at 2 reservoirs. For the 22 reservoirs with measurements reported at the end of June, water levels were below normal target pool or spillway level at 5 reservoirs, above normal target pool or spillway level at 12 reservoirs, and at about full pool level at 5 reservoirs.
In Table 3, the target pool level (elevation) listed for Lake Decatur has been adjusted to reflect the current operating range of the system as posted by the City of Decatur. The target level listed in Table 3 for Lake Decatur represents the midpoint of the seasonal target level range. Water levels within + 0.5 feet are within the normal target operating range for Lake Decatur.
Major Reservoirs. Compared to water levels at the end of May, at the end of June the water level at Lake Shelbyville was 3.5 feet higher, Carlyle Lake was 0.5 feet higher, and Rend Lake was 1.0 foot lower. At the end of June, Lake Shelbyville was 7.0 feet above the summer target level, Carlyle Lake was 1.8 feet above the summer target level, and Rend Lake was 2.1 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 June 2021 mean level for Lake Michigan was 580.5 feet. The monthly mean level one year ago (June 2020) was 582.2 feet. The long-term average lake level for June is 579.3 feet, based on 1918–2020 data. In this period of record, the lowest mean level for Lake Michigan for June occurred in 1964 at 576.6 feet, and the highest mean level for June occurred in 2020 at 582.2 feet. The month-end level of Lake Michigan was 580.6 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Comparison to Period of Record
Shallow groundwater levels in 25 observation wells were slightly below the long-term average for June. Levels were 0.17 feet below average and ranged from 3.93 feet below to 4.44 feet above normal levels (Table 4).
Comparison to May 2021
Shallow groundwater levels were slightly below those of the previous month. Levels averaged 0.09 feet below and ranged from 2.57 feet below to 1.98 feet above May 2021 levels.
Comparison to June 2020
Shallow groundwater levels in June were slightly below levels from one year ago. Levels averaged 0.32 feet below and ranged from 8.04 feet below to 3.93 feet above June 2020 levels.
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, http://rivergages.com, 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://www.isws.illinois.edu/warm