May 2022 Overview
May temperatures were above and precipitation was below the long-term average in Illinois. Mean streamflow statewide was above the median for the month. Shallow groundwater levels were below the long-term depths.
Air temperatures averaged 65.3 degrees F, 2.1 degrees above the 1991–2020 normal for May (Figure 1). Monthly average temperatures ranged from the low 60s in northern Illinois to the high 60s in southern Illinois, between 1 and 5 degrees above normal.
Precipitation statewide in May was 4.07 inches, 0.70 inches below the long-term statewide average (Figure 1). May totals ranged from just under 2 inches in southeast Illinois to over 7 inches on the north side of the St. Louis Metro East.
Soil moisture at 2 inches declined 22 percent in May to a state average of 0.28 water fraction by volume (wfv). Drying continued into the 8-inch depths, but moisture levels remained high at 20 inches and deeper.
Mean provisional streamflow aggregated statewide was above the long-term median flow for May, about 120 percent of median (Figure 1). Monthly mean discharge values in May were normal to above normal.
Water surface levels at the end of May were below the full pool or target level at 6 of 22 reporting reservoirs. At the month’s end, Lake Shelbyville was 0.8 feet above the June 1 target level, Carlyle Lake’s level was 0.8 feet above the June 1 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 0.54 feet below the long-term average at the end of May (Figure 1). Levels averaged 0.63 feet below those in April and 0.19 feet above last year’s levels.
Weather/Climate Information
The following description of temperatures, 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.
May in Illinois was warmer and drier than average statewide.
Temperatures averaged 65.3 degrees F, 2.1 degrees above the 1991–2020 normal for May (Table 1a, Figure 2). Monthly average temperatures ranged from the low 60s in northern Illinois to the high 60s in southern Illinois, between 1 and 5 degrees above normal. The hottest point in the state was Cairo with an average May temperature of 69.4 degrees, and the coldest part of the state was Waukegan with an average May temperature of 58.9 degrees.
While the first week of May delivered the same cooler weather as in April, temperatures quickly ramped up in the second week. In fact, the week of May 9 to 15 was one of the warmest on record for May statewide, with temperatures persistently 8 to 20 degrees above normal. Rockford broke daily high daytime and daily high nighttime temperatures in four consecutive days that week, and both Rockford and Chicago recorded their earliest 70-degree nighttime low temperature on record. The heat was accompanied by summer-like humidity, and dewpoint temperatures regularly reached into the upper 60s and low 70s. Rockford reached its earliest
75-degree dewpoint temperature on record back to 1959. Peoria recorded its earliest 73-degree nighttime low temperature on record back to 1883, which beat the previous record by two weeks.
Altogether, daily high maximum temperature records were broken last month at 131 stations, and daily high minimum temperature records were broken at 125 stations. Three stations broke their all-time May high
temperature records, including a 96-degree high on May 11 in Stockton in Jo Daviess County. Eight stations broke the all-time May high nighttime temperature records, including a 74-degree low in Rockford on May 12.
Daily low maximum temperature records were broken last month at 17 stations, and daily low minimum temperature records were broken at 3 stations.
Modified growing degree days (DD, base 50°, from May 1) ranged from around 300 DD in northern Illinois to around 400 DD in southern Illinois. Degree day accumulation this season has so far (from May 1) been near to 10–20 DD above normal in northern Illinois, and 40–50 DD above normal in southern Illinois.
Precipitation statewide in May was 4.07 inches, 0.70 inches below the long-term statewide average (Table 1a). May totals ranged from just under 2 inches in southeast Illinois to over 7 inches on the north side of the St. Louis Metro East. The wettest point in the state in May was Granite City in Madison County, with just under 8 inches of rain in total. In contrast, Fairfield in Wayne County received just 2.05 inches total in May. The lack of widespread, heavy rain helped move spring fieldwork along very quickly following serious delays in April. Meanwhile, most parts of the state received enough rain to keep soil moisture adequate to surplus to stave off drought.
Severe weather reports: The NOAA Storm Prediction Center recorded 64 severe weather reports for May in Illinois: 11 for tornadoes, 15 for hail, and 38 for wind. (Multiple reports can be generated for a single event). A system of thunderstorms produced nine tornadoes in and around the St. Louis area on May 19, including three tornadoes in southwest Illinois. There were several reports of damage to trees and buildings in Summerfield, Okawville, and Breese but no reported injuries or fatalities. The same storms produced damage from strong winds and hail.
Drought: All three spring months were wetter than normal in northeast Illinois, helping to knock out drought in that region. Parts of McHenry and Lake Counties moved from severe drought in the U.S. Drought Monitor on March 1 to no drought by the end of May. Meanwhile, patches of abnormally dry conditions remain throughout northern, east-central, and southeastern Illinois, but conditions are much improved relative to last year.
Spring (March–May) was overall warmer than normal across most of Illinois. Seasonal temperatures averaged 52.7 degrees, 0.2 degrees above the 1991–2020 normal (Table 1b), ranging from the high 40s in northern Illinois to the mid-50s in southern Illinois. Spring temperatures were within 0.5 degrees of normal in northern Illinois and between 0.5 and 1.0 degrees above normal in southern Illinois.
Spring precipitation averaged 11.97 inches statewide, 0.02 inches above normal (Table 1b). Seasonal totals were highest in southern Illinois and lower overall in northern Illinois. Spring total precipitation ranged from just under 14 inches in southeast Illinois to just over 10 inches in northern Illinois. Only the east-central and southeast climate divisions were drier than normal in climatological spring.
Illinois Climate Network (ICN) (Jennie Atkins)
The Illinois Climate Network (ICN) collects hourly weather and soil information from 19 stations across the state.
Winds were calmer in May, declining 1.4 mph from in April to a monthly average of 7.7 mph, or 0.5 mph higher than the network’s long-term average. ICN Bondville (Champaign County) was the windiest station with a monthly average of 11.9 mph. The highest recorded wind gust was 53.3 mph, reported at ICN Snicarte (Mason County) on May 25.
May temperatures varied greatly throughout the month with a more than 60-degree difference between the monthly maximum and minimum. However, the overall average was near normal at 66 degrees, 15 degrees higher than in April and 3 degrees warmer than the long-term average.
Stations reported monthly lows in the 30s and 40s. The month’s coolest temperature was 35 degrees measured at ICN Freeport on May 4. Highs were in the 90s at all stations with the highest temperature measured at 97 degrees at ICN Big Bend on May 12.
The warmer air temperatures led to rising soil temperatures. Soils ranged from 70 to 77 degrees at the end of May, an increase of 13 to 20 degrees over the month. Under bare soil, temperatures ranged from 39 to 107 degrees at 2 inches and 41 to 97 degrees at 4 inches. Temperatures under sod ranged from 47 to 88 degrees at 4 inches and 48 to 80 degrees at 8 inches.
Precipitation was near normal again in May, averaging 3.89 inches, or 0.11 inches more than in April and 0.13 inches less than the long-term average. Northern Illinois continued to see wetter weather with all stations reporting higher than normal monthly totals. ICN St. Charles (Kane County) had May’s highest total, recording 6.38 inches for the month.
Soils continued to dry in May. Soil moisture at 2 inches declined 22 percent in the month to an average of 0.28 water fraction by volume (wfv). Eastern Illinois had the largest decline with a 34 percent drop. However, despite the drying, moisture levels remained above the wilting points.
Drying continued into the 8-inch depths. However, soil moisture remained high at depths of 20 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 the past record of monthly mean flows at those stations for the same month. Both recent and long-term data are retrieved from 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 above the median value for May (approximately 120 percent of the median) and below the mean for May (approximately 90 percent of the mean). Monthly mean discharge values in May were normal to 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 the 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-April water levels at 20 reservoirs for which levels were reported last month and this month, reported end-of-May water levels were lower at 11 reservoirs, higher at 5 reservoirs, and about the same as at the end of April at 4 reservoirs. For the 22 reservoirs with measurements reported at the end of May, water levels were below the normal target pool or spillway level at 6 reservoirs, above normal the target pool or spillway level at 11 reservoirs, and at about the full pool level at 5 reservoirs.
Major Reservoirs. Compared to water levels at the end of April, at the end of May the water level at Lake Shelbyville was 2.8 feet higher, Carlyle Lake was 1.6 feet higher, and Rend Lake was 1.5 feet lower. At the end of May, Lake Shelbyville was 0.8 feet above the June 1 target level, Carlyle Lake level was 0.8 feet above the June 1 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 May 2022 mean level for Lake Michigan was 579.9 feet. The monthly mean level one year ago (May 2021) was 580.5 feet. The long-term average lake level for May is 579.1 feet, based on 1918–2021 data. In this period of record, the lowest mean level for Lake Michigan for May occurred in 1964 at 576.6 feet, and the highest mean level for May occurred in 2020 at 582.0 feet. The month-end level of Lake Michigan was 580.1 feet. All values are provided by the U.S. Army Corps of Engineers Detroit District.
Groundwater Information (Jennie Atkins)
Water table levels continued to decline at most WARM monitoring wells in May. Seventy-six percent of wells had end-of-month levels that were lower than in April. On average, levels were 0.63 feet below those of the previous month.
Northern well levels were up and down. Water levels rose at several of the wells in May, including Freeport (Stephenson County), Mt. Morris (Ogle County), and St. Charles (Kane County). However, levels fell at other wells. The Galena well (Jo Daviess County) declined 3.19 feet in May.
Water tables were slightly higher than last year’s levels, averaging 0.19 feet above those in May 2021 and ranging from 3.41 feet below to 3.66 feet above.
Levels averaged 1.00 foot below the 15-year average and 0.54 feet below the period of 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://www.isws.illinois.edu/warm