Author: Trent Ford

The preliminary statewide average August temperature was 73.6 degrees, right at the 1991-2020 average and tied for the 63^rd warmest on record going back to 1895. The preliminary statewide average total August precipitation was 2.96 inches, 0.63 inches below the 1991-2020 average and the 49^th wettest on record statewide.

Data are provisional and may change slightly over time.

Big August Heat, but Mild Weather Overall

The final month of climatological summer had mild temperatures overall, except for a very intense heat wave in the final few days of the month. Daily average temperature departures from Galesburg show days in the middle half of the month were 3 to 10 degrees below normal (Figure 1). However, temperatures between Aug. 25 and Aug. 30 were 5 to 12 degrees above normal, bringing back a taste of mid-summer.

August average temperatures ranged from the low 70s in northern and central Illinois to the high 70s in southern Illinois, around 1 degree above normal in most of northern and southern Illinois, and slightly below normal in central Illinois (Figure 2). Most parts of the state pushed into the mid to upper 90s in the final week of the month, and a few spots saw temperatures above 100 degrees, including in Quincy. The intense heat was preceded by a fleeting taste of fall air, and many places saw nighttime temperatures dip into the high 40s, including 45 degrees in Barrington and 46 in Normal. The warmest point in the state last month was Olmstead at 78.4 degrees, and the coolest point was Shabbona at 69.1 degrees.

Overall, the preliminary statewide average August temperature was 73.6 degrees, right at the 1991-2020 average and tied for the 63^rd warmest on record going back to 1895.

Late Season Heat Wave

This summer will not go in the books as a particularly extreme season temperature-wise. Most parts of the state have seen near to slightly fewer than normal days with high temperatures exceeding 90 degrees, and much of July and mid-August were particularly pleasant. However, the largely mild weather was bookended by two intense heat waves, one in June and the other in late August

A large upper-level ridge that had been burning the western U.S. for much of the summer opened in the last week of August and allowed very warm and humid air from the southern U.S. and Gulf of Mexico to move into the Midwest. Daily high temperatures exceeded 95 to 100 degrees across much of the state on Aug. 26 and 27 (Figure 3), and combined with the humidity, pushed heat index values over 110 degrees in many places, including a 115-degree heat index in Chicago. The heat caused buckled roads in parts of central Illinois, stressed air conditioning units, and significantly increased energy demand across the Midwest.

Drier End to the Growing Season

Following the third wettest July on record statewide, August rainfall was a bit harder to come by in Illinois. Total August rainfall ranged from less than 1 inch in parts of southwest and far southeast Illinois to over 5 inches in east-central Illinois. A narrow band of the Interstate 74 corridor from Moline to Danville was wetter than normal last month, while most of the rest of the state was between 1 and 3 inches drier than normal (Figure 4).

The dryness last month was most intense in southern Illinois. August was the 4^th driest on record in Carbondale with 0.69 inches, the 3^rd driest on record in Vandalia with 0.52 inches, and the fifth driest on record in Edwardsville with 0.63 inches. Overall, the preliminary statewide average total August precipitation was 2.96 inches, 0.63 inches below the 1991-2020 average and the 49^th wettest on record statewide.

Summer in Illinois

Climatological summer encompasses June, July, and August, and the season often brings more than its fair share of intense weather. This past season was on the mild side, temperature-wise. June started the season with warmer than normal conditions but was succeeded by a cooler than normal July. August wrapped up the summer with near normal temperatures (Figure 5).

The summer season began with a dry June and some concerns of crop and plant stress from the dryness and heat. However, a very wet July across the state cleared all concerns of drought and flipped the switch in some places to flooding concerns. July was also one of the most intense severe weather months in memory in Illinois. August was more of a mixed bag, with drier conditions in northern and southern Illinois. In all, summer precipitation totals ranged from 6 inches in parts of southern Illinois to over 20 inches in parts of northwest and southwest Illinois. Most of the state ended the season near to slightly wetter than normal, while parts of southern and north-central Illinois were 1 to 2 inches drier than normal this summer (Figure 6). This past season was the 7^th wettest summer on record in Salem with 19.02 inches, and the 4^th wettest summer on record at Carlyle Reservoir with 18.96 inches.

Outlooks

September brings in what is undoubtedly the best season in Illinois, and we have already experienced some fall-like weather this month. The outlooks for the entire month of September, though, show equal chances of above and below normal temperatures for much of the state, while drier than normal conditions are expected to prevail this month. Meanwhile, guidance for climatological fall (September through November) shows stronger signs of above normal temperatures, with mostly equal chances of above and below normal fall precipitation.

The preliminary statewide average July temperature was 73.8 degrees, 1.6 degrees below the 1991-2020 average and tied for 29^th warmest on record going back to 1895. The preliminary statewide total July precipitation was 7.01 inches, 2.95 inches above the 1991-2020 average and the third wettest on record.

Data are provisional and may change slightly over time.

Persistently Mild July Temperatures

July gave us a mixed bag of temperatures, but more days last month were cooler than normal. Daily temperatures from Springfield in Figure 1 show temperatures in the third week of the month were consistently 3 to 10 degrees below normal. While last month also brought some intense heat, it was fleeting.

July average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, between 1 and 2 degrees below normal for most of the state (Figure 2). Persistent northwest flow over the big ridge out west helped keep weather cooler and less humid for much of the month. Impressively cool July nighttime lows included 47 degrees in Taylorville and in Joliet. The coolest place in the state last month was Stockton in Jo Daviess County with an average temperature of 69.8 degrees, and the warmest place in the state in July was Carbondale with an average temperature of 77.0 degrees.  

Overall, the preliminary statewide average July temperature was 73.8 degrees, 1.6 degrees below the 1991-2020 average and tied for 29^th warmest on record going back to 1895. The preliminary statewide total July precipitation was 7.01 inches, 2.95 inches above the 1991-2020 average and the 3^rd wettest on record. This was also the second consecutive July with a statewide average temperature below normal, the first time we have had back-to-back cooler than normal Julys since 2014-15.

A Hurricane and a Derecho Make for a Very Wet Month

Illinois was starting to get a little dry after a drier than normal June. In fact, over half the state was considered abnormally dry in the U.S. Drought Monitor on July 2. The remnants of one of the strongest early season Atlantic hurricanes on record, Beryl, bent up through the Midwest, producing significant rainfall over a 48-hour period across parts of central and east-central Illinois (Figure 3). Parts of Champaign and Douglas counties picked up more than 4 inches in two days from Beryl, dramatically improving soil moisture and streamflow.

Less than a week later, an intense storm system moved across the northern half of the state, producing very heavy rainfall and a derecho that brought severe straight-line winds. Most of the state picked up at least 1 inch of rain over the 48-hour period between July 14 and 16, and multiple spots had over 6 inches, including just over 8 inches in Fulton County (Figure 4). Heavy rain in St. Clair and Washington counties caused significant flooding and forced the opening of a dam spillway near Nashville.

While Illinois’ weather calmed down a bit in the last two weeks of the month, most of the state stayed in an active, stormy pattern. Total July precipitation ranged from around 4 inches in parts of northeast Illinois to over 12 inches in south-central Illinois, between 1 and 7 inches above normal. Several spots in the St. Louis Metro East area picked up more than 13 inches of rain in July, including an incredible 16 inches in Mascoutah. In fact, last month was the wettest July on record in St. Clair County, likely surpassing the previous record by more than 5 inches.

The preliminary statewide total July precipitation was 7.01 inches, 2.95 inches above the 1991-2020 average and the third wettest on record.

Tornadoes by the Dozen

The same storm system on July 15 produced dozens of tornadoes across Illinois, including more than 30 in the Chicago National Weather Service County Warning Area (Figure 6). It was the largest single-day tornado outbreak on record for the Chicago area.

As of July 31, Illinois had an unofficial total of 126 in 2024 according to the National Weather Service. Importantly, this is a preliminary total and may be adjusted later this year as reports are refined. However, if that total is accurate, it would give this year the highest tornado frequency on record for any. The current official annual tornado record for Illinois is 124 in 2006.

Outlooks                                    

August is the beginning of the end of summer but can certainly bring its fair share of heat. August is also an important month for finishing crops as we move toward fall. The most recent Climate Prediction Center outlooks for August show equal chances of above and below normal temperatures in northern Illinois and higher chances of above normal temperatures in southern and central Illinois. August precipitation outlooks are leaning wetter than normal in northern Illinois and near normal for central and southern Illinois (Figure 7).

The newest climatological fall (September through November) outlooks show slightly higher chances of above normal temperatures, with equal chances of a drier or wetter than normal fall (Figure 7). Either way, many local Illinois orchards have apples ready and that is worth celebrating!

The preliminary statewide average June temperature was 74.4 degrees, 2.2 degrees above the 1991-2020 average and the 18^th warmest June on record going back to 1895. The preliminary statewide total June precipitation was 3.39 inches, 1.26 inches below normal and the 46^th driest June on record statewide.

Data are provisional and may change slightly over time.

Summer Indeed

June began with relatively mild temperatures across the state, including a few days with average temperatures that were 3 to 10 degrees below normal. However, as daily temperatures and departures from normal in Chicago show in Figure 1, summer heat kicked in mid-month and persisted until the final few days of June. Daily average temperatures between June 13 and June 25 were 5 to 15 degrees above normal in Chicago and statewide.

The heat wave in mid- to late-June was not particularly extreme in the actual temperatures. Most places in the state reached the mid- to upper-90s several days, and a few places, including Salem and Cahokia Heights, saw high temperatures above 100 degrees. However, these temperatures are not unprecedented for June. However, the heat wave was extreme in how long it lasted, with temperatures 5 to 15 degrees above normal for 10 to 14 consecutive days. For example, Charleston got above 90 degrees on half of the days in June, the most since 1988.

Overall, June average temperatures ranged from the low 70s in northern Illinois to the high 70s in southern Illinois, between 1 and 4 degrees above normal (Figure 2). Most places had daytime high temperatures in the upper 90s and a few places hit or exceeded 100 degrees during the heat wave last month. This included Mt. Vernon, which peaked over 100 degrees for the first time in June since 2012. Meanwhile, many spots in northern Illinois saw nighttime low temperatures dip into the low 40s in early June, including 41 degrees in Lisle and 43 in McHenry. The warmest place in the state last month was Cahokia Heights with an average temperature of 78.3 degrees, and the coolest place in the state in June was Waukegan with an average temperature of 70.2 degrees.

Overall, the preliminary statewide average June temperature was 74.4 degrees, 2.2 degrees above the 1991-2020 average and the 18^th warmest June on record going back to 1895.

Dry June for Most but Not All

June is typically one of the wettest months of the year in Illinois. In some years, June precipitation is consistent across the state, often the case in the wettest of Junes. This year however, the dominant weather pattern in the eastern United States was a large and persistent ridge that built up a big high-pressure system over the mid-south and southeast. The result was a persistent parade of storm systems into the Upper Midwest, causing catastrophic flooding in Minnesota, Iowa, and South Dakota, and largely hot and dry conditions in the southeast. As is often the case, Illinois was caught in between these two extremes, and June near to slightly wetter than normal in northern Illinois, and drier than normal in central and southern Illinois (Figure 3).

June totals ranged from over 8 inches in East Dubuque to only 0.63 inches in Macomb. In fact, last month was the third driest June on record in Macomb, and the driest since 1991. The preliminary statewide total June precipitation was 3.39 inches, 1.26 inches below normal and the 46^th driest June on record statewide.

Drought in Illinois

Illinois was suffering from serious drought this time last year, following extremely dry weather in May and June. By July 1, 2023 93% of the state was in moderate drought and 59% in severe drought. Thankfully, this year we’ve largely avoided serious drought conditions in the state, thanks to abundant (in some cases, surplus) precipitation in April and May. However, the stretches of hot and dry weather last month have begun to quickly deplete soil moisture and drop stream levels in much of central and south-central Illinois. The latest U.S. Drought Monitor map, current as of June 25^th, shows around half of the state is abnormally dry, and a small patch of moderate drought along the Indiana border in east-central Illinois (Figure 4).

July often makes or breaks drought in Illinois. A continuation of hot and dry conditions could accelerate drought impacts to crops, ecosystems, and water supply. However, a July like last year, with abundant (occasionally, too abundant) rainfall can quickly remove concerns of drought issues. On-the-ground reports of drought conditions and impacts are important sources of information for monitoring as we move into the second half of the growing season. Please consider reporting conditions in your area using the Condition Monitoring Observer Reporting (CMOR) system.

Severe Weather

June is one of our most active severe weather months, and while last month was not especially active with severe weather, the northern half of the state was still affected by tornadoes, hail, and severe winds. The NOAA Storm Prediction Center’s preliminary June numbers had 13 tornado reports, including an EF-1 on June 13^th that did damage outside of Gibson City in Ford County and four EF-0 tornadoes in the Chicago western suburbs on June 22^nd. Strong storms also caused several reports of storm damage from strong winds on June 16th. The airport in Moline recorded 65 mph wind gusts that afternoon.

Outlooks

July is climatologically the hottest month of the year in Illinois and can make or break a growing season with its rain (or lack thereof). The latest July outlooks from the Climate Prediction Center continue to show highest chances of above normal temperatures, but have recently leaned closer to near- if not wetter-than normal conditions (Figure 5). It’s always worth noting that July precipitation often comes in the form of isolated thunderstorms that can produce large differences in totals over small distances.

Meanwhile, outlooks for the three-month period between July and September also show highest chances of above normal temperatures and equal chances of wetter and drier than normal conditions. Concerns of Mississippi River levels in late summer and early fall — something on the minds of many after the last two years — have recently been assuaged by flooding in the Upper Midwest. However, the Ohio Valley is currently dealing with emerging drought conditions, something worth watching as we move into the climatological low-flow season for the big river.

The preliminary statewide average May temperature was 66.7 degrees, 3.5 degrees above the 1991-2020 average and the 11^th warmest on record going back to 1895. The preliminary statewide average total May precipitation was 4.85 inches, 0.08 inches above the 1991-2020 average and the 42^nd wettest on record statewide.    

Data are provisional and may change slightly over time

January to April this year was the fourth warmest start to any year on record in Illinois, with only 1921, 2012, and 2017 surpassing 2024. May continued the warm pattern across the state. Daily average temperature and departures from normal in Cahokia Heights show nearly all days last month were warmer than normal, some of which between 10 and 12 degrees above normal (Figure 1). That said, Illinois largely escaped early season extreme heat that we experienced in 2021 and 2022, making for largely pleasant temperatures last month.

May average temperatures ranged from the high 50s in northern Illinois to the low 70s in southern Illinois, between 2 and 4 degrees above normal (Figure 2). Several stations saw their first 90-degree temperatures last month, including 91 degrees in Monmouth and at Chicago’s Midway Airport. Meanwhile, the state was largely spared a late spring freeze in May, with only a handful of stations experiencing temperatures below 40 degrees. Overall, the warmest place in the state last month was Du Quoin, with an average temperature of 72.5 degrees, and the coldest place in the state was Mundelein with an average temperature of 59.2 degrees. Last month was the 7^th warmest May on record in the St. Louis area and in Quincy, the 10^th warmest May on record in Carbondale, and 11^th warmest May on record in Peoria.

Overall, the preliminary statewide average May temperature was 66.7 degrees, 3.5 degrees above the 1991-2020 average and the 11^th warmest on record going back to 1895.

May capped off a very warm climatological spring season. Each of the three spring months brought average temperatures that were 2 to 6 degrees above normal (Figure 3). It was a top five warmest spring on record throughout virtually the entire state, including the 3^rd warmest on record in Chicago and the 2^nd warmest on record in Peoria. For perspective, the spring 2024 average temperature in Peoria of 56.9 degrees is a full 2 degrees less than spring 2012, illustrating just how incredibly warm spring 2012 was. Overall, the preliminary average climatological spring temperature in Illinois was 55.6 degrees which, if confirmed, would tie 2024 for the fourth warmest spring on record statewide. Only 1977, 1991, and 2012 featured warmer springs in the prairie state.

A Mixed Bag of May Precipitation

May is the third wettest month climatologically in Illinois, and the wettest in parts of southern Illinois. However, last month was variable across Illinois, with extremely wet conditions in far southern Illinois and near to slightly drier than normal conditions in central and northern Illinois. Specifically, total May precipitation ranged from around 3.5 inches in west-central Illinois to nearly 10 inches in parts of southeast Illinois. Most areas south of Interstate 64 were 1 to 4 inches wetter than normal last month, while areas farther north were near to 1-2 inches drier than normal (Figure 4).

Unfortunately, much of last month’s precipitation came from thunderstorms that also brought severe weather to Illinois. The NOAA Storm Prediction Center listed 24 tornado reports, 153 severe wind reports, and 56 severe hail reports in Illinois in May. Among these include an EF-3 tornado that tore through parts of Jackson and Williamson Counties, causing significant damage around the Lake of Egypt area.

Climatological spring was wetter than normal for the entire state, with totals ranging from around 12 inches in central Illinois to nearly 18 inches in far southern Illinois. Most of the state was 1 to 4 inches wetter than normal this spring (Figure 5).

June & Summer Outlooks

June is the start of climatological summer and is an exciting month for many reasons. Warm weather is here, schools are out, and cicadas are buzzing (even more than usual this year). Unfortunately, the Climate Prediction Center’s June outlook is less exciting than the month’s weather. The one-month outlook is decidedly undecided in Illinois, with equal chances of warmer, cooler, drier, and wetter weather.

The summer season outlooks (June through August) are a bit more insightful, with higher chances of above normal temperatures this summer. For precipitation, Illinois is squeezed between a band of expected drier than normal conditions to the west and wetter conditions to the east. That setup often, but not always, corresponds with active stormy weather in the summer.