August Caps off a Mild Summer – Illinois State Climatologist

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.

**Figure 1**. Daily August average temperature departures in Galesburg.

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.

**Figure 2**. Maps of (left) August average temperature and (right) August average temperature departures from normal.

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.

**Figure 3.** Map of high temperatures across Illinois on Aug. 27.

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).

**Figure 4**. Maps of (left) August total precipitation and (right) August precipitation departures from normal.

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).

**Figure 5.** Maps show average temperature departures for June, July, and August.

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.

**Figure 6**. Maps show (left) summer total precipitation and (right) summer precipitation departure from normal.

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.