August wrapped up mild, drier Illinois summer

Sep. 2—CHAMPAIGN — The heatwave in late August was intense, but the average temperature and rainfall for the month were nearly normal based on long-term records, according to Illinois State Climatologist Trent Ford at the Illinois State Water Survey.

Likewise, the summer season temperature was within 1 degree of normal with slightly drier conditions.

The preliminary statewide average August temperature was 73.5 degrees, 0.1 degree above the 1991 — 2020 average. Temperatures ranged from the low 70s in northern and central Illinois to the high 70s in southern Illinois.

Around Aug. 20, a large upper-level ridge established over the central U.S., bringing very warm air and high humidity from the southern U.S. and Gulf of Mexico. Daily high temperatures exceeded 95 to 100 degrees across most of the state on Aug. 23 and 24, and, combined with the humidity, pushed heat index values over 110 degrees statewide. Peoria set a new heat index record at 121 degrees, breaking its previous record from 1995.

Rainfall

Moderate drought remained in more than half the state at the start of August. Multiple weather systems brought wetter weather to central and southern Illinois, helping to relieve earlier drought conditions. Total August rainfall ranged from less than 2 inches in parts of northwest Illinois to over 10 inches in southeast Illinois. Most areas of the state south of Interstate 80 were 1 to 5 inches wetter than normal in August, while much of northern Illinois was 1 to 3 inches drier than normal.

The dryness last month was most intense along the Missouri border and in northeast Illinois, where many places had less than 1 inch of total rainfall. Chicago had only 0.71 inches last month, making it the fourth driest May on record there.

The wetter weather in August helped improve crop and pasture conditions across the state, stabilize declining streams and pond levels, and promote ecological health in our natural lands. Despite the recovery, drought likely and significantly impacted crop yield potential this year, and its impact on tree health — especially in urban areas — will not be well known until next year.

The preliminary statewide average total August precipitation was 4.43 inches, 0.87 inches above the 1991 — 2020 average.

Summer season

The summer season began with a very dry June that kicked off drought concerns across the state. Wetter conditions in July and August helped reduce or eliminate drought in central and southern Illinois, but parts of northern Illinois remained dry in the latter part of the season. Summer precipitation totals ranged from 6 inches in parts of northwest Illinois to over 20 inches in parts of southeast Illinois.

Outlooks

Outlooks for September suggest we may need to wait a bit for more consistent fall weather, with higher odds of warmer and drier than normal weather. Meanwhile, guidance for climatological fall is less than confident, with equal chances of warmer, cooler, wetter, and drier than normal weather.

For more information about monthly Illinois weather summaries, current conditions, and climate, visit the Illinois State Climatologist website, http://stateclimatologist.web.illinois.edu/.