The Illinois statewide average temperature for September was 70 degrees, 3.8 degrees above normal and the 12th-warmest September on record, according to Illinois State Climatologist Jim Angel of the Illinois State Water Survey, part of the Prairie Research Institute at Illinois.
The hottest temperature recorded for September was 99 degrees in Springfield on Sept. 5, and the coldest was 30 degrees at Ottawa on Sept. 29.
The statewide average rainfall for September was 4.97 inches, 1.74 inches above normal, so that 2018 was the 19th-wettest September on record. The highest monthly rainfall total for the month was 12.43 inches at Clay City, just south of Effingham.
Rainfall was heaviest in southern Illinois with amounts up to 12 inches and in northwestern and western Illinois with amounts up to 10 inches – all well above normal. A band of drier conditions stretched from St. Louis to Chicago where rainfall totals of 1 to 3 inches were common.
Heavy rains also fell across the upper Midwest in September. The rains were especially heavy in Wisconsin, Iowa and Minnesota, leading to river flooding in Illinois along the Mississippi and rivers flowing out of Wisconsin.
The National Weather Service outlook for October shows Illinois with very strong chances for above-normal temperatures and rainfall.