September brought heat, flooding, and drought conditions to Illinois

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[October 08, 2019]    September 2019 was the 4th warmest September on record since 1895, according to Illinois State Climatologist Trent Ford at the University of Illinois’ Illinois State Water Survey. Precipitation in September varied widely across the state.

The preliminary average statewide September temperature was 71.3 degrees, which is 4.9 degrees above the long-term average. Monthly temperature departures ranged from 2 to 4 degrees warmer than normal in northeast Illinois to over 6 degrees warmer than normal in southwest Illinois.

September average temperatures across the state ranged from 65 degrees in Jo Daviess County to 78 degrees in Lawrence County. The lowest minimum temperature reported in Illinois was 45 degrees in Jo Daviess County on Sept. 5, and the highest maximum temperature was 97 degrees in both Alexander and Pope Counties on Sept. 16.

Well over 100 local daily climate records were broken in Illinois in September, most of which were high daily minimum temperature records. These conditions were attributed to several very warm nights, including the night of Sept. 22, when the nighttime minimum temperature remained above 70 degrees as far north as Elizabeth in Jo Daviess County and Freeport in Stephenson County.

On the night of Sept. 10, the station in Rock Island reported a nighttime minimum temperature of 77 degrees, besting the previous daily record by 3 degrees.


Precipitation in Illinois varied tremendously from north to south across the state. The preliminary average statewide precipitation was 5.34 inches, which is 1.9 inches above the long-term September average. However, the data also show large differences in September precipitation totals across the state, with northern Illinois receiving much more than average precipitation, and southern Illinois receiving much less than average.

Areas of northern and north-central Illinois received in excess of 12 inches of rainfall in September, while areas of southeast Illinois received less than 0.25 inches over the same time period.

Locally, a station near Stockton in Jo Daviess County observed 16.62 inches in September, or nearly 13 inches more than normal, while the station at Smithland Lock & Dam in Pope County recorded only 0.02 inches, or 3.5 inches less than normal.


The latest U.S. Drought Monitor Sept. 24 map shows a pocket of moderate drought covering parts of Champaign, Ford, Iroquois, and Vermilion counties. Concurrently, below normal rainfall and above normal temperatures in the southern part of the state generated abnormally dry conditions in September.

The latest U.S. Drought Monitor depicts abnormally dry conditions for most of Illinois south of I-64, and a pocket of moderate drought from Pope and Hardin counties in the southeast to Perry and Franklin counties in south-central Illinois.

Conditions in southern Illinois have shown some signs of a flash drought, which is a rapidly intensifying drought event, often provoked by an existing precipitation deficit combined with intense heat.

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Reports from Illinois Farm Bureau CropWatchers regarding drought in east-central and southern Illinois are mixed. Some report the dryness and heat have helped late-planted crops reach maturity, while at the same time possibly sacrificing yield.


In contrast to the ongoing drought in southern and east-central Illinois, September was abnormally wet for most of northern and north-central Illinois. Persistent, heavy rains led to flooding impacts in parts of northern Illinois, including the closure of several state parks and significant flooding along the Fox and Des Plaines Rivers, among others.

Areas in northern and north-central Illinois received in excess of 12 inches of rainfall in September. In most parts of Peoria, Woodford, Marshall, and Livingston counties, most of the rainfall totals came in a 24-hour period between Sept. 27 and 28. This event created dangerous flash flooding from Peoria into the southwest Chicago suburbs.

The COOP station in Minonk in Woodford County recorded 9.09 inches of rainfall over that 24-hour period, although that likely fell over a less than 12-hour window. This total approached the 24-hour, 500-year storm total of 9.53 inches and surpassed the 12-hour, 500-year storm total of 8.29 inches.

A 500-year storm total refers to a precipitation accumulation over a given time period (e.g., 12, 24, 48 hours, etc.) and has a 0.2 percent chance of occurring in a given year.

The 9.09-inch total in Minonk broke, and nearly doubled, the all-time 24-hour precipitation total record at that station, which was just over 5 inches (data going back to 1895). Images of flooded fields in Woodford and Marshall Counties suggest this most recent heavy precipitation event may delay harvest.

Short-term temperature forecasts call for continued above average temperatures for the first few days of October and then a regression to cooler, more seasonal conditions. Longer term Climate Forecast System (CFS) forecasts from the National Centers for Environmental Protection show probabilities of a 32-degree freeze in Illinois remain below 30 percent into the third week of October.

Looking into October, the 8 to 14-day outlook from the Climate Prediction Center (CPC) shows elevated probabilities of above normal temperatures and elevated probabilities of above normal precipitation across the state. The CPC monthly outlook for October still shows elevated probabilities for below normal temperatures across the northern half of the state, with equal chances (above normal, normal, below normal precipitation) for all but the very northwest corner of Illinois.

[Lisa Sheppard]


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