Based on preliminary data, September 2015 was the ninth warmest September on record for Illinois. The statewide average temperature was 70.2 degrees, 4 degrees above average. The warmest September on record was 1933 with 72.2 degrees.
The statewide average precipitation for September was 3.42 inches, 0.19 inches above average. However, the precipitation was unevenly distributed around the state as shown in the maps below. Most of the state received between 3 to 5 inches (shades of blue), while western and far southern Illinois were much drier with less than 3 inches of precipitation.
Some of the most sizable September totals came out of Vermillion County, near Hoopeston, where a CoCoRaHS station (IL-VR-17) reported 11.78 inches and where a NWS COOP station reported 11.74 inches. Those two sites are located in the area of pink in the map below. The next largest amount in the state was 8.45 inches at the DuPage County airport.