Illinois’ weather in October was a mixed bag of conditions with temperatures in the 90s, the first fall frost, the first snow of the season and widespread heavy rains, according to Illinois State Climatologist Jim Angel at the Illinois State Water Survey, part of the Prairie Research Institute at Illinois.
The statewide average temperature departure for the first nine days of October was 11.5 degrees above normal. Many stations reported temperatures in the 90s during this period, including a high of 94 degrees in Fairfield on Oct. 7.
After the passage of a strong cold front, temperatures dropped and remained much cooler for the rest of the month. The temperature departure from Oct. 10 to 31 was 5.7 degrees below normal. The warm and cold periods balanced out, leaving the statewide average temperature for the month at 54 degrees, or only 0.4 degrees below normal.
Freezing temperatures arrived across most of Illinois on Oct. 21. The lowest reported reading was 19 degrees at locations near Stockton, Shabbona and Champaign. Historically, the average dates for the first fall frost are in October across the state.
The first snow of the season fell on Oct. 12. Across Illinois, stations reported traces of snow, meaning it melted as it fell. A few areas reported a small amount of accumulation, including Moline and Bloomington with 0.2 inches.
“We do not always see snowfall in October in Illinois, but it does show up in the records from time to time,” Angel said. “However, it is not a reliable indicator of the upcoming winter.”
Precipitation was widespread across Illinois in October. The statewide average was 3.81 inches, 0.57 inches above normal. Amounts of 4 to 7 inches or more were common in the northern half of the state, which is well above normal.
Meanwhile, most of the southern half of the state was drier, with precipitation amounts in the 2- to 4-inch range, which is much closer to normal for October. The largest monthly total was 8.07 inches in Nauvoo in Hancock County.
Another interesting feature of 2018 so far in Illinois has been the impressive precipitation totals in northern Illinois. Several stations have received over 50 inches of precipitation through Oct. 31, including Elizabeth in Jo Daviess County with 54.45 inches, which is 22.66 inches above normal.