Temperatures for the month of October trended above normal over most of central, east central and southeast Illinois. Some scattered showers and thunderstorms helped to ease drought conditions over parts of east central and southeast Illinois during the month, while below normal precipitation occurred over the remainder of the area.
As depicted on the above figure from the Midwest Regional Climate Center, temperatures over central and southeast Illinois averaged above normal for the month of October 2011. Peoria experienced 6 days (4th through the 9th) of 80 degrees or warmer with Springfield eclipsing the 80 degree mark 9 days ( 4th through the 11th, and again on the 25th) in October. Champaign's preliminary data indicated that there were 8 days (4th through the 11th) where the mercury surpassed 80 degrees for the afternoon high temperature. Peoria reported 3 days where the early morning low temperatures were at or below the freezing mark, with the coldest morning being on the 29th of the month when the temperature dropped to 30 degrees. Further south in Springfield, the mercury dropped to a frosty 31 degrees as early as the 2nd of the month, with the coldest morning being on the 28th with an early morning low of 28 degrees. The map below represents the date of the first 32 degree freeze from National Weather Service Cooperative Network sites. The following link will give you a much more detailed look at the Midwest Regional Climate Center's freeze maps for the Fall season.
An overview of the jet stream pattern (figure below) at 300 mb (approximately 30000 feet) over the lower 48 during the month of October depicts a rather highly amplified pattern over the lower Great Lakes as a deep area of low pressure was anchored across the region. The shaded area on the map represents the strongest core of winds at 300 mph, starting at 70 knots, contoured every 20 knots. So in the figure below, winds were up to 90 knots out of the north at 300 mb over extreme east central Illinois at 700 am CDT on the 1st of October. This brought about rather cool conditions to the area to start the month off with. Jet stream depictions courtesy of the California Regional Weather Server at San Francisco State University (http://squall.sfsu.edu/crws.html).
The jet stream pattern changed significantly as we headed into the second week of October as seen in the figure below. The strongest core of winds and associated active weather shifted west, and then north into Canada, leaving our area with above normal temperatures, which lasted for about 10 days. The coolest and wettest weather was located across the Rockies and over the far northeastern sections of the country.
As seen below, the jet stream became more active for the Midwest signalling a change to a cooler pattern at the end of the month.
Precipitation amounts (see figures below from the MRCC) for the most part came up below average over most of the area. The exception being once again over parts of east central and southeast Illinois, where several weather systems during the month brought above normal rainfall.
Due to the beneficial rainfall over parts of east central and southeast Illinois during the month of October, drought conditions have ended, but continue to linger further west. According to the U.S. Drought Monitor from October 25, severe drought conditions existed over portions of central Illinois, roughly from about Lincoln and Springfield westward. Some areas near Galesburg and Peoria were still considered abnormally dry.
Here are links to specific climate summaries for area cities. The summaries for Peoria, Springfield, and Lincoln are the official data for those locations. For the remaining locations, they are supplemental, meaning another site in the area is the official climate station.