Chicago finished September with an average temperature of 21.27°C, making it the 5th-warmest on record, and almost tying September 1971's 21.33°C. Meanwhile, the NCDC predicts warmer-than-normal temperatures through the end of next week.
This shows one of the ways global warming will actually make Chicago a better place to live. If trends continue, we'll continue to have warmer and wetter winters than the historical norms, at the cost of warmer and drier summers. The downside, of course, is drought, and most of Northern Illinois desperately needs rain—especially this close to the harvest. (Just not too much rain, because, you know, harvest.)
For now, though, I love the sunny and comfortable (mid-20s during the day, high-10s overnight) weather this time of year.