Changes in the climate of Illinois can sometimes be seen in the monthly temperature and precipitation and sometimes not. Unlike the other three seasons, fall in Illinois tends to be non-trendy (like my wardrobe). Here are the time series of temperatures and precipitation for September and October for Illinois.
All four plots show a considerable amount of year to year variability. Some decades were more volatile than others. Some of the biggest swings in temperatures in September occurred in the 1920s and 1930s. On the other hand, if you take out the three really wet Octobers of 1919, 1941, and 2009, the October precipitation is fairly consistent from one year to the next and usually within 2 inches of the 1981-2010 normal.
There are no significant trends in temperature over the last century. September precipitation has a small downward trend of 0.5 inches over the last century while October precipitation shows a slight upward trend of 0.6 inches over the last century. As a result, they largely cancel each other out.
We will cover November trends in a separate post, but it’s fair to say that November is much more interesting. The horizontal line in each graph is the 1981-2010 average (aka normal).
NOAA National Centers for Environmental information, Climate at a Glance: U.S. Time Series, Average Temperature and Precipitation, published October 2016, retrieved on October 6, 2016 from http://www.ncdc.noaa.gov/cag/