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Illinois has been cool and dry in September so far. The statewide average temperature was 63.9 degrees, 6.1 degrees below normal. The statewide average precipitation was a minuscule 0.19 inches, 13 percent of normal (maps below). In fact, most of Illinois is now labeled as D0, abnormally dry, with the US Drought Monitor.
This September follows on the heels of a cool, dry August. The updated numbers for August are that the statewide average precipitation was 2.33 inches, 1.26 inches below normal, and the statewide average temperature was 70.5 inches, 3.1 degrees below normal. As a result of the dryness since August, both streamflows and soil moisture are very low, even for this time of year.
However, this may soon change. There are chances of rain over the weekend. And it looks like the pattern over the next two weeks favors above-normal temperatures and precipitation according to the NWS.
September Precipitation Maps
Here are the actual precipitation, departure from normal, and percent of normal, for September 1-15 (click to enlarge). If it had not been for the remnants of Hurricane Irma, there would have been even less rain to report in Illinois.
US Drought Monitor
USGS Streamflow Conditions in Illinois