Cool, Dry September in Illinois

This was posted on Friday but it was somehow not indexed properly in WordPress


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




Hurricane Harvey Over Chicago

Sometimes it is hard to appreciate the scale of the disaster from the rainfall of Hurricane Harvey. To put this into perspective for us in Illinois, here are the 5-day rainfall totals of Hurricane Harvey transposed from Houston to Chicago. Amounts of 40 or more inches would have covered most of the Chicago area and stretched westward towards Rockford and beyond. Amounts of 20 inches or more would have covered most of Illinois north of Interstate 80 and extend into southern Wisconsin.

Imagine …

  • the number of people that would have to be evacuated and how far they would have to go get away from the floodwaters,
  • the pollutants and raw sewage in all that floodwater,
  • the amount of damage to Chicago and the region – damage to businesses, homes, schools, hospitals, airports, highways, railroads, etc.,
  • the amount of time and money it would take for Chicago to ever recover from such a disaster,
  • the ecological damage to Lake Michigan and the Illinois River Basin as these flood waters pushed chemicals and debris far and wide,
  • getting a year’s worth of rain (and more) in 5 days – the statewide average precipitation in Illinois is 40 inches.


By comparison, here are the rainfall amounts from the July 17-18, 1996 storm that hit northern Illinois and set the record for the most rain in 24-hours, 16.94 inches at Aurora. Six people in Illinois died from this event and the damages were in the $600-$700 million in 1996 dollars. While this storm caused substantial damage in the Chicago area, the rainfall totals were much smaller than Hurricane Harvey and covered a smaller area.


Sorry for the way the map looks, it was hand drawn and used a variety of hard-to-find rainfall measurements. Today we have access to much more data,  including CoCoRaHS and radar-based precipitation estimates.


While Illinois can receive rain from tropical systems, they are usually weakened by the time they arrive and far away from abundant sources of moisture like the Gulf of Mexico. Here are the 5-day totals from one of the largest storms to hit Illinois in recent years – the remains of Hurricane Ike in 2008. Although the heavy rain was widespread, the amounts were still much smaller than from Harvey.


Here are some other noteworthy large-scale storms in Illinois. Click to enlarge.


August, Cool and Dry for Illinois


August was cool and dry for most of Illinois. The statewide average temperature was 70.6 degrees, 3 degrees below normal, and the 12th coolest August on record. This followed June and July which were both slightly above normal. The warmest daily reading in Illinois for August was 96 degrees, reported at Effingham on the 21st. The coolest daily reading in Illinois was 42 degrees, reported at Shabbona (DeKalb County) on the 25th.

The statewide average precipitation was 2.41 inches, 1.18 inches below normal. There was a narrow band of 3 to 6 inches between Quincy to just north of Champaign. The rest of the state received 1 to 3 inches, which was 1 to 3 inches below normal. The highest monthly total for August was 7.09 inches, reported at both Augusta and Hoopeston. On a side note, conditions in northern Illinois were quite different than July when that area was particularly wet. Click to enlarge.

Summer (June, July, August)

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