First Half of September in Illinois – Warmer than Average

The first half of September in Illinois was warmer than average, about 3.5 degrees above average.


Precipitation for the first half of September is mixed. A few areas east of St. Louis and along the eastern border with Indiana have seen 3 to 4 inches, which is well above average. Much of the rest of the state has seen a respectable 1 to 3 inches. Western Illinois has been drier.


Hot, Dry Start to September in Illinois

Illinois and the rest of the Midwest are off to a hot start in September. As the map below shows, areas in shades of brown were up to 9 degrees above average. This includes southern Illinois and much of the southern states in the Midwest. The areas in pinks and red were even farther above average, some 9 to 12 degrees above average. This includes most of the northern half of Illinois, as well as northern Missouri, Iowa, Minnesota, Wisconsin, and most of Michigan.

Cooler and more seasonable weather is expected in Illinois after today.


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September Was Cool and Wet for Illinois


September was 1.5 degrees cooler than average and 0.88 inches wetter than average for Illinois. Year to date temperatures were 3.3 degrees below average and the 6th coolest January-September on record. The year to date precipitation was 32.91 inches, 2.5 inches above average.


The statewide average temperature for September in Illinois was 64.4 degrees, 1.5 degrees below the 1981-2010 average.

The statewide average precipitation for September in Illinois was 4.12 inches, 0.88 inches above average. The heaviest rains fell across western and central Illinois with amounts of 6 to 9 inches (map below). A second area of heavy rains occurred south of Interstate 70 near Effingham. The largest monthly total reported so far is Beardstown with 10.72 inches.


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Summer Weather Gives Way to Fall

If you look at the start to September, temperatures have run 3 degrees above average. And yesterday and today have been the hottest yet with temperatures the 80s and 90s across much of state. But that is about to change as a cold front pushes through later today. This weekend and next week will be cooler with highs in the 70s and low 80s.  I am looking forward to that.

Average daily highs in September in Illinois range from the low to mid 70s in northern Illinois to the low 80s in southern Illinois (map below, click to enlarge).


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September – Warm and Dry in Illinois

Strictly speaking, we are not done with September until the end of today. However, I went ahead and pulled together the September statistics because there is no rain in the forecast. All these statistics are based on data that have come in so far and may change slightly in the coming weeks.


The statewide precipitation for September was 1.87 inches, 1.39 inches below average and the 14th driest September on record. As the first two maps below illustrate, dry conditions were widespread. Only a few spots received 3 or more inches in Illinois, including the Chicago area, far western Illinois, and the southeast corner of the state. Several areas in the state received less than an inch of precipitation.

The second map shows the September data as a percent of average. Several areas had less than 25 percent of average (red). Even the “wet” areas had trouble achieving totals close to the 1981-2010 average. Overall, the precipitation departure for September was 57 percent of average.

This is the 6th driest July-September on record for Illinois with 6.64 inches (4.24 inches below average). Ironically, this came on the heels of the wettest January-June on record for Illinois with 28.96 inches (9.13 inches above average).

This is the third month in a row with below-average precipitation. In July, Illinois received 3.14 inches (0.91 inches below average). In August, Illinois received 1.63 inches (1.37 inches below average).


The statewide temperature for Illinois in September was 68.8 degrees, 2.9 degrees above average. It was warm but not record-setting. The warmest September on record was 1925 with 72.7 degrees.

The above-average September reversed the pattern of the last three months (June, July, and August) which were below-average on temperature.

  • June: 71.4° (0.4° below average)
  • July: 72.9° (2.4° below average)
  • August: 72.5° (1.0° below average)
  • September: 68.8° (2.9° above average)

Statewide records began in 1895.



September in Illinois: Cooler and Wetter Than Normal


The statewide average temperature for September in Illinois was 64.6 degrees, 1.6 cooler-than-normal. This was the first cooler-than-normal month of 2012, and the first cooler-than-normal month since September of 2011. See graph below.

The statewide average precipitation for September was 4.9 inches, 1.7 inches wetter-than-normal. This is the first wetter-than-normal month in 2012. See second graph below.

Much of the September rainfall came from the remains of Hurricane Isaac that passed over Illinois on Labor Day weekend. Additional rains fell later, especially in south-central Illinois. In general, areas south of Interstate 80 had monthly totals in the 3 to 12 inch range. A few sites in that region reported over a foot of rain with the largest total at Centralia with 15.89 inches. See first map below.

Precipitation totals north of Interstate 80 were around 1-2 inches. One of the driest spots in the state was Elburn (Kane County IL-KN-30) with only 1.28 inches for the month. Chicago and Rockford were not far behind with O’Hare Airport reporting 1.76 inches while the Rockford airport reported only 1.74 inches for September.

By the end of September, drought conditions had eased somewhat according to the US Drought Monitor.  Only 6.7 percent of the state was in the worst two categories  of drought (D3 and D4). This compares to 70 percent of the state in the two worst categories at the end of August. Even so, 82 percent of the state still remained in some stage of drought at the end of September.


Even with a wet September, the January-September statewide precipitation total of 22.38 inches was 8.34 inches below normal and the fifth driest on record. Here are the top five:

  1. 1988 with 19.49″
  2. 1901 with 19.84″
  3. 1936 with 21.76″
  4. 1940 with 22.17″
  5. 2012 with 22.38″

Remarkably, the precipitation over the last two months has erased the precipitation deficit since January 1 in much of Fayette, Washington, Clinton, Bond, and Montgomery counties. Sizable deficits remain across much of Illinois, especially western and northern Illinois as well as far southern Illinois. See second map below.

The January-September statewide average temperature of 59.6 degrees was 4.1 degrees above normal. It was the second warmest January-September on record and just slightly cooler than the record of 59.7 degrees set in 1921.

Notes: “normal” refers to the 1981-2010 averages. Statewide temperature and precipitation records began in 1895.

Statewide temperature departures from normal for September in Illinois. Click to enlarge.
Statewide precipitation departure from normal for September in Illinois. Click to enlarge.
30-day precipitation total through the morning of September 29 for Illinois. The 30-day map for September 30 failed to run. Click to enlarge.
Year to date precipitation departures from normal through September 29 for Illinois. Click to enlarge.

First Half of September – Wet in Eastern and Southern Illinois

The statewide average precipitation for the first half of September was 3.77 inches, which is already above the monthly normal precipitation of 3.24 inches.

However, that precipitation was not distributed equally around the state. The figure below shows that large areas across eastern and southern Illinois received 3 to 6 inches of rain (shades of dark blue and green). Meanwhile, areas north of Interstate 80 and in western Illinois were much drier with amounts of less than 2 inches. Most of the heavy precipitation fell from the remnants of Hurricane Isaac that passed through Illinois on Labor Day weekend.

Reports from individual stations ranged from 8.08 inches for the CoCoRaHS station Bush (IL-WM-4) in southern Illinois to only 0.57 inches for the CoCoRaHS station Geneva (IL-KN-1) in northeast Illinois.

Precipitation totals for September, 2012, in Illinois through the morning of September 16. Click to enlarge.