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.

Drought Improvements in Illinois

After some sizable rains in August and a nice contribution from TS Isaac over the Labor Day weekend, it should come as no surprise that the US Drought Monitor has revised the drought depiction for Illinois (see Figure). Besides the rainfall, we have seen major gains in soil moisture conditions and stream flow.
Large areas of Illinois were upgraded by one or two categories of the US Drought Monitor. Most notable is the reduction in two most severe drought categories: D3 and D4. The D4 category went from 7.8 percent to zero percent. The D3 category went from 69.6 to 6.96 percent – a ten-fold reduction. D4 was present in parts of 10 counties in northern Illinois. Much of the state still remains in D1 or D2 drought. We have an area in northeast Illinois categorized as just abnormally dry.
While the impact from TS Isaac has been remarkable, concerns remain for some lakes and rivers as well as shallow groundwater sources.  The statewide average precipitation after Isaac is at 20.8 inches, still about 6.3 inches below normal.

US Drought Monitor map for Illinois for the period ending September 4. Click to enlarge.