National Weather Service Weather Forecast Office

The average September sees a total of 3.12 inches of rainfall at Indianapolis. Through the first seven days of September 2012, 4.69 inches of rain have fallen at the Indianapolis International Airport. This is the 4th wettest first week of September on record for the Indianapolis area.

Indianapolis Area  (ThreadEx Station)
Extremes
Highest Total Precipitation inches
Days: 9/1 - 9/7
Length of period: 7 days
Years: 1871-2012

Rank  Value  Ending Date
  1    7.23    9/ 7/2003
  2    6.80    9/ 7/1895
  3    5.15    9/ 7/1879
  4    4.69    9/ 7/2012
  5    4.35    9/ 7/1921

August 2012 was the 8th wettest August on record for the Indianapolis area. If August's rainfall is added to September's first seven days' rainfall, the total of 11.20 inches already ranks as the 10th wettest August+September on record.

Indianapolis Area  (ThreadEx Station)
Extremes
Highest Total Precipitation inches
Days: 8/1 - 9/30
Length of period: 61 days
Years: 1871-2012

Rank  Value  Ending Date
  1   16.11    9/30/1989
  2   15.01    9/30/2003
  3   14.80    9/30/1921
  4   14.30    9/30/1926
  5   12.65    9/30/1879
  6   12.31    9/30/1890
  7   12.11    9/30/1993
  8   12.08    9/30/1896
  9   11.65    9/30/1980
 10   11.20    9/30/2012 (through 9-7-12)

With over 3 weeks to go, 2012 should get even higher in the ranking.
Below is a plot of rainfall for September 2012, September 2011, and the average September rainfall.