Illinois State Water Survey - Publications Search - Publication Abstract, University of Illinois at Urbana-Champaign

Two August 2002 rainstorms, one centered in Illinois and Indiana on August 18-19, and one in Iowa, Illinois, and Wisconsin on August 21-22, created record-setting point rainfalls of >10 inches and >12 inches, respectively. Return intervals of both storms’ heavy rain amounts for 3-, 6-, and 12- hour durations exceeded once in 100-year values. Storm characteristics were similar to those of 36 past rainstorms during 1951-2001 that also were investigated in comparable detail. The similarities included the fact that most of the rain fell over 8 hours at night, storm areas were oriented west-east, and the region with >2 inches covered more than 9,000 square miles. Synoptically, conditions were similar to those of most past rainstorms: the storms developed south of an west-east-oriented front, precipitable water values were exceptionally high, >1.7 inches, and the frontal position and low-level jet stream proximity led to training of thunderstorms along the same path.

However, the August 2002 rainstorms were different than past rainstorms in that the two storm events occurred just 2.5 days apart and in relatively adjacent areas. No other major past storms had occurred in such close time proximity. Both storms occurred where the prior 2.5-month rainfall was much below normal, creating much below normal soil moisture and droughtlike conditions for crops. All 36 previous major assessed rainstorms occurred after prolonged periods of average to much above average rainfall. This pre-storm difference in moisture conditions greatly affected the storms’ impacts, and both August storms produced small economic losses compared to those of comparable prior storms. A much greater percentage of total storm rainfall infiltrated the soil, resulting in less runoff. High early peak flows in rivers where the heaviest rain fell quickly returned to normal levels within 10-22 days. Flooding, mostly near river courses, quickly dissipated, and flood losses were minimal. The major economic impact of the two August storms related to the added soil moisture and, in turn, the positive effects on soybean crops. Soybeans were in the pod-filling stage and shy of soil moisture when the storms occurred, and the rain-filled soils led to increased yields valued at $51 million in Illinois and Iowa.