Record flood-producing rainstorms of 17-18 July 1996 in the Chicago metropolitan area. Part II: Hydrometeorological characteristics of the rainstorms

The rainstorm on 17-18 July 1996 in northern Illinois produced three rainfall records. The 43.0-cm total storm rainfall at Aurora was the greatest point rainfall recorded for storm durations of 24 hours or less in this century in Illinois and most surrounding states. The 27.9-cm storm rainfall recorded in the southwestern part of the Chicago metropolitan area was the heaviest 24-h amount ever recorded in that city. The July 1996 storm also produced the heaviest 24-h mean rainfall recorded in Illinois over areas of 5200 and 13 000 km² immediately surrounding the storm center. An area of approximately 12 000 km² experienced 24-h point rainfall amounts that exceeded those expected to occur, on the average, once in 10 years. Similarly, the 25-, 50-, and 100-yr frequency values were exceeded over areas of 6730, 4920, and 3500 km², respectively. One concern resulting from a major rainfall event such as this storm is its impact on the rainfall frequency analysis. This new information may result in changes in the estimated rainfall amounts at selected return periods, which are used to design water-handling structures. Based on previous research, the Aurora rainfall appears to exceed the 1000-yr return period. However, fitting a statistical distribution to the annual maximum time series and using regional averages minimized the effect of this storm on rainfall frequency estimates.