How Does the Record July 1996 Illinois Rainstorm Affect Probable Maximum Precipitation Estimates?

Abstract

The probable maximum precipitation (PMP), an estimate of the upper limit to storm rainfall, is determined using a catalog of extreme rainstorms. The effect of a recent extreme rainstorm—the northeastern Illinois storm of July 17–18, 1996—on PMP estimates for the Chicago area, is evaluated using standard PMP moisture maximization procedures. The results show that although the storm is a record rainfall event, its maximized precipitation is still below the PMP estimates for the Chicago area. Instead, a storm that occurred in Oklahoma, or perhaps another in Iowa, established the 24-h, 26- km2 (10- mi2 ) PMP estimate for Chicago. In either case, the higher maximized precipitation can be attributed to the combination of a larger storm rainfall amount, and a larger in-place moisture maximization factor. The case study also illustrates potential limitations with using standard PMP procedures. For instance, it is difficult to obtain an objective index to represent the mean moisture availability for this storm, si...