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

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, since it has multiple rain periods. In addition, the estimated precipitable water based on standard PMP assumptions is much greater than that observed with upper-air soundings. Finally, for the same storm to occur under climatological maximum moisture conditions, a fundamental assumption in moisture maximization, dew points in the atmospheric profile would need to be increased beyond the observed air temperatures for the storm, a situation that is physically impossible. [ABSTRACT FROM AUTHOR]