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Probable Maximum Precipitation Study and Evaluation Tool

This study provides gridded Probable Maximum Precipitation (PMP) values for all drainage basins within Pennsylvania, including regions adjacent to the state that also provide runoff into drainage basins within Pennsylvania. PMP values provided in this study supersede the current HMR PMP depths from Hydrometeorological Reports (HMRs) 40 (Goodyear and Riedel, 1965); HMR 51 (Schreiner and Riedel, 1978); and HMR 52 (Hansen et al., 1982).

PMP is a deterministic estimate of the theoretical maximum depth of precipitation that can occur over a specified area, at a given time of year. Parameters to estimate PMP were developed using the storm based, deterministic approach as discussed in the HMRs and subsequently refined in the numerous site-specific, statewide, and regional PMP studies completed since the early 1990’s.

Methods used to derive PMP values for this study included consideration of numerous extreme rainfall events that have been appropriately adjusted to each grid point and representing each storm type, local, general, and tropical. This large number of storm events provided enough data from which to derive the PMP. The process of combining maximized storm events by storm type into a hypothetical PMP design storm resulted in a reliable PMP estimation by combining the worst-case combination of meteorological factors in a physically possible manner.

Deliverables from the PMP Study


All deliverables for the Probable Maximum Precipitation Study are being provided herein with the expressed understanding that the Pennsylvania Department of Environmental Protection (DEP) and the Commonwealth of Pennsylvania are releasing these products at the user’s own risk. The Probable Maximum Precipitation Study is intended to supplement existing requirements. Nothing in the Probable Maximum Precipitation Study shall affect existing statutory or regulatory requirements. It is the owner and its engineer who are expected to take all steps necessary to ensure proper use and understanding of these products and that all information is being properly interpreted and applied. DEP reserves the right to deviate from the process outlined above if circumstances warrant.