Probable Maximum Precipitation (PMP): From Hydrometeorological Reports (HMRs) and Site-Specific Studies to Numerical Weather Modeling and Probabilistic Estimates

Defined as “the depth of precipitation which can be reached but not exceeded under known meteorological circumstances,” maximum possible precipitation (MPP) was developed starting in the 1940s by the U.S. Weather Bureau (HMR 23, 1947) and published in NOAA’s Hydrometeorological Reports (HMRs) series. The term probable maximum precipitation (PMP) replaced MPP in the 1950s as HMR 33 (Riedel et al., 1956). Both MPP and PMP were based on data from historic extreme rainfall events together with procedures to increase the rainfall to its plausible maximum. This approach is often referred to as the storm-based method. Recent HMRs and site-specific PMP studies have incorporated new data (e.g. radar-derived precipitation and longer periods of record for both precipitation and dew point temperatures) along with improvements in analysis procedures. In the meantime, high-resolution numerical
models have become increasingly capable of reproducing observed rainfall amounts and patterns, and model-based PMP
approaches have been under development for several years. These efforts essentially follow the storm-based logic but use atmospheric physics equations to incorporate our current understanding of rainfall production. Use of atmospheric models replaces some of the assumptions required by earlier methods by explicitly computing rainfall amounts along with spatial and temporal rainfall distributions. Building on these developments and to address remaining shortcomings, a June 2024 report from the National Academies of Sciences, Engineering and Medicine (NASEM) on “Modernizing PMP” recommends a probabilistic definition of PMP using extreme precipitation calculations based on rare storms from large ensemble atmospheric model simulations (NASEM, 2024). This paper reviews HMR and site-specific methods for PMP estimation, emerging alternatives using numerical modeling, and summarizes the recommendations from the recent NASEM report.