Probable maximum precipitation (PMP) is the theoretically greatest depth of precipitation that is meteorologically possible for a given area and duration. It is used in designing critical infrastructure such as dam spillways.
Probable maximum precipitation (PMP) is defined as the greatest depth of precipitation for a given duration that is physically possible over a specific drainage area at a particular location and time of year, based on the most severe combination of meteorological conditions that are reasonably possible. PMP estimation involves maximizing observed storm events by adjusting moisture content to the theoretical maximum (based on maximum persisting dew point temperatures) and transposing major storms from nearby regions to the target area. The PMP is a key input for computing the probable maximum flood (PMF), which is used to design spillways and other safety-critical features of large dams, nuclear power plants, and other facilities where failure could have catastrophic consequences. In the United States, the National Weather Service (now NOAA) has published Hydrometeorological Reports (HMRs) that provide PMP values for various regions and storm durations. PMP values are typically much larger than 100-year or even 1,000-year return period precipitation estimates. For example, the 72-hour PMP for some locations in the eastern United States can exceed 1,000 mm. PMP concepts are being reevaluated in light of climate change, as warmer temperatures increase atmospheric moisture-holding capacity, potentially increasing the upper limits of precipitation.