The Probable Maximum Precipitation (PMP) is an estimate of the highest rainfall that could fall in a specific area and timeframe. Engineers rely on the PMP to design infrastructure in your community, from dams and levees to roads and railways. The PMP is an integral part of hydrology, hydraulics, and dam safety in Maryland.

In April 2025, the Maryland Dam Safety program completed an update of statewide PMP data through a two-year PMP study conducted by Applied Weather Associates (AWA). Parameters to estimate PMP were developed using the storm based, deterministic approach as discussed in the NOAA Hydrometeorological Reports (HMRs) and subsequently refined in the numerous site-specific, statewide, and regional PMP studies completed since the early 1990’s. The Maryland PMP 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.

This study provides gridded PMP values for all drainage basins within Maryland, including regions adjacent to the state that also provide runoff into drainage basins within Maryland.

Effective April 8, 2025, the selection and usage of PMP values in the design of dams will be governed by the results of the Maryland PMP study. PMP values provided in the Maryland PMP study supersede the current HMR PMP depths from HMR 40 (Goodyear and Riedel, 1965); HMR 51 (Schreiner and Riedel, 1978); and HMR 52 (Hansen et al., 1982).

In comparison to the HMRs, the new study:

• Provides reproducible results.
• Uses a gridded dataset within the GIS platform that relies on computer analysis rather than human interpretation of contours.
• Reviews 50 years of new data—including 24 new storms for analysis that were not accounted for in the HMRs.
• Includes temporal and spatial distributions of PMP rainfall events.

Data, assumptions, and analysis techniques used in this study have been reviewed and accepted by the Steering Committee (which included representation from the National Weather Service, Academia, Natural Resources Conservation Service, United States Army Corps of Engineers, and the North Dakota Department of Water Resources). Additionally, significant input was also provided by other study participants including the Federal Energy Regulatory Commission and numerous private consultants.

Results of this analysis reflect the current standard of practice used for defining PMP, including comprehensive storm analyses procedures, extensive use of geographic information systems (GIS), explicit quantification of orographic effects, updated maximum dew point for storm maximization, and improved understanding of the weather and climate related to extreme rainfall throughout the region.

All deliverables for the Probable Maximum Precipitation Study are being provided herein with the expressed understanding that the Maryland Department of the Environment and the State of Maryland are releasing these products at the user’s own risk. 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.