Probable Maximum Precipitation (PMP)
The current Probable Maximum Precipitation (PMP) dataset covering North Dakota was derived in the 1970s and 1980s as part of Hydrometeorological Reports (HMRs) completed by the National Ocean and Atmospheric Administration (NOAA). HMR-51 consisted of the continental US east of the 105th meridian, while HMR-48 was done specifically for the Red River of the North and the Souris River. HMR-48 PMP values include influences attributable to snowmelt, which has proven to influence runoff and flooding.
Since the completion of the HMRs, North Dakota has experienced a wet-cycle that was not prevalent during the climate record used in previous studies. This period consisted of a number of large spring floods and precipitation events. Many of the historic flooding events in North Dakota occurred due to melting snow or rain on snow events, most recently in 1997, 2009, 2010, and 2011.
This study’s purpose is to develop more representative PMP estimates for evaluating flood safety, assessing flood risk, and calibrating event-specific hydrological models.
Currently, the manner in which PMP values are calculated is inconsistent and cumbersome, from the use of hand calculations to coarse chart estimations. This study aims to provide the public a user-friendly tool to estimate PMP values.
This project is managed by a Review Board. The Review Board consists of both state and federal agencies with direct knowledge of the sciences and methods involved in a PMP analysis. Headed by the Water Commission, the remaining members of the Review Board are the National Weather Service (NWS) offices of Bismarck, ND and Grand Forks, ND, the Natural Resources Conservation Services (NRCS) office in Bismarck, ND, the North Dakota State Climatologist at the North Dakota State University (NDSU), and the United States Army Corps of Engineers (USACE) Omaha District. This board was developed with the intent to guide the NDSWC program, maintain the analysis integrity through participation in meetings and discussions, and review the deliverables and final products.
Probable Maximum Precipitation Study Update
The probable maximum precipitation (PMP) study, being conducted by Applied Weather Associates (AWA), reached its halfway mark in May of 2020. The project will provide updated, state-of-the-science PMP data and analysis techniques to hydrology in the state of North Dakota. The project is following the protocols established by NOAA for HMR 48, 51, and 52 published in 1973, 1978, and 1982, respectively. However, not only will data used to determine the PMP be updated with data through 2020, an additional 40 years, but it will also expand the “Cool Season PMP” (rain on snow events) throughout the entire state as opposed to just the Red River and Souris (Mouse) River basins. The events looked at in the project vary in size and duration from 1 square mile to 10,000 square miles and 1 hour to 120 hours.
The warm season PMP development, covering the period from June through September, is virtually complete and now includes storms from ND, surrounding states, and southern Canada. Storms from Grand Forks County (2000) and Cass County (1975) are 2 examples that have been added. These events are rain events occurring during the warm season.
The cool season storm study continues to take shape. Within the cool season study, a state-wide map of snow water equivalent has been developed. This data depicts the 1% exceedance of water held in the snowpack for varying periods during the spring months, March through May. The storm list has been finalized and the first runs of cool season PMP events have been analyzed.
The project is on track to be completed by May of 2021 with new PMP data and an accompanying GIS tool which will provide all the output in one simplified interface. In addition, this database can be continually updated if and when new storms occur, or updated methods are developed. This will prevent work developed during this project from becoming outdated.