The Probable Maximum Precipitation (PMP) is an integral part of hydrology, hydraulics, and dam safety in North Dakota. It relies on up-to-date meteorological data and processes to ensure the best estimate of a natural maximum rainfall. In June 2021, the DWR completed an update of statewide PMP data through a two-year PMP study conducted by Applied Weather Associates (AWA). Additionally, the DWR continues to develop updated dam design guidelines for North Dakota. While these design guidelines are still in development, the selection and usage of PMP values in the design of dams will be governed by interim guidance.
These new datasets update the PMP values originally conveyed in the Hydrometeorological Reports (HMRs) covering North Dakota, derived in the 1970s and 1980s, that were completed by the National Ocean and Atmospheric Administration (NOAA). HMR-51 consisted of the continental U.S. 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 affect runoff and flooding.
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, development of snow water equivalent (SWE) and temperature time series climatologies, and improved understanding of the weather and climate related to extreme rainfall and rain-on-snow throughout the region.
In comparison to the HMRs, the new study provides:
- Reproducible Results
- Gridded dataset within GIS platform relies on computer analysis versus human interpretation of contours
- Reviewed 40 years of new data that resulted in including 24 new storms for analysis that were not accounted for in the HMRs
- PMP results available statewide for summer rain events and spring rain-on-snow events
- Includes a 1% exceedance daily SWE dataset for March through April which provides statewide snowmelt data
- Includes temporal and spatial distributions of PMP rainfall events
The new PMP estimates are available statewide, including those drainage basins extending across the state border such as the Mouse River, Red River, and Missouri River to Fort Peck. Within that area of interest, PMP values can be estimated for a variety of events. Consistent with the previous HMR studies, summertime rain events lasting up to 120-hours are available. In addition, springtime events or rain on snow events can now be estimated statewide rather than being limited to the Red River and Souris River.
When compared to previous PMP depths provided in HMR 51, the updated values from this study resulted in a wide range of reductions for most basin sizes and storm durations, with some regions resulting in minor increases. PMP depths are highest in southeastern North Dakota into southwestern Minnesota, and lowest in the far western study region. These spatial variations in PMP depth, match the general weather patterns of the region related to moisture availability, topography, and storm dynamics.
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.
A fact sheet with basic information regarding the PMP study is available here.
Dam Design Implications
PMP analysis is used in dam design. The DWR has regulatory authority for the issuance of construction permits for the construction or modification of dams, holding ponds, lagoons, or dugouts in accordance with North Dakota Century Code § 61-16.1-38. PMP analysis is required as part of obtaining a construction permit for certain classifications of structures. Interim guidance has been developed that discusses these requirements in detail, including interim freeboard design criteria for spillways.
During the interim period until the ongoing Dam Design Guidelines are finalized, dam construction permit applicants have the flexibility to either use the Study or the HMRs to select the appropriate PMP value. Based on the applicant’s PMP selected source, either FEMA’s P-94 or DWR’s 1985 North Dakota Dam Design Handbook will guide dam freeboard requirements. The interim guidance also provides technical direction on performing a PMP analysis using the study results.
The more accurate PMP values provided by this study benefit dam safety in North Dakota by providing a more complete picture of the potential risk posed by extreme PMP events. Generally, the more accurate PMP values will equate to less design and construction costs, while still meeting dam safety requirements. Dam owners can also use this data to develop a better understanding of the current level of risk for their existing dams.
The interim guidance for use of the PMP study is available here.
PMP Completed Study for North Dakota
Please find below the final report of the PMP study completed by AWA and the accompanying GIS tool.
PMP GIS Tools/Manual/Scripts
For more information on Probable Maximum Precipitation, please contact Karen Goff, Dam Safety Program Manager at (701) 328-4953 or e-mail.