Updating Probable Maximum Precipitation Procedures for Oregon

The Oregon Water Resources Department (OWRD) regulates more than 950 dams across the state with 173 designated as High-Hazard Potential Dams. OWRD is updating procedures to estimate Probable Maximum Precipitation (PMP) across the state. The current standard relies on NWS HMR-57, published in 1994, which applied methods developed in the 1980s. There are two phases to the effort. The first phase investigated how atmospheric river-based precipitation is influenced by ocean temperature, air temperature, and other factors, especially those measured over time to assess trend. The first phase also included an in-depth review of the methods and assumptions used in HMR-57. The second phase of this project is developing updated procedures for PMP estimation across the state and developing guidelines on generating estimates of the associated Probable Maximum Flood. State of the art tools, including numerical weather models, are used to better capture and maximize historical extreme events, and more appropriately estimate PMP in Oregon. This presentation reports findings to date.