Extreme Precipitation and Hydrometeorology Investigations and Needs

In lieu of standards-based hydrometeorologic and hydrologic methods for evaluating dam safety, such as the Probable Maximum Precipitation (PMP) and Probable Maximum Flood (PMF), Reclamation is now utilizing risk analysis to assess its 350 water storage dams in the western United States. Reclamation has formed a Probabilistic Flood Hazard Cadre (Flood Cadre) to estimate probabilistic flood hazards for dam safety. The Flood Cadre is currently developing programs for collecting data and methods for estimating probabilities of extreme storms and floods. Because Reclamation is no longer relying on PMP and PMF, we now are focusing our efforts on the physical understanding of extreme storms and floods, and the estimation of extreme probabilities of these events. This includes making annual exceedance probability estimates (AEPs) of precipitation and floods to about 1 in 10,000, and include uncertainty, for input to risk analyses.

A review of extreme storm and flood data in the Rocky Mountain region (especially in Colorado, Wyoming, Utah, Idaho, and Montana) indicates that there are major deficiencies in our understanding of the spatial and temporal distributions of extreme flood-producing storms at all basin scales (50 to 60,000 km2). There are major areas in the intermountain west where extreme precipitation is poorly defined and where Reclamation needs much more information on extreme storms, hydrometeorology, and climatology. Simple data comparisons show a lack of documented extreme flood-producing storms at regional scales and high elevations (greater than 2,300 m). An example from the North Platte River basin in Wyoming demonstrates the challenges.

There are clear opportunities and needs to share data and ideas to solve this difficult problem. New opportunities to improve existing extreme storm and flood data, such as cataloging and analyzing WSR-88D radar data and cloud-to-ground lightning data are feasible and should be explored.