A Summary of U.S. Watershed Precipitation Forecast Skill and the National Forecast-Informed Reservoir Operations Expansion Pathfinder Effort

Abstract Forecast-Informed Reservoir Operations (FIRO) seeks to improve the efficiency and effectiveness of reservoir operations by leveraging reliable and skillful meteorological and hydrological forecasts to inform water management decisions. FIRO aims to provide increased operational flexibility to optimize competing resource objectives within a catchment area, its reservoir, and regions downstream. While there are many physical, hydrological, environmental, and engineering considerations that make FIRO potentially viable versus impractical at any given reservoir and dam, the potential success of FIRO nationwide relies fundamentally on quantitative precipitation forecast (QPF) and streamflow forecast skill from numerical weather prediction and/or operational forecast guidance. This study demonstrates that western U.S. watersheds in California, Oregon, and Washington exhibit on average the highest QPF skill based on the critical success index (CSI) for extreme watershed precipitation days (quantified by mean areal precipitation values above the top 1% of wet days locally). Watersheds in New England and the mid-Atlantic exhibit similarly high QPF skill but reveal a more rapid decrease in skill as lead times increase. The national variability in QPF skill is reflected within the U.S. Army Corps of Engineers (USACE) portfolio of dams with highest watershed QPF skill for sites within the Sacramento, Portland, Seattle, San Francisco, Los Angeles, Wilmington, Charleston, Baltimore, and New England Districts, among others. The results of this study motivate further national QPF skill analyses as part of the “FIRO Phase III: National Expansion Pathfinder” effort to develop and apply a FIRO screening process nationally across the USACE portfolio of dams. Significance Statement The viability of Forecast-Informed Reservoir Operations (FIRO) at a given reservoir relies in part on precipitation and streamflow forecast skill. This study illustrates the spatial characteristics of precipitation forecast skill for watersheds across the United States and summarizes the results by the hydrologic region and for U.S. Army Corps of Engineers (USACE) districts in support of FIRO Phase III: the National Expansion Pathfinder effort.