West-WRF 34-Year Reforecast: Description and Validation

This study presents a high-resolution regional reforecast based on the Weather Research and Forecasting (WRF) model, tailored for the prediction of extreme hydrometeorological events over the Western U.S. (West-WRF) spanning 34 cool seasons (1 December to 31 March) from 1986 to 2019. The West-WRF reforecast has a 9-km domain covering Western North America and the Eastern Pacific Ocean and a 3-km domain covering much of California. The West-WRF reforecast is generated by dynamically downscaling the control member of the Global Ensemble Forecasting System (GEFS) v10 reforecast. Verification of near-surface temperature, wind, and humidity highlight the added value in the reforecast compared to GEFS. Analysis of geopotential height indicates that West-WRF reduces the bias throughout much of the troposphere during early lead times. The West-WRF reforecast also shows clear improvement in atmospheric river characteristics (intensity and landfall) over GEFS. Analysis of mean areal precipitation (MAP) shows that at the basin-scale, the reforecast can improve MAP compared to GEFS and reveals a consistent low bias in the reforecast for a coastal watershed (Russian) and a high bias observed in a Northern Sierra watershed (Yuba). The reforecast has a dry bias in seasonal precipitation in the northern Central Valley and Coastal Mountain ranges, and a wet bias in the Northern Sierra Nevada, consistent with other operational high resolution (< 25 km) regional models. The applications of this high-resolution multi-year reforecast include process-based studies, assessment of model performance, and machine learning applications.