Incorporating weather and climate predictions from NCEP GFS and CFS into operational water supply forecasts for the Western U.S

Predictions spring and summer runoff volumes -- termed “water supply forecasts” -- are issued throughout each water year to help water and energy managers allocate resources or operate reservoir systems efficiently. In recent years, the National Weather Service (NWS) Colorado Basin River Forecast Center (RFC) has augmented its traditional statistical methods for water supply forecasting by implementing operational model-based Ensemble Streamflow Prediction (ESP) forecasts, which are now made on a weekly basis. ESP forecasts largely represent future climate with a climatological ensemble, though some variations occur in practice. The NWS Office of Hydrologic Development (OHD) has developed a new approach for integrating both weather forecasts from a frozen version of the current NCEP GFS model and climate forecasts from the current NCEP CFS model, into the ESP method. Using a series of hindcasts spanning several decades, we compare streamflow forecasts produced via the new approach with those from climatological ESP, for a set of test catchments in the western U.S. We further describe the results of several objective approaches to achieve a multi-model combination of these forecasts with the statistical water supply forecasts from the NRCS National Water and Climate Center.