A Seasonal Forecasting System for the Sacramento River Basin in California: Coupled Atmospheric–Hydrologic Numerical Model WEHY-HCM Integrated with a Lead Time-Dependent Seasonal Statistical Filter

To forecast monthly river flows at Sacramento River Basin in California during February–July period of a year, a numerical atmospheric/hydrologic modeling system was coupled with a statistical updating system to create a seasonal flow forecasting system. This system first dynamically downscales global forecasts of Climate Forecast System Version 2 (CFSV2) to Sacramento River Basin at 9 km resolution by Weather Research Forecast (WRF) atmospheric model. Then, the refined climate forecasts from WRF are input into WEHY-HCM which is made up of the WEHY hydrology model, coupled to the WRF model, to develop monthly flow forecasts at various sub-basin outlets of Sacramento River during February–July period. These numerical model-based forecasts are then updated by a new seasonal exponential smoothing filter that is based on forecast lead times in order to issue the final monthly forecasts. This forecasting system is demonstrated in one of the subbasins of the Sacramento River.