Developing an Extended Streamflow Forecast for the Pacific Northwest

This paper presents the results of a forecasting procedure developed for the reservoirs that supply water to the cities and suburbs of Everett, Tacoma, and Seattle, Washington. A six-month streamflow forecast is developed that incorporates weather forecasts provided by the National Center for Environmental Prediction (NCEP). The NCEP data are bias corrected, spatially distributed, and temporally downscaled to produce monthly forecasts for temperature and precipitation for four river basins. These forecasts are then used as inputs into a distributed hydrology model that translates the future climate conditions and the antecedent conditions into an ensemble of future streamflows. These streamflows are then incorporated into systems operation models to determine operation of the four regional water supply reservoirs. This paper also evaluates of the quality of the forecasts and describes how these ensemble forecasts can be incorporated into decision making.