NWS Water Supply Forecasting

The National Weather Service (NWS) has a long history in forecasting seasonal run off volumes in the western United States where snow melt is the dominant component. The NWS (known as the Weather Bureau at the time) first opened the Water Supply Unit in Salt Lake City in 1947 to issue ten seasonal supply forecasts. Today, official forecasts are generated by six River Forecast Centers (RFCs) and coordinated with other agencies. NWS forecasts rely on two fundamental tools: Statistical Water Supply (SWS) and Ensemble Streamflow Prediction (ESP). SWS employs multivariate regression equations based primarily on the observed snow pack to date as well as other predictors such as climate indices and antecedent conditions to predict the seasonal run off volumes. ESP, by contrast, uses the continuous hydrologic model at the RFCs together with some prediction of future weather and climate to generate a forecast seasonal streamflow volume. NWS hydrologic forecasters contribute their expertise to produce forecasts at least monthly between January and June across the western United States. Recent and ongoing advances in forecast technology include improvements to ESP that increasingly account for meteorological and climate forecasts and include measures of uncertainty. Comparisons are shown between the various ESP methods accounting for climate and weather forecasts. Ongoing development is taking place in product delivery as well. A new web presence is being developed to present a consolidated picture of all NWS water supply forecasts as well as allow forecast users access to information from each forecast tool. Forecast verification information is also being developed to aid users, forecasters, and managers in making the best decisions possible based on the forecast