Short Lead-Time Hydrologic Ensemble Forecasts from Numerical Weather Prediction Model Ensembles

U.S. National Weather Service (NWS) River Forecast Centers (RFCs) have produced hydrologic forecast ensembles for many years using historical precipitation and temperature as the forcing data for the ensembles. This has worked well for long lead-time ensembles of greater than thirty days. However, at least in the eastern United States, there is more interest in shorter lead-time ensembles, particularly in the period of three to seven days that convey hydrologic forecast uncertainty resulting directly from the uncertainties in precipitation and temperature forecasts. This paper will demonstrate the use of numerical weather prediction (NWP) model ensembles as the forcing for hydrologic models within the NWS River Forecast System (NWSRFS) Ensemble Streamflow Prediction (ESP) system. This approach makes it possible to produce probabilistic hydrologic forecasts that incorporate the skill and uncertainty of atmospheric models. While there are significant issues in directly using the meteorological ensembles, including biases, spatial resolution of the available data, and treatment of only a portion of all uncertainty sources, the approach allows for the generation of information for a time frame at which there is little current probabilistic hydrologic guidance available. This paper presents a description of a joint project among three NWS RFCs to incorporate ensemble forecasts from a variety of meteorological model ensembles. These include the National Oceanic and Atmospheric Administration (NOAA) NWS National Centers for Environmental Prediction (NCEP) Global Ensemble Forecast System (GEFS) and Short Range Ensemble Forecast System (SREF). The paper briefly outlines procedures to convert the incoming gridded ensembles into the necessary hydrologic inputs and discusses a variety of potential products that are being generated. We present a recent case study as a representative example. Finally, there is a discussion on how this project fits in with future ensemble forecast work including the NWS Office of Hydrologic Development (OHD), Hydrology Laboratory, Hydrologic Ensemble Forecast System (HEFS).