HEFS models the total uncertainty in hydrologic forecasts as a combination of different specific sources of uncertainty, including the meteorological and hydrologic uncertainties. The HEFS Meteorological Ensemble Forecast Processor (MEFP) produces reliable and skillful precipitation and temperature ensemble forecasts using forcing information from weather and climate models. A hydrologic processor produces ensemble streamflow forecasts using a suite of hydrologic and hydraulic models, which are forced by the MEFP forecasts. The ensemble of streamflow forecasts are post-processed with the Ensemble Post-Processor, in order to remove systematic biases and to account for the total hydrologic uncertainty. HEFS also includes the Ensemble Verification System for hindcasting and verification and the Graphics Generator to create configurable plots for analysis and delivery to the public.
Throughout the transition period, five RFCs will implement and test the HEFS in real-time over a large number of basins, for which extended scientific evaluation is also being conducted. In an early application of HEFS, New York City Department of Environmental Protection will use HEFS outputs to more efficiently manage the New York City water supply.