What is HEFS?

The Hydrologic Ensemble Forecasting System (HEFS) is a probabilistic forecasting tool currently under development by the National Weather Service (NWS).  The goals of HEFS are to provide hydrologic forecasts including an analysis of “probable outcomes” and to minimize biases in the atmospheric models and in the hydrologic models.  Although HEFS does not currently play a significant role in the development of the ABRFC Water Supply forecasts, we want to provide our partners the opportunity become familiar with the type of output that will be provided by the next generation of NWS hydrologic forecasting tools.

HEFS uses the current conditions in the NWS hydrologic model, but then uses short and long range weather forecasts and known climatology as additional model inputs.   HEFS attempts to quantify and bias-correct for the known meteorological uncertainty within the atmospheric model.  The meteorological model output is adjusted to reduce the bias associated with the possible outcomes, resulting in a set of possible weather conditions.  These possible conditions are used in the hydrologic model to create a set of possible hydrologic events.  The hydrologic output is compared to historical data and adjustments are made to reduce known error and bias levels in the hydrologic model.

The HEFS graphics on our webpage and are based upon the exceedance probabilities for the quantity being forecast (the same interpretation as the ESP charts).  For water supply, the quantity is the volume of water (either native or observed) that will be available for a given point along a river during a certain time during the year.   See the below for further information on interpreting our ESP graphics.