Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 4.0 The Hydrologic Ensemble Forecast Service (HEFS) » 4.4 The HEFS Compared to Previous Guidance

Goal: Improve NWS Hydrologic Services
Feature | EPS (Old Service) | HEFS (New Service)
Forecast Time Horizon | Weeks to seasons | Hours to years, depending on the input forecasts
Input Forecasting (“Forcing”) | Historical climate data (i.e. weather observations) with some variations between RFCs | Short-, medium-, and long-range weather forecasts
Uncertainty Modeling | Climate-based; no accounting for hydrologic uncertainty or bias; suitable for long-range forecasting only | Captures uncertainty from multiple sources and corrects for biases in forcing and flow at all forecast lead times
Products | Limited number of graphical products (focused on long-range) and verification | A wide array of data and user-tailored products are planned, including standard verification

Until recently, NWS ensemble streamflow forecasts within NOAA’s Advanced Hydrologic PredictionService (AHPS) were produced exclusively with the Ensemble Streamflow Prediction (ESP) system. The HEFS offers advances in ensemble forecasting that were lacking in ESP, such as:

- Use of short, medium, and long-range NWP model forecasts, such as QPF
- More comprehensive accounting for uncertainty from both the meteorological forcing and the hydrologic modeling
- More products, including verification and user-tailored products