Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 4.0 The Hydrologic Ensemble Forecast Service (HEFS) » 4.1 The HEFS Design

The flowchart shows the design of the HEFS. The HEFS utilizes the same hydrologic models that NOAA/NWS River Forecast Centers (RFCs) use to produce their operational single-valued forecasts. These hydrologic models are embedded within the Community Hydrologic Prediction System (CHPS). For a detailed discussion of this design, access the following sites.

The left side of the flowchart shows the time resolution of the input forcing data. The HEFS provides forecasts for a range of forecast time horizons from hours and days to weeks and months. The HEFS is designed to merge the forcings from different time horizons and produce seamless hydrologic forecast products.

The MEFP (meteorological ensemble forecast processor) and EnsPost (Ensemble Post-processor) boxes represent the HEFS functions that quantify uncertainty. In broad terms, hydrologic forecasts deal with two kinds of uncertainty: the uncertainty associated with the meteorological forcing, and the uncertainty associated with the hydrologic response. Both can have biases. The HEFS quantifies the composite uncertainty associated with the meteorological forcing and hydrologic response, while reducing biases.

The Ensemble Verification Service (EVS) is used to verify predictions from the HEFS. The EVS will help with the long term improvement of the HEFS products.