Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 1.0 Introduction

Hydrologic events, such as floods and droughts, are among the highest impact phenomena in the United States and around the world. Reducing their impact is an important goal of NOAA’s National Weather Service. A necessary step in supporting weather readiness and decision making is to provide forecasts that include explicit estimates of the forecast uncertainties. This information enables users to perform risk assessments and take actions appropriate for their vulnerability.

In this lesson, we will introduce the Hydrologic Ensemble Forecast Service (HEFS). Before we get to the specifics of the HEFS, we will examine some basics of hydrologic ensemble forecasting that underlie it.

After completing this lesson, you should be able to:

1. Describe the need for ensemble forecast information in hydrology
2. Compare and contrast the ensemble forecasts to single-valued forecasts
3. Illustrate how an ensemble forecast is used to build probabilistic guidance
4. Describe the major sources of uncertainty in HEFS (forcing vs hydro model)
5. Identify the main functions for different components of the HEFS
6. Describe the advantages of the HEFS over ESP
7. Interpret HEFS products