Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 2.0 Ensemble Forecasts in Hydrology » 2.3 Need for Uncertainty Information

Forecasters know that there is uncertainty in single-valued forecasts. But since a single-valued forecast cannot quantify the uncertainty, forecasters are not able to communicate it to decision makers in a way that lets them mitigate against risk.

In contrast, an ensemble forecast provides multiple, equally likely predictions that can be used to explicitly quantify the probability of a future event, such as flooding, for which some mitigating action is needed. Probability guidance derived from ensemble forecasts can provide more actionable information to users with decision-making responsibility.

### Single-valued forecast

### The HEFS Probabilistic Forecast

Note that the single-valued operational forecast uses only short-term quantitative precipitation forecasts (QPF), perhaps only 24 hours, while the HEFS does not curtail the forecast precipitation. Click the tabs to see the impact on the hydrologic forecasts.

Ensemble forecasts have become a valuable tool in weather and hydrologic forecasting due to the recognition that single-valued or “deterministic” forecasts do not provide essential decision-making guidance about forecast uncertainty. As stated in a 2006 report from the National Research Council, “uncertainty is thus a fundamental characteristic of weather, seasonal climate, and hydrological prediction, and no forecast is complete without a description of its uncertainty.”

As of this writing (early 2017), the HEFS only has a limited number of displays in operation.The number of HEFS guidance products is expected to increase as implementation continues and more user feedback is collected. This lesson provides a good foundation from which forecasters can make the best use of ensemble forecast information.