Using the Hydrologic Ensemble Forecast Service (HEFS)

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### 2.0 Ensemble Forecasts in Hydrology » 2.1 Single-valued vs. Ensemble Forecasts

An ensemble forecast is, effectively, a collection of single-valued forecasts of the same variable at the same locations and times. For example, a single-valued forecast from a particular model may show the expected evolution of river stage at a specified point. If we run the model multiple times, each with slightly different inputs (such as precipitation or soil moisture states), we will have an ensemble forecast of river stage for that location and time period. An ensemble can also be created using the same inputs from multiple models. Each single-valued forecast is an ensemble member.

#### Question

In the ensemble forecast above, which member has a better chance of verifying against the observations, member 1 or member 2? (Choose the best response.)

a) Member 1
b) Member 2
c) They have the same chance

The correct answer is c.

In a forecast system like the HEFS, each ensemble member has an equally likely chance of verifying as the correct forecast. A dense cluster of ensemble members suggests that values within the cluster have a higher probability of occurring, but not that an individual member is more likely than another. Therefore, ensemble member #2 is no more likely than ensemble member #1.

Please make a selection.