Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 2.0 Ensemble Forecasts in Hydrology » 2.2 Flood Risk Example

Probabilistic guidance can be generated from an ensemble forecast to assist forecasters and end users with visualizing the range of possible outcomes and their associated probabilities.

#### Question

In this single-valued forecast, what is the probability of reaching flood stage? (Choose the best response.)

a) 0%
b) A small, non-zero chance
c) ~50% chance
d) >75% chance
e) A small chance for major flood

The correct answer is a.

There are no absolutely correct answers since your experience and familiarity with the situation play a role in developing the forecast. Given the situation, you may say there’s a small, non-zero chance because the forecast peaks close to flood stage and you know that that there’s uncertainty in the forecast. However, if you believe that the single-valued forecast is perfectly accurate, you would give a 0% chance.

Please make a selection.

This HEFS graphic provides probability guidance for the same time period and initialization time as the single-valued forecast above.

#### Question

Based on this product, what should you forecast for reaching flood stage? (Choose all that apply.)

a) 0%
b) A small, non-zero chance
c) ~50% chance
d) >75% chance
e) A small chance for major flood

The correct answers are d, e.

The probabilistic forecast provides quantitative guidance about the forecast uncertainties. But like the single-valued forecast, it doesn’t provide an absolutely correct answer. It does, however, suggest a much greater chance of reaching flood stage (75%) and a small chance (10-25%) of reaching major flood stage.

Please make a selection.