Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 5.0 HEFS Examples » 5.2 Focusing on Specific Thresholds

Probabilistic guidance is often used to asses the risk of certain high or low thresholds. In this graphic, the labels for different flood severity levels have been superimposed on the HEFS probabilistic time series. Although the HEFS does not currently have official tabular products, the HEFS information can be used to generate probabilities of reaching or exceeding important thresholds.

Stage | Action (2.4m, 7.9ft) | Flood (3.4m,11.1ft) | Major Flood (5.1m, 17.0ft)
Probability | 40% | 25% | 3%

The table shows the probabilities of reaching Action, Flood, or Major Flood stage during the 10-day forecast period. Tabular information could also be generated with more time resolution rather than cumulative over the forecast period.

We showed this dataset earlier with the HEFS EnsPost functionality that identifies uncertainty associated with the hydrologic model and adds appropriate spread to the forecast. Here are the values for those cumulative probabilities.

Stage | Action (2.4m, 7.9ft) | Flood (3.4m,11.1ft) | Major Flood (5.1m, 17.0ft)
Probability | 55% | 38% | 16%

Although major flooding was not observed, nearby basins did experience major flooding. Users who are sensitive to such high water can benefit from guidance when small chances of such an event is possible.