Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 5.0 HEFS Examples » 5.3 Time Evolution of the HEFS Guidance

Here’s the HEFS probabilistic guidance compared with the operational single-valued forecast time series for a different basin (Illinois River at Tahlequah, Oklahoma) for the same 26 December 2015 storm event. The overlays show the single-value (deterministic) operational forecast in black, and the observations in green.

#### Question

Would the HEFS probabilistic guidance have given you more useful information than the single-value forecast? (Choose the best response.)

a) Yes
b) No
c) Not enough information to answer

The correct answer is c.

This question does not have a correct answer, but you may have noted that the single-valued forecast called for a major flood level. It may be considered a better forecast because the HEFS only showed very small probabilities of a major flood.

Remember that this represents a rather short lead time, with the rapid rise in river levels beginning in 24 hours. If we look at the evolution in the days leading up to the forecast, we see that the HEFS gives more information than shown on this one plot.

### 21 Dec

### 22 Dec

### 23 Dec

### 24 Dec

### 25 Dec

### 26 Dec

Please make a selection.

#### Question

Would the HEFS probabilistic guidance have provided you with more useful information than the single-value forecast? (Choose the best response.)

a) Yes
b) No

The correct answer is a.

This time the answer is yes. Because the HEFS uses QPF from short and medium-range model forecasts, its guidance can alert users to a potential significant or dangerous event several days out. Although this case has an example of a very useful single-valued forecast within 24 hours of the event, the HEFS guidance can provide very important longer lead times for major events.

Please make a selection.