Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 5.0 HEFS Examples » 5.1 HEFS vs. Single-value Operational

Let’s return to the standard 10-day forecast time series from the HEFS for the Little River case in Tecumseh, Oklahoma. The overlays show the single-value (deterministic) operational forecast in black, and the observations in green.

#### Question 1 of 3

What does the operational single-valued forecast show compared to the observations? (Choose all that apply.)

a) The same level of peak flood category
b) A different number of peaks
c) Lower stages through December 28
d) A different timing of the initial rise

The correct answers are b, d.

The single-valued forecast had an earlier initial rise and more peaks than the observations.

Please make a selection.

#### Question 2 of 3

What does the HEFS probabilistic guidance show? (Choose all that apply.)

a) A 10-25% chance of the peak observed value at the time of the observed peak
b) A >50% chance of the peak observed value at the time of the observed peak
c) The HEFS median is a good forecast of peak magnitude
d) At least a 5% chance of higher flood category than the observed

The correct answers are a, d.

The HEFS probabilistic guidance shows that the observed stage fell within the 10-25% exceedance probability at the time of observed peak. During the period of high flow, the observations were well above the ensemble median, and the probabilistic guidance showed at least a 5% chance of reaching moderate flood stage.

Please make a selection.

#### Question 3 of 3

The operational single-valued forecast has higher discharge values than all of the probabilistic guidance at the time of the peak observed discharge. (Choose the best response.)

a) True
b) False

The correct answer is b.

In this case, the operational single-valued forecast was higher than the observed value throughout the period of high flows through 28 December.

### Ensemble Traces issued 26 December 2015

### Probabilistic Guidance from the HEFS with Single-valued Forecast and Observations

In the HEFS plot through 28 December, the single-valued forecast was at a greater or equal discharge value than the blue shaded areas. However the single-valued forecast was not greater than all of the probabilistic guidance. Recall that a few ensemble members lie in the unshaded area above the 5% exceedance probability. In the ensemble traces plot, we can see that at least one member peaked at a discharge level that was greater than the single-value forecast.

Please make a selection.