Using the Hydrologic Ensemble Forecast Service (HEFS)

### Produced by The COMET® Program

### 3.0 Developing Products from Hydrologic Ensembles » 3.4 Probabilistic Guidance

### Middle 50% from the ensemble, and the median

### Box plot

The information in the box and whiskers plot can be presented in a time series graphic of stage (or discharge), like the one shown in the first tab. We’ll look at how this kind of HEFS probabilistic guidance is constructed and how to interpret it.

The blue shading in the time series shows the middle 50% of possible outcomes according to the ensemble, which is depicted by the box portion of the box plot (tab 2). The dark blue horizontal line is the median. The probability that the outcome will be above the blue shading is <25%. Likewise, the chance of it being below the blue shading is ≤25%. Therefore, the blue shading depicts the probability of exceedance from 75% to 25%.

The medium shade of blue enveloping the darker blue shading represents the middle 80% of possible outcomes according to the ensemble. This interval represents exceedance probabilities ranging from 90% (bottom) to 10% (top).

Finally, the light shade enveloping the medium and dark blue shading represents the middle 90% of possible outcomes according to the ensemble. This interval represents exceedance probabilities ranging from 95% (bottom) to 5% (top), and is consistent with the whiskers on the box plot.

#### Question 1 of 3

The ensemble forecast system suggests that the discharge at point A has a _____ chance of occurring. (Choose the best response.)

a) 50%
b) 75%
c) 25-50%
d) 50-75%

The correct answer is d.

The ensemble forecast system suggest that the discharge at point A has a 50-75% chance of occurring.

Please make a selection.

#### Question 2 of 3

The forecast system suggest that the discharge at point B has a _____ chance of occurring. (Choose the best response.)

a) 0%
b) 5%
c) < 5%
d) < 1%

The correct answer is c.

The ensemble forecast system suggest that the discharge at point B has less than a 5% chance of occurring.

Please make a selection.

#### Question 3 of 3

The orange dashed line depicts the ensemble member with a 5% exceedance probability. (Choose the best response.)

a) True
b) False

The correct answer is b.

The orange dashed line does not necessarily depict a single ensemble member. It may be tempting to interpret the boundaries between shading in the probability plots as hydrographs (in which case, the line would represent the 5% exceedance hydrograph). But that’s not the correct way to interpret these plots. At any given time, the 5% exceedance probability is where 5% of ensemble members lie along and above that value. The actual member that meets this criterion will likely change from one lead time to the next.

Please make a selection.