3.0 Developing Products from Hydrologic Ensembles » 3.1 Multiple Single-valued Forecasts

Consider this single-valued forecast hydrograph for a 10-day period. The peak stage reaches well above moderate flood stage during the first 48 hours of the forecast period. Making the most effective use of this forecast requires some ability to quantify that uncertainty.

Here are the 48 single-valued forecasts that make up the ensemble for that location and 10-day period. They are displayed in an “ensemble traces” plot, sometimes called a “spaghetti plot.” Note the differences in timing and magnitude associated with the peak stage and rate of change. In addition, some members show high stage occurring after the first 48 hours.

Question

From the same graphic (above) does it appear that ensemble members in the top 5% reach moderate or major flood stage during the forecast peak(s)? (Choose the best response.)

a) Yes, both moderate and major flood stages are reached

b) Yes, just moderate but not major flood stage is reached

c) No, neither moderate nor major flood stages are reached

The correct answer is b.

The graphic shows the middle 50% of ensemble members in the blue shading. The entire middle 50% of forecasts remains below flood stage. The top 5% of ensemble members for the 48-member ensemble is represented by the top 2 members at any given time. In the figure this is approximated with the 5% exceedance probability in dashed orange (the top two members lie above this line). The highest 5% of members are forecasting a moderate flood peak but not a major flood.

Please make a selection.