ROC Scores: Flow Forecasts

If we look at the ROC score plots for the two points we are examining, we can get a feel for how forecast discrimination skill changes with lead time for the high flow threshold.

At the headwater point in Williamsburg, the forecast discrimination skill decreases with increasing lead time. The forecast discrimination skill seems to drop most rapidly between 60 and 72 hours. This is when the climatologically-generated QPF input, which has little or no discrimination skill, is beginning to have a big influence on the flow forecasts.

Look at the ROC score plot for the downstream point at Newport.

ROC Score as a Function of Lead Time for Top 20% of Observed Flows at Williamsburg and Newport

Question

How does the forecast discrimination skill for the downstream point at Newport compare with the upstream point? (Choose the best answer.)

The correct answer is d.

So the ability of the forecast system to discriminate between events and non-events decreases with lead time. The switch from QPF input to climatologically-generated QPF is a main factor in the evolution of forecast discrimination skill. The impact of climatologically-generated QPF occurs at later lead times for the downstream point.