Spread-bias Diagram: Lead Hour 6
Let's examine the spread bias plots for the downstream point at Newport. We will look at four different lead times, corresponding to the box plot lead times that we examined.
In the 6-hour lead time, we see a great example of what underspread forecast ensembles look like in the spread bias plot. The plotted point on the left axis indicates that 56% of the observations fall completely below the ensemble range. Thus, there is major overforecasting. These 56% of points would correspond to box plots that lie completely above the zero line.
The point on the right axis indicates that about 40% of the observations fall completely above the ensemble forecast range. This is underforecasting.
So if we do the math, we had 56% of the observations below the ensemble distribution, and 40% above the ensemble distribution. Adding 56 and 40 shows that 96 percent of observations did not fall within the ensemble distribution. That means only 4% did. Therefore, the ensemble forecast distribution is severely underspread. Recall, this is the time period where the box plots were very tiny indicating that lack of spread.
