Let me try again...
In any single run there are big variations from day to day - ups and downs. If each perturbation ends up with a completely different evolution from the others in the timing of ridges and troughs, then the mean of all perturbations will be fairly flat (even if generally it might be lower or higher than the seasonal norm).
If on the other hand most of the perturbations end up with, say, a rising trend in week one, then a dip, then another rise in week 2, then the mean line will also vary up and down and be less flat.
This evening’s ENS mean line wiggles a lot. That means that most, albeit not all, perts are following the same general pattern over the 15 days rather than averaging each other out. Earlier this week we had a lot of very flat ensemble means which give the impression of stable weather when in fact they hide huge variability.
Originally Posted by: TimS