Ensemble forecast
Models & data Ensemble prediction system EPS
An ensemble is a collection of slightly different weather calculations. Their agreement and spread show plausible developments and help describe forecast uncertainty.
Several possible futures
The atmosphere’s state is never known perfectly everywhere. An ensemble therefore runs several calculations with slightly altered starting conditions and sometimes different model assumptions. Each calculation is a member. These members are not extra weather stations.
Reading the lines
- Close together: The calculations support similar developments for that variable.
- Widely spread: Several quite different outcomes remain possible.
- Mean or median: Describes the centre, not necessarily the outcome that will occur.
An average can hide the important part
If half the members predict no rain and the other half predict 10 mm, their average is 5 mm. Yet no member in that example predicts 5 mm. It is worth looking beyond the thick central line.
What probabilities can tell us
The fraction of members exceeding a threshold helps estimate probabilities. Members are not fully independent, though, and can share model errors. Even a narrow spread is not a guarantee. Uncertainty also depends on the question: temperature may be fairly settled while a shower’s track remains unclear.