howsure

What you're looking at

A forecast model is run many times from slightly different starting conditions, because we never know today's weather precisely enough to be sure about next Tuesday's. Each run is a member. The number you normally see is the middle of them.

The fan widens with lead time because small differences today become large ones later. That's not a flaw in the model — it's the atmosphere. What's a flaw is showing day seven in the same typeface as tomorrow.

One honest caveat: ensemble spread is not the same as forecast error, and it's known to be too narrow. Real errors are typically larger than the spread suggests, so treat the fan as a floor on the uncertainty rather than the whole of it.