[s1artibartfast]

Climate. Models aren't really specific enough to predict a new 500 year storm in a specific location.

Those thresholds and definitions are based on the data record, and already encoded into regulation and a 100 years of construction.

What we see instead is Regulators simply increasing the requirements from a X year storm to a 2X year storm, and leaving the definitions. This is what I have seen with the California building code

[beerandt]

You're on the right track, but your framing is still off wrt how the engineering design process works.

Assuming that designing for a 500-yr storm has anything to do with 'predicting what a 'future 500-yr storm' (or 25-yr or 100-yr) looks like is dead wrong. Irrelevant.

The 'definitions' are not left alone, they are updated as time goes on. But with historical data, and they are not extrapolated/predicted out into the future.

Engineers (PEs) design by taking known criteria and then applying probabilities and factors. They do not predict criteria. It's a subtle but important distinction.

A 500yr event, by definition, is actually the one year probability of a 1/500 chance event.

And it's up to the designing engineer to choose and state whatever the assumptions are that go into that.

But a levee designed this year will use this years current 'storm definition' just as it uses this year's building code. Not a future one.

(Sometimes the storm/ event definitions seem stale because things like flood maps might only get updated every few decades.)