Historical data only. Rule of thumb is usually 30 yrs minimum of records/data from usgs/ nws or similar weather stations. But it's often less. Then it's just

Historical data only. Rule of thumb is usually 30 yrs minimum of records/data from usgs/ nws or similar weather stations. But it's often less. Then it's just a probability eqn, and there are various rules for making adjustments based on things like distance away from the weather gages. But the more fundamental thing is you are setting a static design benchmark, not a future or perpetual target. Once that benchmark is set, you apply factors/probabilities of various failure modes. Or you can just choose a larger design-storm. But you don't say 'I'm designing for what the 2025 or 2074 or 2124 100-yr storm will be'.