Helene’s size shocked me but the storm surge for Katrina was unusually extreme. It was a well organized Category 5 and then weakened to a strong 3 right before landfall. 

To compare with Helene, which was similar in terms of (east to west) diameter but covered much more area overall, with category 4 winds at landfall: the Weather Channel was making a big deal out of the 8ft storm surges. During Katrina, the Mississippi Gulf Coast had a 28 foot storm surge. (The Miss. Gulf Coast isn’t that geographically different from the Fla. big bend region but that plays a role too.)

Helene’s unusual movement speed kept it strong very far inland and caused massive issues in places that rarely see tropical weather. Harvey was the opposite: it stalled over Houston and dumped days of rain on a major metropolis.

I wish we could update the Saffir Simpson scale to something that takes into account more variables. There are other measurements but no storm is identical in terms of damage potential. A category 5 can not even make landfall whereas something like Hurricane Sandy was a category 1 (or equivalent since it wasn’t technically still a hurricane) when it hit NYC and caused massive damage and flooded subway systems. Sometimes, a storm hitting a place that isn’t used to them can knock over all the trees or flood rivers while a similar storm would be nothing to Miami or New Orleans.