They got 36cm of rain in short order. Places that aren't in the 100-year flood plan still got drowned.
The area is primarily rolling hills. So every time it flattens out just a little bit every tiny creek just overflowed.
A couple hundred miles inland you can't just expect everyone to build only on the top of the highest location.
This is deep in climate change denier country. I suspect we're about to get some climate change converts, after it's way too late.