Comment by yorwba

Given 50 states and independent weather in each state, on average one state would experience each "once every 500 years" extreme weather event every decade. Of course in reality weather is not independent across political borders, but there are also many more locations where flood levels can be measured than just one per state. So depending on the details "once every 500 years" may not be as rare as it sounds, even without deviation from historical patterns.