Attention tracks a hurricane's wind speed, and the storm is often treated as finished once it weakens after landfall. Freshwater flooding well inland has repeatedly proven to be the more lethal hazard.
Why wind fades and rain does not
A hurricane's winds are sustained by heat drawn from warm ocean water, and that supply is cut off the moment the circulation moves over land.
Friction over rough terrain removes energy quickly, so the wind field weakens within hours of the center coming ashore.
The moisture already in the system is unaffected by either process. A weakened storm still holds a deep column of tropical moisture and continues to rain heavily.
How slow movement multiplies totals
Rainfall accumulation at a fixed point depends on how long the rain lasts as much as on how hard it falls.
A storm that stalls or drifts slowly inland can hold heavy rain over the same basins for a day or more, producing totals that a faster storm would spread across several states.
Systems often slow after landfall because the steering flow that carried them across the ocean weakens once they move into a different pattern inland.
Why terrain concentrates the rainfall
Moist air forced up a slope cools and condenses, so rainfall totals on windward hillsides substantially exceed those on flat ground nearby.
Steep terrain also routes runoff into narrow valleys quickly, which turns high totals into a rapid rise rather than a gradual one.
Mountain regions hundreds of miles inland have consequently recorded some of the most damaging flooding from systems that were no longer hurricanes when they arrived.
How antecedent conditions decide the outcome
The same rainfall produces very different flooding depending on how much water the ground can still absorb when the storm arrives.
A wet preceding month, or an earlier system passing over the same basins, leaves saturated soil that converts nearly all rainfall into runoff.
Back-to-back systems following similar paths are therefore far more dangerous than the same two storms would be separated by a dry interval.
Why warnings shift as the storm moves inland
Hurricane and tropical storm warnings are wind products, and they are discontinued as the wind threat ends even while the rainfall threat is building.
Responsibility passes to flash flood and river flood warnings issued by local offices, which are keyed to basins rather than to the storm.
The change in product names can read as an all-clear, which is why advisories emphasize that the flooding hazard continues after the tropical warnings end.