The second punch is storm surge. The powerful winds of the hurricane — with some small assist from pressure variations inside and outside of the hurricane — cause massive walls of water to build up along the outer edge of the storm. These "water walls" can crash ashore with devastating impact, sometimes (as in the case of Super Storm Sandy and Hurricane Katrina) causing far more damage than the winds themselves. One cubic yard of sea water weighs more than 1,700 pounds and can wreak havoc on areas subjected to storm surge.

The third punch – and the one that's talked about the least – is flood. If a hurricane "blows through" quickly, then wind and surge will cause the most damage. However, if a hurricane decides to take a few days to leave town — and dumps dozens of inches of rain on an area — then flood becomes a major concern. This is exactly what happened with Hurricane Harvey, where much of the $125 billion in damage was caused by catastrophic flooding.

What's the risk from Florence?

In the Southeast, here's what we see in terms of risk for the counties that are under mandatory evacuation.

Although it's difficult to predict exactly when and where Florence will land, one thing is clear: It's a giant storm ready to inflict serious damage to the East Coast of the US. We're seeing damage possibilities that are far in excess of what some other risk companies are suggesting — just as we did with Hurricane Harvey.

Let's hope that we're all wrong and that the storm peters out. But given what we see so far, Florence is going to wallop the coast and flood risk — if the storm stalls — will be Harvey-like. If you're in a low-lying area, it's an especially good idea to evacuate. With extreme flooding, you won't be able to leave and first responders won't be able to get to you.

Bob Frady is CEO of HazardHub, a provider of property-level hazard risk databases. He can be reached at [email protected].