Forecasters are warning of significant storm surge as Hurricane Dorian slowly moves up the east coast.
The National Hurricane Center and the Coastal Emergency Risk Assessment warn of as much as 8 feet of storm surge from Isle of Palms to Myrtle Beach in South Carolina, and up to 10 feet in some parts of the Pamlico Sound. Predictions could change if Dorian alters course or increases in intensity.
But what is storm surge in the first place? The Hurricane Center, part of the National Oceanic and Atmospheric Administration, calls storm surge "the greatest threat to life and property from a hurricane."
In short, storm surge is the water level height above what would be expected from the astronomical tides, and should not be confused with storm tide. For example, if the high tide is 2 feet above the average water level, and the storm surge is an additional 15 feet, the storm tide would actually be at 17 feet. The reverse would be true during low tide.
That said, astronomical tides play a significant role in how damaging storm surge can be, and that is certainly the case with Hurricane Dorian's potential impact in North Carolina.
"It looks like the storm will get to Cape Fear and north of Cape Fear right about the time of high tide," says Rick Luettich, the director of the Institute of Marine Sciences at UNC Chapel Hill in Morehead City. "Those are areas where the storm surge is apt to cause inland flooding and substantial erosion, perhaps erosion of the dunes."
Luettich says that storm surge flooding from Dorian could be even worse inland where the Pamlico Sound meets the Neuse River, in communities like Oriental, Minnesott Beach, and Merrimon.
Continental shelf elevation also plays a major role in storm surge. A steep shelf can blunt some of the worst surge effects in a way that a shallow shelf cannot. North Carolina, says Luettich, has an especially flat topography.
It might be easy to underestimate the force of water, particularly during storm surge. Just one cubic foot of water weighs more than 60 pounds, so each wave of surge can bring many tons of force behind it.