Hurricane Irma Poses Serious Storm Surge Threat, But Path Remains 'Uncertain'

Hurricane Irma's winds are among the highest ever seen for an Atlantic hurricane, and those winds bring the threat of significant storm surge and high waves. But experts say the extent of that threat depends on where the hurricane goes, and the local characteristics of the coast and seafloor.

Everyone from the Florida Panhandle to South Carolina should be on alert to "see some really serious storm surge depending on this track," said Hal Needham, a storm-surge specialist and founder of Marine Weather and Climate, a private company that helps communities improve resiliency against coastal hazards.

Right now, that track is uncertain, but for any one spot, "a little change in the track can really change your surge potential," he told Live Science. [Hurricane Irma Photos: Images of a Monster Storm]

Rising waters

Storm surge is the rise in sea level that happens along a coast as the howling winds of a hurricane create a buildup of water. Intense storms like Irma — which is packing winds of 185 mph (298 km/h) at its core and is among the strongest hurricanes ever measured in the Atlantic — generate more surge than weaker storms do, as surge increases exponentially with winds. Storm surge accounts for nearly half of all hurricane-related deaths, according to the National Oceanic and Atmospheric Administration.

Storm size also plays a role, with bigger storms both amplifying surge levels and bringing the potential for storm surge to a wider area. Irma is relatively big for a Category 5 storm — the highest ranking on the Saffir-Simpson scale of hurricane strength — so it poses a significant threat, Needham said.

But how big storm surge is in a given area also depends on local characteristics, he said. Storm surge is higher in areas with shallow coastal waters and where the coastline is concave, which focuses the incoming water.

Conversely, the waves that a hurricane's winds whip up tend to be highest where there are deep coastal waters, he said.

The difference has to do with how water of different depths reacts to the energy of a hurricane. In deeper waters, the energy the storm generates doesn't break until it reaches land, "so it forms these monster waves" that can reach 50 feet (15 meters) or higher, Needham said. In contrast, in shallow water, the energy breaks many times before it reaches shore, so waves aren't as high, and surge dominates. [Hurricane Irma: Everything You Need to Know About This Monster Storm]

In Irma's crosshairs

The waters off the Leeward Islands and other islands in the Caribbean region are relatively deep, so although surge was expected to be significant there, it won't be as high as it might be along a shallower coastline. The National Hurricane Center forecast storm surge of 7 to 11 feet (2 to 3 m) for the northern Leeward Islands and the British and U.S. Virgin Islands, and 15 to 20 feet (4.5 to 6 m) for Turks and Caicos and the southeastern Bahamas.

"These super intense storms are pushing a tremendous amount of storm surge, so even areas without shallow bathymetry [depth] can observe locally high surges," Needham said in an email.

Large waves will act on top of that surge, the NHC warned. Such waves can also push water far inland, Needham said, and waves can pile on top of one another, with one wave breaking before the water from a previous one has washed back to sea.

Waves and surge can also outrun a storm, as happened when Hurricane Ike hit Texas and Louisiana in 2008, Needham said. Many people were waiting to evacuate before the worst winds hit but were caught when waves inundated the area and cut off evacuation routes.

For Hurricane Irma, the main question mark for storm surge is Florida, as it's uncertain what track the storm will take as it approaches the mainland U.S.

"All eyes are on Florida, where we could see a substantial surge," Needham said.

Forecast for Florida

Right now, the Florida Keys look to be the most likely part of the state to see some impact. But, as with the Leeward Islands, the relatively deep waters around the Florida Keys would mean lower storm-surge levels, Needham said, though it doesn't take much surge to overrun the low-lying roads there and cut off evacuation routes. [A History of Destruction: 8 Great Hurricanes]

If Irma makes its expected northward turn earlier along its path, it could rake along the east coast of Florida. This would likely lead to the worst-case storm-surge scenario, Needham said, because the counterclockwise circulation of the storm would keep winds pushing water onshore for an extended period of time. By contrast, if the storm were to parallel Florida's west coast, that same circulation would mean winds along the coast would move offshore, pushing water away.

The coast south of Palm Beach has deeper waters, but there are some areas, such as Biscayne Bay, where the shape of the coastline concentrates the surge, Needham said. That area saw the highest storm surge when Hurricane Andrew came ashore in 1992, the last Category 5 storm to make landfall in the U.S.

North of Palm Beach, the water is shallower; this area likely will have the biggest storm-surge threat if Irma moves up that way, Needham said. Of particular concern are the coasts of Georgia and South Carolina, where the coastline, like in Biscayne Bay, tends to amplify the surge.

It is also possible that Irma could make landfall in South Florida and move up through the middle of the state, which would put the biggest storm-surge threat at the state’s southern tip.

Experts are also concerned because the population of Florida has grown substantially in the past decade, and the state's coasts are considerably more built up than during the highest surges many spots have previously seen.

In Palm Beach, for example, the highest surge on record was 11 feet (3 m), in 1947. If Irma takes a worst-case path, the storm could easily produce a surge of 17 feet (5 m) there, Needham said.

In nearby Miami, the highest surge on record was 15 feet (4.5 m), in 1926. Needham cited a picture of that flooding that shows what such a surge can do.

"You just see the tops of palm trees sticking out," he said.

Original article on Live Science.