Why Irma's Dreaded Storm Surge Didn't Materialize

Editors Note: The following article is a reprint from the Tuesday, September 12th New York Times. It's a good read and clear reminder of just how fortunate we were, this time.

Across coastal Florida, the dreaded storm surge from Hurricane Irma — caused when ferocious winds pile up ocean water and push it onshore — was not as bad as forecast. While some areas were hard hit, notably the Florida Keys and Marco Island, residents of neighborhoods north to Fort Myers, Sarasota and Tampa Bay were expressing relief.

That bit of good fortune was the product of some meteorological luck.

Because a hurricane’s winds blow counterclockwise, the precise path of the storm matters greatly for determining storm surge. Had Irma lingered far enough off Florida’s Gulf Coast, its eastern wall, where the strongest winds occur, could have shoved six to nine feet of water into parts of Fort Myers and Naples, while swamping Tampa Bay, St. Petersburg and likely Sarasota as well.

At the last minute, Irma unexpectedly veered inland right before it got to Naples, taking its eastern wall safely away from the ocean. That meant that as the storm tracked north over Naples, Fort Myers and Tampa Bay, the winds at the head of the storm were moving west and actually pulling water away from the shoreline. In Tampa, water levels dropped five feet below normal, and bewildered spectators walked out to see beaches sucked dry. In Sarasota, a manatee became stranded.

Then, once the eye of the hurricane had passed through those areas, the back side of the storm hit, pulling water east toward the coast. But by this point, the storm’s winds were weakening, and the resulting surge was not nearly as strong as feared.

That weakening was readily apparent in Fort Myers. When it passed over the city at about 7:15 p.m., the center of the storm, rather than being a well-formed eye, was a jumbled mass of thinner clouds. This suggested that the hurricane’s cyclonic structure was beginning to come apart.

Some parts of Fort Myers and Naples saw sea levels surge four to five feet above normal levels — a damaging flood, but less than early warnings had suggested.

Along the Southwest Florida coast, only Marco Island, south of Naples, appeared to have suffered from significant flooding. Police officers on Monday went door to door on the island, checking on residents who stayed behind to ride out the storm. No deaths or serious injuries were reported.

In Fort Myers early Monday morning, there were few signs of flood damage. Across the Caloosahatchee River in North Fort Myers, residents returned to mobile home parks to find them dry and intact, with what damage there was coming largely from wind, not water.

Tampa, Sarasota and St. Petersburg saw just two to three feet of storm surge, according to data from Hal Needham, a storm surge expert and founder of the private firm Marine Weather & Climate.

“That initial draining of water acted as a crucial buffer,” said Rick Luettich, director of the University of North Carolina’s Institute of Marine Sciences and an expert on storm surge. “By the time the back side of Irma hit, the storm was further inland and not quite as strong.”

Yet because Irma was so unusually large, its fierce winds also extended all the way to the east coast of Florida, pushing water inland there. Dr. Needham estimated that salt water levels rose four feet above normal in Miami — the 10th highest level seen since 1880.  “There’s always a lot of uncertainty with storms that run south to north,” said Mr. Watson. “If the storm shifts just 50 miles east or west, that can make a huge difference in terms of damage.”

Experts also warn that Florida’s west coast should not get too complacent after this bout of relative good fortune. If a major hurricane were to travel from the Gulf of Mexico and barrel northeast directly into Sarasota or Tampa Bay, that could produce surges of 15 feet or more.

“The west coast of Florida still has plenty of disasters that are waiting to happen,” said Mr. Luettich. “This was really a best-case scenario, from a track perspective, for a storm this strong.”