Florida’s Gulf Coast is vulnerable to storm surge. Idalia is pushing up to 15 feet of it

When Tropical Storm Idalia barrels into Florida’s Gulf Coast on Wednesday, forecasters predict it will bring up to 15 feet of storm surge ashore in the Big Bend region — one of the most surge-prone areas in the state.

The National Hurricane Center’s advisory at 11 a.m. Tuesday predicted the biggest surge, which could peak between 10 and 15 feet, would hit between the Aucilla River and Yankeetown. This sparsely populated area includes parts of Taylor, Dixie and Levy counties. Although much of the coastline is made up of undeveloped nature preserves — which may limit the property damage and risk to human life — there are populated coastal areas at serious risk, including Cedar Key.

Florida’s Gulf Coast is particularly vulnerable to storm surge thanks to a wide, shallow stretch of seafloor called the West Florida Shelf. The shelf extends up to 200 miles off the state’s west coast and runs from the panhandle to the Florida Keys. The ocean here is less than 100 meters (330 feet) deep.

Read More: Idalia strengthens to a hurricane. It could be a Category 3 by the time it hits Florida

“You got really shallow water for a long ways off shore, which allows the wind to drag water toward the coastline more effectively,” said Mark Luther, an associate professor of oceanography at the University of South Florida. “The shallower the water is, the more potential there is for the wind to pile water up against the shoreline.”

The Big Bend, which sits in the crook of the coastline between the panhandle and the peninsula, is particularly prone to storm surge because the land on either side funnels water into it.

“We saw that with Hurricane Dennis in 2005,” said Florida state climatologist David Zierden. “The eye of the hurricane made landfall way over on the west side of Pensacola, nearly 200 miles away, but due to the shallow shelf and the geography of the big bend, here in Apalachee Bay we had 10 feet of surge, as much or more than where the hurricane made landfall.”

Surge risk is exacerbated by both sea level rise and this year’s king tides, which will bring high tides near Idalia’s expected path to their highest level of the year this week. Climate change has already raised Florida sea levels 8 inches since 1950, according to the Florida Climate Center. If the storm strikes at high tide, it will have an additional head start on flooding.

“Every little bit adds to the flooding potential,” said Zierden.

Scientists warn that storm surge risk is widespread and hurricane track forecasts are imprecise. Last year, Hurricane Ian turned south at the last minute, directing the worst of its fury toward Fort Myers instead of Tampa Bay. That area of Southwest Florida, much more heavily populated, suffered billions in damage, much of it from storm surge, and dozens of deaths. Lee County, home to Fort Myers Beach, had a death toll topping 70 alone, many from drowning in the coastal flooding.

Read More: How will Tropical Storm Idalia affect South Florida weather? What the forecast says

“The biggest thing we learned is don’t focus too much on the predicted tracks because that could change at any minute,” said Luther. “If you’re anywhere close to that cone of uncertainty in those hurricane maps that the Hurricane Center puts out, be prepared to get out.”

Surge risk rises on Florida’s Gulf Coast

The wide stretch of shallow water west of Florida is a perfect breeding ground for storm surge. When a hurricane passes over the ocean, its strong winds push water near the surface in one direction, creating large waves. Out in the deep ocean, deeper layers of water below the surface will flow in the opposite direction and keep the sea level more or less the same. But when the ocean is very shallow, as it is to the west of Florida, a strong hurricane can push all the water in the same direction, causing it to pile up high into a large storm surge.

“Since the seafloor gradually slopes up near the coastline, there’s just no other place for that water to go other than to be pushed up toward the coast,” said Accuweather chief meteorologist Jonathan Porter.

When that wall of water hits Florida’s Gulf coast, there are many bays, inlets and estuaries that help funnel flood waters farther inland. During Hurricane Ian, for instance, the Caloosahatchee River helped flood waters move 14 miles upstream from the coast into downtown Fort Myers.

Storm surge risk remains uncertain

Idalia’s storm surge risk is widespread. The National Hurricane Center has warned about potential storm surge as far south as the Florida Keys and as far north as Mexico Beach in the middle of the panhandle. With the storm still a day or two out from landfall, there’s still plenty of uncertainty about where it will hit.

“Even if it’s well behaved and follows the forecast, the impacts could extend well outside of that cone,” said Zierden.

Tampa Bay, in particular, is at risk. For one thing, as Idalia approaches the coast, Tampa Bay is forecast to be on the upper right or “dirty” side of the storm, where winds and waves are most intense. Plus, Tampa Bay is sort of like a miniature version of the West Florida Shelf: a wide stretch of shallow water where waves can pile up on themselves and create an extra tall storm surge.

The Hurricane Center has warned Tampa Bay is at risk for a 4-7 foot storm surge.

Tides will play a particularly big role in determining the size of Idalia’s storm surge because the storm will strike at the height of King Tide season. That means high tides will be extra high — but it also means low tides will be extra low. There will be an almost four foot swing in sea level depending on the tide.

The Hurricane Center’s peak storm surge forecasts assume a worst-case scenario where the storm hits at high tide.

This climate report is funded by Florida International University and the David and Christina Martin Family Foundation in partnership with Journalism Funding Partners. The Miami Herald retains editorial control of all content.