Storm surge is the abnormal rise in water levels during a storm, as heavy hurricane winds push a bulge of water toward shore as depths become shallower. While winds are the primary cause of storm surge, it’s also affected by a storm’s angle of approach, the shape of the ocean floor and the low pressure within a storm, which slightly aids the bulging effect.
Because of the way storm surge can quickly inundate coastal locations and penetrate well inland, it’s typically one of the deadliest threats from a hurricane.
Part of the problem is the region’s topography. Florida’s western coastline along the Gulf of Mexico isn’t very deep, and it features a gentle underwater slope.
“The continental shelf is quite shallow,” Fritz said. “It doesn’t take a lot of force.”
Chris Slocum, a physical scientist with the National Oceanic and Atmospheric Administration’s Center for Satellite Applications and Research, said Hurricane Milton is approaching from the southeast — an angle that will allow it to directly push water up the continental shelf.
On top of that, sea level rise as a result of climate change has also increased the region’s risk of flooding.
It’s not clear exactly where the storm will make landfall, and small changes can make a substantial difference for particular areas, like Tampa Bay.
On Tuesday, Milton “wobbled,” according to an early evening forecast from the National Hurricane Center. The newest modeling suggests it could strike south of Tampa Bay rather than make a direct hit, which could spare the city from the worst effects.
In a forecast discussion, the National Hurricane Center said forecasts can be off by about 70 miles when a storm is 36 hours from potential landfall.
“We still can’t pinpoint an exact landfall location, especially if additional wobbles occur in the short term,” forecasters said.
A major hurricane hasn’t hit Tampa Bay directly since 1921. A 2015 report from the risk modeling firm Karen Clark & Co. ranked it as the place most vulnerable to storm surge flooding from a hurricane in the U.S. Its underwater terrain, in particular, can act like a giant funnel, channeling and trapping floodwaters in the bay.
The city’s extensive urban development over the past century puts more people and coastal structures in harm’s way. The metropolitan area is home to more than 3 million residents.
“Milton has the potential to be one of the most destructive hurricanes on record for west-central Florida,” the National Hurricane Center said Tuesday in an advisory.
Milton is expected to grow larger, and its winds should weaken as it nears land; both factors could influence how high storm surge gets.
“More intense storms are going to be able to move more water, and large storms are going to move more water,” Slocum said.
Local officials in Pinellas County, which includes the cities of Clearwater and St. Petersburg, called the forecast storm surge “not survivable” and urged residents to heed mandatory evacuation orders.
“This is the ocean coming into your living rooms,” Cathie Perkins, director of Pinellas County Emergency Management, said Tuesday in a news briefing. “This is fast, rising water with a lot of pressure behind it. So don’t think that you’re going to be able to ride that out.”
Even Florida’s east coast could get storm surge, since Milton is expected to traverse across the peninsula and remain a hurricane as it moves over the Atlantic Ocean once again. As it does, heavy winds will draw in on the low-pressure system, potentially pulling water onshore.
Northeast Florida could get 3 to 5 feet of storm surge, the hurricane center projects.
Storm surge is a serious concern with any major hurricane, which NOAA classifies as Category 3 or above. But even lower-ranked storms can generate catastrophic storm surge.
Hurricane Katrina, which reached Category 5 intensity but made landfall in Louisiana in 2005 as a Category 3 storm, produced a record 27.8-foot storm surge. In 2008, Hurricane Ike pummeled the Texas Gulf Coast as a Category 2 storm and generated a 15-foot storm surge in Galveston Island.