A massive swell of ocean water pushed by Hurricane Isaias was expected to come ashore along hundreds of miles of the Gulf Coast on Friday night, causing coastal flooding as the storm crosses onto land.
The storm surge, a rise in the ocean caused by the hurricane’s winds, is expected to flood normally dry coastal areas, inundating homes and roads. To make matters worse, the surge could coincide with a high tide.
The threat of a storm surge stretched about 350 miles from southeast Louisiana to Florida, with the most dangerous conditions expected in the western half of the Florida Panhandle, where forecasters predict the water could rise six to nine feet. Destin, Fort Walton, Navarre and Pensacola are among the communities at the highest risk.
Officials put out a flurry of evacuation orders for the area, asking residents to leave their homes and head to higher ground before Isaias, the first Atlantic hurricane of the season, arrives on Friday night.
“The storm surge will come in very quickly, and those water levels will rise very rapidly,” said Jason Beaman, the meteorologist-in-charge at the National Weather Service office in Mobile, Ala. “If people stay within these areas, they could get trapped by the water and find themselves in a very dangerous and life-threatening situation.”
A storm surge is a complex phenomenon. Its height and intensity depend on several factors, including the storm’s size and angle of approach. Another element is the depth of the continental shelf, the ledge of land extending into the ocean, where the surge happens. Storm surges are generally more intense when the shelf is shallow, as it is along the Gulf Coast, and less intense when it’s deep.
Jill Trepanier, a hurricane climatologist at Louisiana State University, said that the column of water beneath the hurricane becomes compressed as the storm reaches a shallow continental shelf.
“The water spreads outward, but also upward, and that’s where that storm surge ultimately comes from, it’s that upward swell of this large momentous push of water toward the coast,” Ms. Trepanier said.
The Gulf Coast is also highly vulnerable because the land is relatively flat, and so the rising water easily comes up on shore into areas with homes and hotels. The water also can push farther inland on the Gulf Coast than in some other places because the land is laced with rivers, inlets, estuaries and bays.
Ms. Trepanier said it is difficult to predict whether a storm surge will reach developed areas with homes and hotels, but when it does, the inundation of water often can be surprising.
“You have a lot of development right at the coast because we love our beachfront property, “ she said. “It’s a gorgeous place but highly vulnerable.”
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