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Hurricane Irma: the storm surge threat, explained in 400 words

Zack Beauchamp is a senior correspondent at Vox, where he covers ideology and challenges to democracy, both at home and abroad. Before coming to Vox in 2014, he edited TP Ideas, a section of Think Progress devoted to the ideas shaping our political world.

Hurricane Irma made landfall on the Florida Keys as a Category 4 hurricane on Sunday morning, with winds of roughly 130 mph. As the day went on, Irma moved closer and closer to the rest of the state — with the Tampa Bay area and other parts of Florida’s western Gulf Coast likely to be hit particularly hard as it goes north.

Owing both to the speed of the storm’s winds and its sheer size, Hurricane Irma is likely to cause a devastating storm surge across the region. And it’s this rising tide of floodwater — not the wind knocking down buildings — that is “greatest threat to life” from this storm, per the National Oceanic and Atmospheric Administration (NOAA).

“Storm surge is water from the ocean that is pushed toward the shore by the force of the winds swirling around the hurricane,” NOAA’s website explains. “Storm surge combined with waves can cause extensive damage. It can severely erode beaches and coastal highways. The pounding waves can take out boats and buildings.”

Bill Read, the former director of NOAA’s National Hurricane Center, told the Capital Weather Gang that "the Keys through Tampa will likely experience the worst storm surge event that area has seen in generations.”

Here’s a graphical representation of what Read is talking about, based on Sunday morning NOAA data. It shows a pretty massive storm surge all across Southwestern Florida:

(NOAA/Zack Beauchamp/Vox)

The largest storm surge is projected to hit the most Southwestern parts of the state, like the Naples-Marco Island Metropolitan Area (population 322,000). But the threat to the Tampa metropolitan area, at the very top of the map, is also extremely dire.

Much of the Tampa area, whose population is about 4 million, is built right on the water (as you can see on the below map). The area, which hasn’t seen a hurricane of Irma’s strength since 1924, has little in the way of coastal defenses to block waters from sweeping up and covering the city. This vulnerable urban planning, together with the region’s relatively dense population, means that it is tremendously vulnerable to Irma’s storm surges.

(City of Tampa)

“The metropolitan area is the most vulnerable in the United States to flooding and damage if a major hurricane ever scores a direct hit,” Darryl Fears wrote in a richly reported Washington Post piece published in late July on the risk a major hurricane poses to Tampa. A 2013 World Bank study that ranked cities according to their vulnerability to major storms placed Tampa number seven — not among American cities, but among all cities in the world.

We are not really prepared for how bad this could get.