Hurricane Irma broke records for the duration of its incredible winds this week. Since then, the storm has weakened to Category 4, with sustained winds of about 150 miles per hour—but those are still incredibly powerful gusts. The danger posed by these winds lies not only in their capacity to topple aboveground structures but also in how they whip up the ocean. In particular, the potential for storm surge—a dramatic rising of the sea—could extend the destructive effects of the already deadly event.
Because Irma is so huge, hurricane-force winds are stretching almost 150 miles across the storm. All that wind damages houses and causes power outages once it reaches land. But it also riles up the ocean it travels over. That includes pushing water out around the storm, and pushing water down through the water column.
When a hurricane is over open ocean, the wind-shaped water is not a big deal. But as the storm approaches shore, there's not as much depth for the downward moving water to travel through, nor as much space for the outward moving water to disperse to. Instead, the water piles up and flows onto land. The water from the storm surge is in addition to normal tides and rainfall.
While storm surge is one of the key threats of Hurricane Irma, Florida is actually better placed to cope with it than places like the Texas and Louisiana coasts. That's because the seafloor around Florida slopes more steeply, which heads off the surge.
Storm surge is particularly dangerous because it moves quickly—water can rise feet in minutes. And it isn't gentle: The sheer force of the horizontal movement of water, which weighs almost a ton at about 1,700 pounds per cubic yard, is enough to sweep cars away. The storm surge is a key reason why Hurricane Katrina was so deadly and Superstorm Sandy so destructive.
To predict storm surges, the National Oceanic and Atmospheric Administration uses a computer model called Sea, Lake and Overland Surges from Hurricanes, or SLOSH, which maps features along the U.S. coast that shape storm surge and its risks, like inlets and ports. To measure storm surge during storms, the U.S. Geological Survey installs temporary storm-tide sensors which augment its standard network of coastal monitoring.
The National Hurricane Center imposes storm surge watches and warnings in addition to hurricane watches and warnings. A storm surge warning, like the one currently in effect for the Florida Keys and Jupiter Inlet, indicates life-threatening danger within the next 36 hours.
During the last extraordinarily large hurricane to hit Florida, Andrew in 1992, storm surge ended up being less deadly than forecasters had feared, likely due to coastal evacuation and a ridge of high ground formed from millennia-old coral reef, which blocked the water. It's too early to tell whether the same good luck will meet Irma.