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Why a storm surge can be the deadliest part of a hurricane

It starts to build up before the hurricane makes landfall.

In late August 2005, hurricane Katrina was moving its way across the Gulf of Mexico. It was classified as a Category 3 storm. Dangerous, but in a region with a long history of Category 4 and 5 hurricanes, hardly something residents hadn’t seen before. Or so it seemed. Katrina was only one day away from the Louisiana coast when the mayor of New Orleans issued an evacuation order. Unfortunately, it was too late.

The sea level had already risen, in a phenomenon called storm surge. Water rapidly rose up against the city's levees, a series of walls designed to keep the area from flooding. Before Katrina made landfall, the levees broke. A wall of water rushed into the city, trapping thousands. What followed was one of the worst natural disasters in US history.

Storm surge was the main cause of death during Hurricane Katrina. In fact, it can be the most dangerous part of any hurricane, and it is only partly determined by wind speed (the aspect of a storm on which the hurricane categories are based). It occurs when strong winds from an approaching hurricane push water into the shore.

What makes this rise particularly dangerous is that it starts to build up before the hurricane makes landfall. So the coastal flooding from it can make evacuation procedures and the impact of a hurricane much worse.

For instance, in 2008, hurricane Ike caused a big storm surge around Galveston, TX, a day before it made landfall. The rising water cut off evacuation routes, stranding hundreds. More recently, the National Hurricane Center issued dire warnings for the storm surge accompanying Hurricane Irma, the most powerful storm to rage through the Atlantic Ocean.

A storm surge can also be unpredictable. Rising sea levels caused by global warming increase the risk of storm surge. But there are many factors that can change its size and strength. For example, the largest storm surge recorded before Katrina was about 22 feet, during Hurricane Camille in 1969. But Camille was a category 5 storm with 190 mph winds. Katrina was only a category 3 with 130 mph winds, but it had a storm surge of about 28 feet.

Katrina was twice as wide as Camille, which dramatically increased the storm surge. So in addition to wind speeds, meteorologists predict storm surge by taking into account the size of the storm, its direction, angle of approach, atmospheric pressure and the shape and slope of the coastline. These calculations help forecasters warn communities at risk, but they’re often not enough to prevent damage on the ground.

In the US, the Eastern and Southeastern coastlines are among the most vulnerable areas for storm surges. Along the East Coast, hurricane Sandy produced a massive storm surge in 2012. On the Gulf Coast, places like Galveston, Texas and New Orleans have seen multiple hurricanes so they've built some infrastructure to help defend against excessive flooding. Levees, canals and seawalls are designed to stop or redirect rising water away from cities. But even those can be inadequate, when faced with an especially strong hurricane, like Irma.

What really concerns experts, though, are places that don’t experience a lot of hurricanes but are still vulnerable to storm surge, like the coast of Georgia and Northern Florida. These areas have shallow water, which means sea level can rise faster and water can reach further inland making the flooding worse. But they’ve seen fewer hurricanes than the Gulf Coast and they are likely to be less prepared.

So when a major hurricane like Irma hits low-lying areas like these, the storm surge can be the first and deadliest thing headed their way.