Having strengthened with astonishing speed into a Category 4 storm Wednesday, Hurricane Laura will make landfall in Texas and Louisiana sometime early Thursday morning. With the landfall comes a dreaded storm surge—a rise in water level generated by a storm—that scientists say could spread seawater up to 30 miles inland, an inundation the National Hurricane Center just called “unsurvivable.”
The surge will be particularly dangerous along the coast, but it will remain a threat as the water moves inland. “You have very large currents, very large and dangerous waves pretty far inland along the immediate coastline,” says Brian Zachry, Joint Hurricane Testbed director at the National Hurricane Center. “And if you’re talking about a surge of 15 to 20 feet with very large waves, you just can’t survive that.”
“Even if you go inland,” Zachry adds, “as water gets over the tops of banks of rivers and other estuaries and such, that water can also have some velocity to it. As you see in flash flooding from rainfall, you can get swept away in that.”
For context, 2005’s Hurricane Katrina, a Category 5 storm, had an 18- to 23-foot storm surge. “This storm looks like it will be comparable as far as the levels of storm surge that we’re seeing,” says Mike Chesterfield, a meteorologist at the Weather Channel.
The size of a hurricane’s storm surge depends on a number of factors, “which makes the prediction of storm surge difficult until close to landfall,” writes Katie Peek, a coastal research scientist at Western Carolina University, in an email to WIRED. This includes wind speeds, how fast the storm itself is moving, and atmospheric pressure. “Where a storm makes landfall is also important, as shallower waters offshore and the shape of the coast play a part as well,” Peek writes. “In the case of Laura, the storm is moving through warm, shallow waters and projected to make landfall near an embayment (the shoreline is concave like a bowl) which can cause the waters to further ‘pile up’ along the shore.”
And it isn’t just the fact that the hurricane’s winds are pushing water horizontally onto shore—the storm actually lifts the water vertically. “In the center of a hurricane, you get incredibly low pressures, which actually allows a little bubble to form underneath the hurricane,” says Chesterfield. “The winds come and pick up that water and just pile it up on land. It’s a smaller factor when compared to wind, but it definitely does play a role.”
Not helping matters is the fact that warm water—which is particularly warm in the Gulf of Mexico right now—physically expands, taking up more space than cold water. And this storm could arrive during high tide, which might also add a bit to the surge.
That could mean a veritable wall of water barreling inland, overwhelming anything in its path. “Storm surge itself is and does remain the deadliest aspect of hurricanes,” says Chesterfield. “If you put yourself in a situation where there’s even 10 feet of storm surge, chances of you getting out in one piece are fairly small. But when you get up to 20 feet, there is no home structure, anyway, that’s going to keep you safe.”
This is particularly problematic where Laura could hit—in low-lying parts of Louisiana like the small cities of Houma and Morgan City. And across the state’s coast, inlets and river channels can carry the water farther inland. “You’re on the swamp, essentially,” says Jeremy Porter, head of research and development at First Street Foundation, which analyzes flood risk in the United States. Small cities are not well suited to fend off a storm surge like this. “They just don’t have the infrastructure, because they’re less populated,” Porter adds. “So there’s risk in having a lot of population, but there’s also risk of not, because you don’t have the tax base to build the infrastructure to actually protect yourself from these types of events.”