Tropical storm forming by Louisiana may test New Orleans river levees

Bourbon Street is halfway under water today.

A weather system over the Gulf of Mexico dumped six to nine inches of rain on New Orleans by Wednesday evening, forcing the National Weather Service (NWS) to declare a “flash flood emergency” as waters rose.

The ongoing thunderstorms are expected to worsen over the coming days, possibly developing into a tropical storm or even a Category 1 hurricane (which would be named Barry) that will likely head toward land by the weekend.

This could be only the third time in the last 168 years (since researchers started keeping track) that a hurricane hits Louisiana in July, meteorologist Eric Holthaus wrote in the New Republic. Typically, August and September are peak hurricane season in the Gulf.

The potential storm poses a significant threat to the city of New Orleans, since the Mississippi River, which snakes by the city, has been continuously flooding the surrounding land since January. Currently, the water sits at a height of 16 feet.

New Orleans has levees in place to keep the river from flooding its banks and swamping nearby neighborhoods. But those levees are only 20 feet high in some places. By Friday afternoon, the river is forecast to crest at a near-record height of 19 or 20 feet. If that happens, it’d be the highest level the Mississippi has reached in New Orleans since at least 1950, according to the NWS.

As of 5 p.m. ET Wednesday, the NWS had issued a storm surge watch for the area of the Louisiana coast between the mouth of the Pearl River and Intracoastal City. (That stretch includes New Orleans.)

A hurricane watch is also in effect for the area from the mouth of the Mississippi River to Cameron, Louisiana.

Louisiana Gov. John Bel Edwards declared a state of emergency in anticipation the impending weather front, which could dump as much as 20 inches (approximately 76 centimeters) of rain in the state over the coming days, according to the National Hurricane Center.

The biggest test of Mississippi River levees since 1927

In 2005, Hurricane Katrina — one of the deadliest storms in US history — killed over 1,800 people when storm surge levees along canals in New Orleans failed.

The Mississippi River levees, which were built in 1927, stayed intact during that storm. But this week might prove to be their biggest test ever. Gov. Edwards warned that there could be “a considerable amount of overtopping” of levees in Plaquemines Parish, a suburban district southeast of New Orleans.

“Right now 19 feet is the official forecast, and we can manage that,” David Ramirez, the chief of water management for the Army Corps of Engineers’ New Orleans District, told Slate.

But Ramirez added that his team is closely monitoring the lowest points of the levees.

“The levees protect the city up to 20 feet, but 19 is close and doesn’t include waves splashing up and so on. It’s too close for comfort for us. And that surge could be more or could be less,” he said. “If things change and it gets higher, at some point, there’s only so much we can do.”

We’re likely to see more frequent and wetter hurricanes

This past year was the hottest on record for Earth’s oceans and the fourth warmest for the planet.

As ocean temperatures continue to increase, we’ll likely see more coastal flooding because of sea-level rise (since water, like most things, expands when heated) and more severe hurricanes. That’s because hurricanes’ wind speed is influenced by the temperature of the water below. A 1-degree Fahrenheit rise in ocean temperature can increase a storm’s wind speed by 15 to 20 miles per hour, according to Yale Climate Connections.

Read More: The oceans are the hottest they’ve been since we started measuring — which means we should prepare for more disastrous flooding and storms

Currently, water temperatures in the Gulf of Mexico are at near-record levels, Holthaus wrote.

What’s more, as the planet keeps warming, Earth’s atmosphere will be able to hold more moisture. That increases the likelihood of intense rainfall in already wet areas, according to Holthaus.