Photo: AP

If there is one thing you need to know about Tropical Storm Barry, it is this: The storm bearing down on Louisiana is forecast to be a flooding disaster.

Basically every aspect of the setup for Barry is lined up to unleash multiple types of flooding that could last for days on end, potentially placing the flooding on par with Hurricanes Florence and Harvey. The Gulf Coast from the Florida Panhandle to Louisiana is covered in a panoply of flood warnings from the National Weather Service. The variety and extent could expand in the coming days as Barry gathers strength and its path becomes more clear.

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The reason for Barry’s prolific flood forecast has very little to do with its official strength. While the category it reaches on the Saffir-Simpson scale as its winds speed up will be the primary way Barry is defined meteorologically, the rainfall and storm surge will define how much damage this storm does.

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Rain, which is expected to begin in earnest on Saturday, is the biggest concern, with Barry forecast to drop 10-15 inches of rain over a wide area of Louisiana and Mississippi and locally higher amounts of 20 inches or more possible. That would be bad news enough. An unnamed storm dumped more than 20 inches of rain across a large portion of Louisiana in August 2016, causing $10 billion in damage. Baton Rouge was particularly hard-hit as rain hammered the state capital.

Barry may not match the exact rainfall totals from that storm, which topped 31 inches, but the rain that does fall will do so on already swollen rivers and soggy land that can’t hold an iota more water. That can lead to two types of flooding: your standard but still terrible standing water floods, and flash floods with rushing water that can wash away cars, people, and infrastructure in their path. Both already racked New Orleans and the surrounding area on Wednesday, which is in part why everything is so soggy ahead of Barry.

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The timing of Barry is also particularly bad from a river flood perspective. The Mississippi River is still shuttling record-setting spring floodwater from the Midwest to the Gulf of Mexico. The high state of the river coupled with an approaching hurricane “would be the first time we’ve had the hurricane hit the state with rising rivers” according to a tweet from Louisiana Governor John Bel Edwards.

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“Hurricanes/tropical storms usually impact Louisiana later in the season,” Hal Needham, a hurricane expert and founder of Marine Weather and Climate, told Earther in an email. “Barry is arriving earlier and encountering Mississippi River levels that are high.”

Any rain that falls will engorge the river further, and could threaten to overtop levees in New Orleans. If that happens—or if a levee fails—the result would be catastrophic for the city. And that brings us to yet another type of flood the Gulf Coast could see. As floodwaters rush down rivers and creeks toward the Gulf of Mexico, they’ll run into a wall of saltwater trying to come up as Barry plows inland.

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Barry’s storm surge is expected to be relatively minor compared to, say, Hurricane Michael from last year, because the storm is likely to be smaller and less organized, according to Needham. But Barry will raise water levels along the coast and push at least some surge inland, essentially damming freshwater coming downstream in place and leading to something known as compound flooding. Something similar happened during Hurricane Florence last year where water levels stayed elevated even after the storm had passed.

“Even if a community is not flooded by saltwater, elevated salt water levels along the coast can contribute to flooding by slowing the drainage process,” Needham said.

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This whole setup also neatly illustrates how climate change and the way we’ve developed can create overlapping risks that will only increase in the coming decades. Heavy precipitation events like the ones that inundated the Midwest this spring and are now clogging the Mississippi River in Louisiana are becoming more common in every corner of the U.S. The warmer atmosphere human carbon pollution has created simply holds more water, leading to more prolific downpours. While no research has been done to attribute any aspect of Barry to climate change, the expected heavy rain also certainly fits that pattern. And attribution research for Harvey two years ago showed that climate change did increase the odds of the storm’s record-setting rainfall.

At the same time, the infrastructure we rely on has also added to the flooding woes we’re facing today. In Houston, for example, paving over the prairie grass that used to absorb floodwaters with concrete that accelerates runoff and keeps waters aboveground contributed to Harvey-related woes. Ditto for New Orleans, which is partly located below sea level and relies on a series of levees largely built in the 20th century. And with days of rain ahead, we’re about to see if last century’s protections can keep this century’s water at bay.

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