Benches and walkways are flooded by water along the river in Morgan City, Louisiana, ahead of Hurricane Barry on Saturday.

Just this week in New Orleans, an extreme storm unleashed about 8 inches of rain in a few hours, causing flash floods that shut down the city. Since then, New Orleans has been on high alert about calamitous flooding, including possible levee topping due to Barry, but those local concerns have largely abated as the storm has developed .

“I think the key is the intensity of the rainfall. Will it overwhelm us?” Twilley said. “These drainage systems just can't handle those intense rainfall events.”

“The thing to remember about South Louisiana is it’s flat,” Robert Twilley, executive director of the Louisiana Sea Grant College Program, told BuzzFeed News. When water “can’t run off, it sits there and piles up.”

Hurricane Barry is predicted as of Saturday morning to dump between 10 and 20 inches of rain over southern Louisiana and Mississippi, raising concerns about widespread flooding because of the flat landscape and already swollen rivers.

Barry’s rain threat is no surprise to climate researchers, who have found that the warming of the planet contributed to the costliest hurricanes in US history — including Katrina, Harvey, Maria, and Irma — all of which grew to much wetter storms than if they had formed under cooler conditions.

In 2017, Hurricane Harvey dropped more than 40 inches of rain in eastern Texas over four days; it was a $125 billion disaster, the second-costliest hurricane in US history. This storm’s historic rainfall levels were made as much as 38% more likely because of man-made warming, according to a 2017 study in the journal Geophysical Research Letters.

“When the air is warmer, it can hold more water vapor. So that when a storm moves through an area that has warmer, wetter air, there’s more of a source for the precipitation,” study author Mark Risser of Lawrence Berkeley National Laboratory told BuzzFeed News. There’s been 1 degree Celsius of warming in the Gulf of Mexico — “that’s what is contributing to the increased intensity of the storms,” he added.

Several similar studies published since then confirm this trend, including a 2018 analysis in the journal Nature that found climate change made it far more likely that Katrina, Maria, and Irma would produce so much rainfall. And another study, notably released ahead of Florence’s landfall in North Carolina last year, predicted strong rains that were made more likely by man-made warming.

Although there’s now consensus that warming is already making storms developing in the Atlantic Ocean and Gulf of Mexico wetter, and thus more intense, there’s no clear evidence that the climate crisis is strengthening wind speeds or the total number of storms forming each year.

Also, an area of active research is focusing on whether and how climate change is affecting what’s called “compounding” events, when more than one weather disaster strikes at the same time, resulting in a combined worse impact. This is what is playing out now in Louisiana, with Barry’s landfall hitting when rivers are already high.

“Barry appears to be playing out very much in the same direction, with the added impact of pre-storm flooding,” climate researcher Katharine Hayhoe told BuzzFeed News in an email. “Though the formal attribution remains to be done, [it] is likely related to the observed increase in heavy precipitation.”