People across southern Louisiana are spending the weekend worried about flooding. The water is coming from every direction: the Mississippi River is swollen with rain that fell weeks ago farther north, and a storm called Barry is pushing ocean water onshore while it drops more rain from above.

It's a situation driven by climate change, and one that Louisiana has never dealt with, at least in recorded history. And it's raising questions about whether New Orleans and other communities are prepared for such an onslaught.

"It is noteworthy that we're in our 260th day of a flood fight on the Mississippi River, the longest in history, and that this is the first time in history a hurricane will strike Louisiana while the Mississippi River has been at flood stage," said Louisiana Gov. John Bel Edwards in response to a question about climate change at a Friday news conference.

"If we anticipate that this could happen with more frequency going forward, then it has to inform a lot of things we do in the state of Louisiana to prepare for disasters in the future," the governor continued.

Warm Water, Warm Air

The storm called Barry formed over hotter-than-usual water in the Gulf of Mexico, and that helped it gain strength and pick up moisture.

That makes Barry the latest in a string of recent tropical storms and hurricanes whose greatest threat is rain, not wind; most notably Harvey in Texas and Florence in the Carolinas.

Studies of those previous hurricanes, as well as other storms, have found that warm water and warm air both contribute to deadly flooding. The warm water evaporates and the warm air acts like a sponge for moisture that then falls as extreme rain. A study published last year found that hurricanes including Katrina, Irma and Maria are dumping about 5 to 10% more rain than they would have if global warming wasn't happening.

Another study found that the amount of rain that fell on the Houston area during Hurricane Harvey in 2017 was equal to the amount of water that evaporated from the Gulf into the storm as it formed.

Barry has another thing in common with recent storms: it's moving extremely slowly. On Saturday morning, it was traveling toward land at just a few miles per hour.

A study published last year found that slower tropical cyclones — which include hurricanes and tropical storms — are getting more common. Researchers looked at tropical cyclones around the world and found they have slowed down 10% in the past 70 years.

When storms move slowly over the water, it can give them more time to gain strength and pick up moisture, but the real danger is when storms move slowly after they make landfall, dumping rain on one area for hours or even days.

If Barry were to stall over southern Louisiana this weekend, it could drop more than 15 inches of rain. As a result, flash flood watches and warnings are in effect for the entire region.

The Wettest Year

The rain from Barry is falling onto a Lower Mississippi River region that is already saturated with water from the wettest 12-month period on record.

The rain started months ago, hundreds of miles north of Louisiana. Waves of extreme rain have battered communities along the Mississippi River and its tributaries since February, from the Dakotas and Minnesota down through Nebraska, Oklahoma, Illinois and Missouri.

Unlike Barry, the storms did not have names, but they, nonetheless, flooded homes and farm land across an enormous swath of the Central U.S. It's the latest, and one of the most extreme, examples of an uptick in the number of extreme rain events in many parts of the U.S. as the earth gets hotter.

"Increasing precipitation, especially heavy rain events, has increased the overall flood risk," according to the most recent National Climate Assessment.

The water from this spring's rains flowed downstream, into the Mississippi River and down toward the Gulf of Mexico. As a result, the Mississippi River in New Orleans was already high when Barry arrived, pushing ocean water upstream as storm surge, and dumping rain onto the region.

The initial storm surge did not cause the river to flood overnight on Friday — good news for low-lying New Orleans. But, as rain falls throughout the weekend, the river is forecast to keep rising, putting even more pressure on the levee system that protects the city.

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