The possibility of a punishing storm surge meeting a swollen Mississippi in New Orleans is a different threat, one that could become more common as the planet warms, spawning longer-lasting floods and earlier hurricanes.

Barry was the first hurricane to menace when the river was as high as it was in July, Boyett said.

In 1929, the year construction started on the spillway that caps the river’s height at New Orleans, the Mississippi topped at 19.99 feet (6.1 meters) in June, Boyett said. But that year saw only five Atlantic tropical systems, with two hurricanes in the Gulf, National Hurricane Center data show — and both stayed away from Louisiana. NOAA forecasters now expect 10 to 17 named storms this year, including five to nine hurricanes.

Opening spillways upriver from New Orleans can’t fix this, because they were designed to keep water flowing at a manageable rate, not to quickly drop river levels, which could cause mudslides when levees don’t dry out as fast as the water falls, Boyett said.

The changing climate means this problem could become an annual threat.

“Flooding is never a one-time thing. We’re just waiting for the next one,” said Pinter, an associate director of the University of California Davis Center for Watershed Sciences. “Given model predictions for climate change and rising sea levels and suggestions that hurricanes are maybe getting more intense, it’s something we have to keep an eye on.”