Overnight storms dropped 10 to 15 inches of rain on parts of the North Shore in about six hours Thursday night.
That’s heavy enough to be considered somewhere between a 100- and 500-year storm, the National Weather Service said, though the agency prefers not to use those designations anymore.
According to radar estimates, the heaviest rain fell along the St. Tammany-Tangipahoa Parish line, where one gauge in St. Tammany registered about 16 inches overnight, according to Mike Efferson, a meteorologist with the National Weather Service office in Slidell.
St. Charles parish also experienced extensive flooding. More than 120 homes took on water. St. Charles Parish saw rainfall totals in the 8- to 12-inch range, Efferson estimated.
Thursday night’s storm system was kinder to the South Shore than up north. Totals in the city of New Orleans ranged between two and four inches, Efferson said. That still caused some isolated street flooding, though, and sent residents outside to move their cars to higher ground in the middle of the night.
Rain and storms remain in the weekend forecast, with at least a 30% chance through Monday. Efferson said there’s another risk of heavy rainfall Sunday, possibly with severe weather.
How we talk about storms is changing.
Efferson said the National Weather Service is moving away from labels like “100-year storm” and “200-year storm” because they give people a “false sense of security.”
“Because if it happens once they think it can’t happen for another hundred years or five hundred years, but that is not the case,” he said.
Instead, the weather service prefers to classify storms by their “annual exceedance probability.” In other words, referring to storms by how likely they are to happen in a given year.
The conversion takes a little bit of math. A 100-year storm, for example, is an event that has a 1 percent chance of occurring in a given year. A 200-year storm has a 0.5 percent chance, and so on.
Efferson said Thursday night’s storm falls somewhere in the 0.2 to 1 percent range. That means, under the old terminology, the hardest-hit areas could have experienced a 500-year storm.
Extreme rainfall events are becoming more and more common across the state.
While the state’s average rainfall has remained relatively constant for several decades, about 60 to 65 inches per year, the rain is falling in more intense spurts, Louisiana State Climatologist and Louisiana State University professor Barry Keim told New Orleans Public Radio last year.
“Instead of having storms lasting for six, seven or eight hours,” he said, “storms are now lasting three, four and five hours. … So basically, the rain is being dumped in shorter periods of time.”
Keim said climate change is at least partially responsible for the increased storm intensity.