A tropical rainstorm — which fell just short of being named “Helene” — brought devastating flooding to portions of North Carolina on Monday, deluging streets and leaving neighborhoods under feet of water. More than 20 inches of rain fell by early afternoon, qualifying the event as a 1,000-year rain event — or one so rare it has a 0.1 percent chance of happening in any given year.

Numerous locations in far-southeastern North Carolina received more than a foot of rain.

According to the National Weather Service, 17.7 inches in 12 hours is the threshold for what constitutes such a statistically rare rain event in the region. Carolina Beach, 14 miles south of Wilmington, N.C., registered 20.81 inches, while Southport — 10 more miles south — received 17 to 19 inches.

Fire crews in Wilmington conducted numerous high-water rescues: 29 adults, two children, five dogs and three cats. The storm also brought a minor surge and overwashed Highway 12 along North Carolina’s Outer Banks. The highway was closed Tuesday morning in Ocracoke. Ferry service between Hatteras and Ocracoke was also suspended.

A sneaky storm without a name

Forecasters had been calling for 4 to 8 inches of rain, with localized amounts up to 10 inches, ahead of “Potential Tropical Cyclone 8.” The system was initially anticipated to tighten into a tropical storm, but instead it remained loose and poorly organized — despite producing tropical-storm-force winds and torrential flooding rains. Winds at Wilmington International Airport gusted to 60 mph and at Wrightsville Beach to 67 mph.

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The lack of a named storm left some residents caught off-guard. Schools in Carolina Beach didn’t cancel classes, and some parents were stuck for hours trying to pick up students amid rising floodwaters. New Hanover County dismissed students two hours early Monday and held classes remotely Tuesday.

Lynn Barbee, the mayor of Carolina Beach, wrote on Facebook: “We need our own name for this storm. Anonymous 2024 just doesn’t do it justice.” He also offered advice for residents beginning the clean-up process.

“I’ve seen a number of videos of people walking thru floodwaters,” he wrote. “I have to admit I’ve done it when I had to, but the bacteria counts will be high and can cause health issues. If you have to, you have to, but please try to avoid it. I’m not a physician but if you have been in those waters, keep your hands away from your face and wash up as soon as possible.”

Authorities in Southport, meanwhile, implemented a curfew until 7 a.m. Tuesday.

What made the storm so bad?

The system was almost a tropical storm. There were two main limiting factors. Instead of a clear-cut center of circulation, the storm had a stretched-out axis of spin, or “vorticity,” and it never tightened into a singular vortex. It also was attached to a very weak frontal boundary, or temperature difference with distance, which meant it wouldn’t be called fully tropical.

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Still, the atmosphere was tropical. Every column of air was holding 2.02 inches of water. Meteorologists call that quantity the air’s PWAT, or precipitable water. That’s in the 90th percentile for the date, meaning the atmosphere was unusually moisture-loaded. By evening, the PWAT rose to 2.34 inches, just shy of the daily record of 2.39 inches.

Ben Noll, a New Zealand-based meteorologist, noted that PWAT values near Wilmington have increased 6.1 percent over the last 80 years. That rise in moisture is in tandem with an observed temperature increase; a warmer world is a wetter world, and while flooding would have happened regardless, there was more moisture for storms to work with.

But part of what made the flooding so extreme was just flat-out bad luck. The low-pressure system was propagating west-northwest along a dissipating frontal boundary. That meant it was slow-moving. It also strung a feeder band of downpours over areas south of Wilmington, leading to “training,” or downpours that passed repeatedly over the same areas. Rainfall rates of 3 inches per hour were common in the downpours; Southside reported 2.95 inches in a single hour.

What’s next for the storm?

The low-pressure system has mostly weakened, its deficit of lower air pressures “filling in” like a coffee whirlpool when you stop stirring. The fluid stops dipping in the middle, and the spin slows down. That means it’s not able to draw in so much moist air off the southeast coast.

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For now, it’s still generating some scattered downpours inland. The remnant low-pressure center is anchored over the mountains of southwestern North Carolina. Heavy downpours and thunderstorms are pinwheeling northwestward ashore into the Outer Banks and Virginia Tidewater. That could give those regions another inch or two of rain. Flood watches span the area from northeastern North Carolina to just south of Fredericksburg, Va., into Tuesday night.