Debby shows there's more to a storm than wind scale: 'Impacts are going to be from water'

Hurricane Debby’s approach to landfall and potential for up to 30 inches of rain in some locations in the Southeast perfectly illustrates a problem meteorologist Barry Goldsmith wrestled with back in 2012.

Goldsmith, a meteorologist with the National Weather Service in Texas, said he went looking for "a catchy slogan" to help a broad audience understand why the traditional wind category system for labeling hurricanes does not account for how much rain might fall or how devastating storm surge can be. At the 2012 National Hurricane Conference, he said he presented this phrase: "There’s more to the story than the category."

"It's a great message," Brian McNoldy, a senior research scientist at the University of Miami’s Rosenstiel School, wrote on X in July. He's used the phrase over and over again for years, hoping to impress upon people just how much rain and storm surge matter.

The message has proven true time and again, and it’s especially true for Debby. The storm’s true threats promise to be storm surge and massive rainfall from Florida’s Gulf Coast to Charleston, South Carolina.

"This is another example of a storm where the primary impacts are going to be from water, rather than wind," said James Franklin, a retired branch chief of the hurricane specialist unit at the National Hurricane Center. "So while the forecast category (borderline 1/2) will matter to those near and to the right of the landfall location, there will be tremendous impacts elsewhere."

That's not to say the Saffir-Simpson Hurricane Wind Scale system isn't useful, especially for personal preparations. "A Category 5 gets you surge, rain, and wind damage," Franklin posted on X in August, adding that he's not comfortable with the message that category doesn't matter. "Category is a pretty good proxy for the worst winds you might experience."

But categorizing hurricanes doesn't tell the whole story. It doesn't convey the rain, surge, or speed of forward movement, experts such as Craig Fugate, the former director of the Federal Emergency Management Agency, have noted.

In some cases, people might mistakenly feel a false sense of security with the lower category storms, thinking, for example, that it’s "just a Cat 1," experts warn.

Why forward speed matters

Debby's label won't include the wall of water up to 10 feet high above the ground that could reach the coast along Florida’s Big Bend overnight Sunday and during the day on Monday. Nor does it convey the incredible flooding that could accompany the forecast rainfall along Debby’s path.

"That’s the question, how much rain falls and where," Fugate told USA TODAY on Sunday. "Based on the current forecast, some areas may see over a foot of rain as the storm moves through (Northeast Florida.)" And Fugate noted even higher amounts are forecast along the coasts of South Carolina and northeast Georgia.

At one point Sunday, the forecast for the South Carolina and Georgia coasts said up to 30 inches of rain was possible with the storm, but by mid-afternoon, the weather service had scaled that forecast back down.

A hurricane moving through an area at 20 mph leaves far less damage than a storm battering an area if it stalls or crawls through at 8 to 10 mph. It also drops far less rain than a storm that passes quickly.

Hurricane scientists have found in recent research that Atlantic storms appear to be slowing down before landfall, which piles up the rainfall and flooding issues.

A changing climate:2024 hurricane season breaks an unusual record, thanks to hot water

Should the wind scale be changed?

Several researchers have suggested taking a look at different ways to accurately convey risks.

Jennifer Collins, a professor in the Geosciences School at the University of South Florida, has co-authored research on how the public perceives the wind scale compared to other proposed alternatives. She was a co-author of a paper in 2021 that proposed a Tropical Cyclone Severity Scale, to include wind speeds, rain, and storm surge to improve public perception of the hazards.

Debby is "another example of how the category of a storm really doesn’t indicate the severity of the impacts," Collins said Sunday. "I’m very concerned about the likelihood of flooding, particularly to areas impacted by past tropical cyclones."

Collins said it’s important for the public to look at all the other products the hurricane center displays on its website.

While characterizing a storm by wind conveys the threat of its strongest winds, it tells you very little about the storm's size or the rain threat, said Phil Klotzbach, a senior research scientist at Colorado State University. "We argue that pressure is a better metric to categorize hurricanes."

It correlates better with the fatalities and damages caused by storms and gives you an idea of storm size in addition to storm intensity, Klotzbach said Sunday.

Rainfall should just be treated separately, he said, in part because of the challenges of conveying the differences in flood potential, such as a slow-moving storm or a very large storm.

Other examples of troublesome tropical storms or 'Cat One' hurricanes

Hurricane Beryl – A Category 5 in the Caribbean, Beryl made landfall on July 8 near Matagorda, Texas, as a Category 1. It caused widespread flooding with rainfall totals as high as 13 and 14 inches, leaving millions without electricity. A week after the storm, at least 300,000 customers in the metro Houston area remained without power.

Tropical Storm Allison – In June 2001, Allison flooded Texas with up to 40 inches of rain in Houston.

Debby in 2012 – Tropical Storm Debby caused an estimated $250 million in damages and claimed eight lives.

Dinah Voyles Pulver covers climate change and the environment for USA TODAY. She's been writing about hurricanes, tornadoes, and violent weather for more than 30 years. Reach her at [email protected] or @dinahvp.