In its brief time this week as a hurricane, Tropical Storm Debby only reached Category 1, not enough to be considered a major hurricane. That hasn’t stopped the storm from causing plenty of havoc as it moves across Georgia and South Carolina. The National Hurricane Center predicted “potentially historic” rainfall of up to 25 inches in some places.
If it feels like there’s a disconnect here, it’s because the category rating can never fully capture how destructive a storm is, said Brian McNoldy, a senior research associate who studies hurricanes at the University of Miami’s Rosenstiel School of Marine, Atmospheric and Earth Science.
A hurricane’s rating on the Saffir-Simpson scale, which runs from 1 to 5, is based only on its maximum sustained wind speed, not on the rain, storm surges or tornadoes it produces.
Factors in the environment, including the warmth of the ocean, can influence both a storm’s rain and its winds. But the main reason for Debby’s extreme precipitation is simply how slowly it is crawling across the Southeast, Mr. McNoldy said. And that leisurely pace is the result of the large systems of atmospheric currents that are steering the storm.
“The primary ingredient for being a rainmaker is just the duration,” Mr. McNoldy said. “If it’s moving right along, you’re going to get less rain.”
Debby is certainly loaded with a lot of water. One big reason: The waters of the Gulf of Mexico and the Caribbean have been abnormally hot for months. This warmth causes more of the water to evaporate, providing storms with more moisture to unleash as rain. The extra heat in the ocean also gives storms more energy with which they could potentially generate stronger winds.
But other factors stopped Debby’s winds from intensifying to the highest levels, said Kim Wood, an associate professor of hydrology and atmospheric sciences at the University of Arizona. Before making landfall in Florida this week, the storm crossed Cuba, where the mountainous terrain very likely disrupted the low-level winds that channel energy from the sea surface into the storm.
“The water was plenty warm, but the storm itself had to be organized,” Dr. Wood said.
The fact that hurricanes can vary so much in how they cause harm makes communicating the risks tricky, said Jennifer Collins, a professor of geosciences at the University of South Florida.
“If you tell people that a Category 5 is coming, they are more inclined to evacuate and/or prepare,” she wrote in an email. “But we don’t tend to see the same response when informing people of a Category 1 or 2 hurricane” or a tropical storm, she said, even though the threats to lives and property can be just as great.
Past hurricanes, including Harvey in 2017 and Florence in 2018, wrought much of their devastation after being downgraded.
“Ultimately, we need to message that huge rain and associated catastrophic flooding events occur even when they are weaker storms,” Dr. Collins said.
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