Researchers propose a more holistic way of classifying hurricanes

Researchers at the University of South Florida, as well as universities in the Netherlands, have developed and tested a new hurricane warning schema that goes beyond wind speed and highlights the dangers of storm surge and heavy rainfall. They said it could help people make safer choices during impending storms.

The new study, published last week in Nature’s Scientific Reports journal, tested whether people can better understand hurricane risks when emergency warnings include more than just the storm’s wind speed. The current standard system— the Saffir–Simpson hurricane wind scale — classifies hurricanes based on their maximum sustained wind speeds, assigning them a number from 1 to 5. Other risks, like flooding from heavy rain and storm surges, which often cause more damage than wind, are not necessarily represented.

The researchers created a new system called the Tropical Cyclone Severity Scale, or TCSS, which combines wind, rainfall and surge, the water pushed to land by the force of a storm’s winds.

Jennifer Collins, a professor in the College of Geosciences at the University of South Florida and one of the study’s authors, said this new warning system could give the public clearer, more specific warnings, such as by highlighting when flooding is the bigger danger, even if winds are moderate.

“So wind actually only accounts for 8% of fatalities, but yet people are making their evacuation decisions that scale,” Collins said. “Storm surge, however, accounts for 49% of fatalities and rainfall accounts for 27%, so it just makes much more sense to have a more inclusive scale.”

People who saw TCSS warnings were more likely to recognize the biggest danger, plan protective actions like evacuating, and generally understand the risks more clearly. Collins said changing behavior is the area in which the new system is most effective.

“People use the Category [scale] to consider their evacuation behavior, so time and time again in my evacuation studies, I’ve heard people say, ‘Oh, if it’s a Cat 1 or Cat 2, no, I’m not going to evacuate,’ or, ‘I’m only going to evacuate for a Cat 3 or higher,’ which is a major hurricane,” she explained.

Hurricane Florence, which hit the Carolinas in 2018, peaked in the Atlantic Ocean as Category 4, with maximum wind speeds at 150 mph. The hurricane weakened as it made landfall and officials changed it to Category 1. Collins said that last-minute shift in classification cost lives because it interrupted evacuations.

“Many of those people who evacuated, a whole bunch of them returned, and those who were in the process of evacuation decided not to evacuate, and that was very deadly, because Florence caused a lot of flooding and loss of life,” Collins said.

She added that under their proposed TCSS scale, Hurricane Florence would have remained a Category 5 storm due to its surge and expected rainfall.

“So if we had told people and communicated it’s a Cat 5, people would have been more aware of their risk and not come back,” she said. “If they had evacuated, they would have stayed away. And those who hadn’t evacuated yet, I think a lot more would evacuated. So I truly believe that our scale has a lot of potential to save more lives.”

Nadia Bloemendaal, a Dutch researcher who worked on the study, said that the TCSS scale could also have provided a more accurate warning ahead of Hurricane Helene, which the Southeast last year.

“Due to Helene’s substantial storm surge threat along the Florida coastline, even at Tampa, it would have classified as a Category 2 for the storm surge threat alone! And the further up you go along the coastline, the more this Category increases,” Bloemendaal wrote in an email to StateScoop.

Collins said the study’s research team reached out to the National Hurricane Center, which still uses the Saffir–Simpson Hurricane Wind Scale, to discuss potentially adopting and implementing the new system, but hasn’t heard back from the agency yet.

“Some some people resist change, but times have changed, and our understanding of hurricanes and risk has changed,” Collins said. “So I think we should get with the times and not be stuck with something that we used a couple of decades ago.”