Storm surge and rainfall — not wind — cause the majority of hurricane deaths, yet are absent from the current warning system. Researchers developed and tested the Tropical Cyclone Severity Scale, which incorporates all three hazards and can rate storms up to Category 6. Study shows people are more likely to correctly identify risks and evacuate when informed using the new scale. TAMPA, Fla. (Aug. 26, 2025) – Wind alone does not account for all hurricane-related fatalities. Storm surge and rainfall do as well. Yet the current warning system – the Saffir-Simpson Hurricane Wind Scale – measures a storm’s strength solely by wind speed.
A new research paper published in Nature Scientific Reports and co-authored by Jennifer Collins, a hurricane researcher and University of South Florida geosciences professor, argues that in order to keep people safe, it’s time for a change. The authors propose replacing the SSHWS with the Tropical Cyclone Severity Scale, which incorporates storm surge and rainfall in addition to wind.
“Frequently, people use the storm’s category to decide whether to evacuate,” Collins said. “That’s incredibly dangerous because if they hear it’s only a tropical storm or Category 1, too often no alarm bells go off, and they see no cause for concern.”
The paper cites hurricanes Katrina and Florence as examples. In 2005, Katrina was listed as a Category 3 based on wind speed, but most of the 1,800 deaths and $125 billion in damage were caused by storm surge and rainfall. Similarly, in 2018, Florence made landfall as a Category 1, yet its massive flooding killed 55 people.
It also cites a 2014 study by Edward Rappaport, then-deputy director of the National Hurricane Center, that stated wind is responsible for 8% of hurricane-related fatalities, while storm surge accounts for 49% and rainfall 27%.
“There have been too many instances of incredible loss of life and destruction because a low category number on the SSHWS, or even if it’s listed as a tropical storm, did not match the danger of the storm,” Collins said. “Based on our research, we now know that people are more likely to evacuate if they understand the dangers from rainfall and storm surge in addition to wind.”
Here’s how the TCSS works:
Wind, rain and storm surge are each given a number from 1 to 5 based on predicted severity, but the final combined number can reach as high as 6 to communicate the additional risk from multiple extreme hazards. If one hazard is worse than the others, the hurricane’s final category is at least that high. For example, if wind and rainfall are rated 2 but storm surge is 4, the final category is at least 4. If two hazards are rated Category 3 or higher, the final number increases by one. So, if storm surge is 4 but wind and rainfall are 5, the hurricane is predicted as Category 6. The TCSS will then list the hazard-specific categories, as well as an overall category. To test public response to these warnings, Collins and her research team conducted an online experiment with 4,000 residents along the Gulf and East coasts – regions frequently threatened by hurricanes.
Participants received forecasts for 10 fictional hurricanes affecting their communities; half received warnings using the SSHWS, and half using the TCSS.
“We were interested in whether better identification of the main hazard would improve precautionary measures,” the paper explains, “and whether the overall category difference would increase evacuation intent for more dangerous storms.”
Questions the participants were asked included:
What is the main danger of the storm – wind, rain or storm surge? On a scale of 1–5, how risky would it be to stay in your home? On a scale of 1–5, would you evacuate? Those who received the TCSS were more likely to correctly identify the main hazard and significantly more likely to evacuate for non-wind hazards.
“The higher category is important,” Collins said. “According to my evacuation research, many people base their decision to evacuate on that number, not just on the details of the hazard.”
The National Hurricane Center began using the Saffir-Simpson Hurricane Scale in 1971, utilizing pressure, storm surge and wind measurements. But in 2012, that was modified to include only the wind component and was named the Saffir-Simpson Hurricane Wind Scale.
“Without satisfactory and complete information, individuals are likely to miscalculate their personal risk or even be moved to inaction,” the paper reads.
Collins, a native of England who moved from Pennsylvania to Florida in 2005, longed to study evacuation behavior during hurricanes.
After an approximately 10-year lull in hurricanes impacting Florida, she was able to conduct her studies due to Hurricane Matthew in 2016, and then subsequent hurricanes including Irma in 2017 and Ian in 2022. In 2024, hurricanes Helene and Milton provided ample opportunity for her team to study hurricane evacuation behavior in Florida.
At a 2017 hurricane conference, she met University of Amsterdam co-author Nadia Bloemendaal, who shared similar interests. Together with the other co-authors, Jantse Mol from the University of Amsterdam and Hans de and Dianna Amasino of Tilburg University, they developed the TCSS, publishing their findings in 2021 and moving on to testing its effectiveness.
Next, Collins and her co-authors plan to present their research to the National Hurricane Center. While others have suggested changes to the hurricane scale, Collins believes their work offers strong evidence supporting such a shift.
“Change is hard for any institution that’s been doing the same thing for years,” Collins said. “But I’m fairly optimistic that now is the time. We now know many people make decisions based on the category messaging, so we need to ensure that we are communicating with a scale which is more realistic of the severity of the hurricane, considering other hazards which commonly occur, particularly from storm surge and rainfall flooding, which is considered in our scale.”
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