Do We Really Need a Category 6 for Hurricanes?

For more than 50 years, forecasters at the National Hurricane Center used the Saffir-Simpson Hurricane Wind Scale (SSHWS) to classify hurricane strength. This scale, which ranks hurricanes from Category 1 to Category 5, is based on only one metric: maximum sustained wind speed. That wasn’t always the case. Until 2012, the SSHWS also took central pressure and storm surge into account, but the NHC eliminated these factors to reduce public confusion. The trouble is, rising global temperatures are exacerbating multiple hurricane hazards, not just wind speed. In recent years, exceptionally intense storms, such as Milton, Patricia, and Typhoon Haiyan, have sparked a debate over whether it’s time to create a Category 6. For this Giz asks, we asked multiple experts which side of that debate they’re on. While some are more open to the idea than others, all agreed that simply adding a Category 6 to the SSHWS isn’t the answer—even though several past hurricanes have exceeded Category 5 wind speeds. Instead, some argue that communicating hurricane risks in a warming world may require rethinking the scale entirely, while others believe the existing system should remain unchanged. Jennifer Collins A professor in the School of Geosciences at the University of South Florida who co-developed an alternative to the SSHWS. The current SSHWS—as the name implies—is just based on wind. Regarding that scale, my thoughts on a Category 6 is that it is not needed when a Category 5 on that scale would lead to total destruction anyway. There was a lot of discussion about this in the scientific community about a decade ago, and I believe that to be the general consensus. Our newly proposed scale, the Tropical Cyclone Severity Scale (TCSS), considers that wind generally only accounts for 10% of fatalities. Storm surge accounts for roughly 50% and rain about 30%. Our scale includes all three of these hurricane hazards, assigning each one a category between 1 and 5. Then, it gives an overall category which can never be lower than the highest category given to the hazards. For example, Hurricane Florence in 2018 would be a Cat 1 at landfall for wind, a Cat 4 for storm surge, and a Cat 5 for rainfall. So, its overall score would be a Cat 5. If you consider the flooding and loss of life, I believe people who lived through it would agree that calling it a Cat 1—which the SSHWH did—does not adequately reflect the other hazards they experienced. People underestimate the risk of a low-category hurricane, or even a tropical storm, when they consider their evacuation decision, according to my previous evacuation research. The proposed TCSS also reflects the high potential risk of two or more hazards. We consider a hazard high risk when its category is classified as a 3 or higher (equal to the definition of a Major Hurricane on the SSHWS). Whenever at least two high-risk hazards have the same category and the third hazard has a lower category, this bumps the hurricane’s overall category up by 1. So, a tropical cyclone with a Cat 3 score for both wind and storm surge, but a Cat 1 score for rainfall, would be classified as a Cat 4. As such, a high-risk tropical cyclone can be classified as a Cat 6 on the TCSS in two scenarios. Either at least two of the hazards are Cat 5s, or two hazards are Cat 4s and one is a Cat 5. This is intended to warn the public of a hurricane with multiple extreme hazards. Brian McNoldy A senior research associate at the University of Miami’s Rosenstiel School of Marine, Atmospheric, and Earth Science who has tracked and written about tropical Atlantic activity since 1996. My general thought is that adding a Cat 6 is not necessary and would not add any value to the current suite of information out there. Since 1980, Cat 5 hurricanes have only accounted for about 5% of all named storms globally. In the Atlantic specifically, they have accounted for 4%. I’m not convinced that splitting that tiny number of storms into even smaller bins has any advantages. The most intense Atlantic hurricane on record was Allen in 1980, with maximum sustained winds of 190 miles per hour. None have reached that mark since then. If the threshold for a Cat 6 is at least 193-mile-per-hour winds as proposed in this study, for example, no Atlantic hurricanes to-date would qualify. Furthermore, partitioning those small numbers into even smaller numbers does not change risk communication. The National Hurricane Center describes the aftermath of a landfalling Cat 5 hurricane as such: “Catastrophic damage will occur: A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.” What additional risk communication would there be for a Category 6 if one should make landfall? Liz Ritchie-Tyo A professor at Monash University’s School of Earth, Atmosphere and Environment who also serves as deputy director of the university’s ARC Centre of Excellence for Weather of the 21st Century. The answer is “no.” The SSHWS was developed in the U.S. by a hurricane specialist and a wind engineer to put maximum sustained wind thresholds on levels of damage at landfall. If a hurricane is a Cat 5 on the SSHWS, that means catastrophic damage is expected. Thus, a new “Cat 6” just doesn’t make sense in terms of communicating threat levels. All hurricane classification systems for all tropical cyclone basins are based on maximum sustained wind speeds. Whether it’s the SSHWS in the north Atlantic and the eastern North Pacific, or other scales in the western North Pacific, Indian Ocean, and South Pacific, but the main idea is the same: once a hurricane reaches the top category, catastrophic damage is likely if the hurricane makes landfall. The main limitation of the current classification systems is not that they don’t go high enough to adequately communicate the threat, it’s that they are based solely on that wind threshold, which does not capture all the possible hazards associated with a landfalling hurricane. What we really need is a new “multi-factor” categorization system that can communicate the threat of multiple hazards, namely wind, storm surge, rainfall, flooding, and landslides. Depending on the location of landfall, different hazards will be more important. Along coastlines, wind, waves, and storm surge are extremely important, while flooding and mudslides caused by heavy rain are more important further inland. What’s more, maximum sustained wind speed does not capture the aerial extent of the storm-force winds that create waves and storm surge. The larger the area of these damaging winds, the greater the potential impact of storm surge. Similarly, the maximum wind intensity is not directly correlated with heavy rainfall. Though it’s true that Cat 4 and 5 hurricanes produce heavy rain, Cat 1 hurricanes can also produce heavy rain. Therefore, a multi-factor categorization system that can communicate the risk of various hazards would be more useful than adding an extra category to the current scale. Daniel Brown, NHC Hurricane Specialist Branch Chief NHC Hurricane Specialist Branch Chief. In this role, he oversees the unit that issues tropical cyclone forecasts and warnings for the Atlantic and eastern North Pacific hurricane basins. Currently, there are no efforts underway within NOAA to modify the SSHWS or add a new Category 6. Storm categories only communicate the wind hazard. When warning the public about the dangers associated with tropical systems, the National Hurricane Center communicates the wide range of hazards, including storm surge, wind, rainfall, tornadoes and rip currents. We do not over-emphasize the wind hazard by placing too much focus on the category, because most deaths caused by tropical cyclones are due to a water hazard. Storm surge, rainfall and inland flooding, and hazardous surf cause about 90% of tropical cyclone direct fatalities in the U.S. Further, the Saffir-Simpson scale’s Category 5 already captures the worst possible damage, which is labeled as “Catastrophic Damage.” Regardless of whether storms are getting stronger, the damage can’t get any worse than “Catastrophic.” Mark Bourassa Professor of meteorology at Florida State University’s Center for Ocean-Atmospheric Prediction Studies with expertise in air/sea interactions, surface water waves, identification of tropical disturbances, and possible precursors to tropical cyclones.