Is It Time to Scrap the Saffir-Simpson Hurricane Rating Scale?
MIT Tropical Expert: “The Saffir-Simpson scale is deeply deficient and should be discarded.”
Science evolves over time. Our understanding of the natural world improves as new technology comes online, leveraging more granular, reliable, repeatable data sets. The science of meteorology, like every other science, is a moving target. The Fujita tornado rating scale was upgraded and enhanced in 2006 to better reflect observed wind damage. NOAA is launching WSSI, the Winter Storm Severity Index, a new, experimental winter storm rating scale with five categories. Sound familiar?
Is it time to rethink the Saffir-Simpson rating scale for hurricanes, which only predicts damage from wind? In recent years nearly 90% of tropical cyclone deaths in the United States were not the result of wind, but water: storm surge, extreme rains and inland flooding.
93% of additional greenhouse gas warming is going into the world’s oceans, which are steadily warming. That’s not a prediction from a climate model — but rather sticking a thermometer in the water. The warmer the water, the greater the potential for rapid intensification. As the tropics warm, the air overhead holds more water vapor — jet fuel for Texas-size storms that get their energy from warm oceans. Another factor to consider: a potential slowing of the forward motion of tropical systems, triggered by a rapidly warming arctic and apparent shifts in upper level steering winds. The combination of slower storms and warmer seas and higher moisture content may be creating a perfect storm for historic flooding from tropical cyclones.
Hurricane Harvey stalled off the coast of Texas in 2017, unleashing up to 60.58” of rain. Hurricane Florence stalled over the Carolinas in September, 2018, dumping about 9 trillion gallons of water on North Carolina. In September of 2019 a weak Tropical Storm Imelda parked itself off the Texas coast again, dumping up to 43” rain; the fifth wettest tropical cyclone on record for the USA. Fluke or trend? It may be premature to make sweeping pronouncements, but the hurricane threat matrix appears to be evolving, as a warmer, wetter climate flavors all extreme weather events.
Hurricane experts are divided. Dr. Kerry Emanuel, Professor of Atmospheric Science at MIT, says two camps are emerging among hurricane experts, those who wish to modify the Saffir-Simpson scale, and those who want to scrap it and start over. Emanuel is in the second camp. He believes we should rate the threat, not the storm. Specifically, he envisions a new rating system that emphasizes threats to a specific location, as opposed to the current Saffir-Simpson scale, which is specific to the storm itself. “Most loss of life and damage in hurricanes is caused by water, not wind. In failing to rate the water damage, the Saffir Simpson scale is deeply deficient and should be discarded.” Emanuel envisions a better way to gauge risk, one that is location dependent. “Rather than rating the storm we should rate the total threat to specific places using, for example, a simple color-coded system, supplemented by concise, thoughtfully composed descriptions of the threat.” he said in a recent interview.
Dr. Phil Klotzbach, Research Scientist in the Department of Atmospheric Science at Colorado State University, argues we should keep the Saffir-Simpson scale, but make necessary refinements. “Instead of ranking hurricanes by Vmax (wind speed) we should rank them by minimum sea level pressure.” Klotzback said, describing a new paper submitted to The Bulletin of the AMS, showing that MSLP (minimum sea level pressure) is a better predictor of continental U.S. landfalling hurricane damage than wind speed. “Basically, when MSLP is dropping your storm is typically getting a stronger Vmax, is growing in size, or a combination of the two.” He said. Klotzbach argues that MSLP is much easier to quantify in measuring a hurricane than Vmax, and available in real-time with aircraft reconnaissance. He agrees that rainfall has to be part of any equation. “When storms are threatening land, I think it makes sense to have an additional rainfall threat metric. You could use some sort of area-weighted QPF forecast” he wrote in an e-mail. “Rainfall is just such a tricky problem, since it’s a function of so many parameters, including translation speed, topography and potential mid-latitude interactions.”
What do the experts at NHC, NOAA’s National Hurricane Center, think? “No one scale can represent all the impacts for every location” says Dennis Feltgen, Communications and Public Affairs Officer at NOAA’s National Hurricane Center. With the addition of new products, including Storm Surge Watches and Warnings, and the Potential Storm Surge Inundation Map, NHC is attempting to shift from a storm-centric to a location-centric threat matrix. “We’ve worked to separate the impacts to best represent areas on the immediate coast from those hundreds of miles inland” Feltgen explained.
Social engineering comes into play. Americans inherently understand that a Category 4 hurricane is much more dangerous than a Category 1 storm. “Even though the Saffir-Simpson scale only describes the potential damage from wind, it still remains well recognized by the public.” Feltgen adds a caveat: there is no room for complacency or hurricane-amnesia. Hurricane Florence weakened into a minimal hurricane on the Saffir-Simpson scale, at a time when inland rains were sparking historic flooding. Hurricane Harvey was another painful data point. Although extreme rainfall amounts were accurately predicted days in advance, the impacts for coastal Texas expanded in aerial coverage over time, catching many people off-guard. Feltgen sums up the challenge. “The last thing we want to do is create confusion, so we plan on having social science and behavioral science data before ever making any potential changes to the scale.”
Consumers have multiple options for trusted information, and — on average — days of lead time to make good choices and smart decisions. Businesses, increasingly, are tapping weather data APIs to personalize forecasts for specific facilities and supply chains, to lower risk to staff, customers and operations. As always, predicting hurricanes is more straightforward than predicting human nature. When push comes to shove and evacuation orders are issued, will people do the right thing?
Two camps remain: keep the Saffir-Simpson scale, but add additional products based on location that set better expectations for coastal and inland flooding? Or do we toss out Saffir-Simpson altogether and start fresh, with a clean sheet of white paper? Experts remain divided, but a rapidly morphing climate will require new solutions to protect Americans living on the coast. Because what worked in the 1980s probably won’t work in the 2020s. New times may require new tools.
- Paul Douglas is co-author of “Caring for Creation: An Evangelical’s Guide to Climate Change and a Healthy Environment”, and co-founder and Chief Meteorologist at AerisWeather, located in Minneapolis Minnesota.
