20 Years After Hurricane Katrina: Is It Time For A New Approach To Hurricane Classification?

Today is the 20th anniversary of Hurricane Katrina making landfall in Louisiana, which, although only classified as a Category 3 storm due to wind speeds, wreaked havoc as it brought with it enormous storm surges that ultimately claimed nearly 2,000 lives and became the costliest storm in US history. It seems focusing on a hurricane’s wind strength alone is not sufficient to capture its destructive potential. As hurricanes continue to become stronger with human-made climate change, there is a need to change our approaches, scientists suggest in a new study.

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This is not the first time this has been raised. Hurricanes are becoming increasingly deadly and are now forming far more rapidly than they used to. This has led some scientists to suggest that a new Category 6 should be introduced to capture these monster storms.

Under the current classification system, based on the Saffir-Simpson Hurricane Wind Scale (SSHWS), hurricanes are categorized by their maximum sustained wind speed on a scale of 1 to 5. According to this, a Category 1 hurricane has sustained winds of 74 to 95 miles per hour (119 to 153 kilometers per hour), which is considered “very dangerous” and can “produce some damage”. In contrast, a Category 5 hurricane has sustained wind speeds of 157mph (252 km/h) or higher and will bring with it “catastrophic damage”.

But when a hurricane arrives, it is not necessarily the winds that bring the most life-threatening or long-lasting problems. This is often caused by large storm surges – the rising water that is created by wind and atmospheric pressures that come with storms – and heavy rainfall.

For instance, according to a study from 2014, winds are responsible for 8 percent of hurricane-related deaths, while storm surges account for 49 percent and rainfall for 27 percent.

“Frequently, people use the storm’s category to decide whether to evacuate,” Professor Jennifer Collins, a hurricane researcher at the University of South Florida’s School of Geosciences and co-author of the new study, explained in a statement.

“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.”

In the new study, Collins and colleagues cite Hurricane Katrina and Hurricane Florence, which made landfall in the Carolinas in 2018, as two examples of where the existing approach falls short. In 2005, Katrina was listed as a Category 3 storm based on its wind speed, but most of the 1,800 deaths and $125 billion in damages were caused by the rains and the storm surge.

Florence, which was classified as a Category 1 storm, produced extensive flooding when it struck the US East Coast, killing 55 people and causing catastrophic damage.

“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 added. “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.”

An alternative to the traditional SSHWS is the new Tropical Cyclone Severity Scale (TCSS), which has been designed to include factors such as rainfall and storm surge, offering people a more comprehensive assessment of the risks.

Here’s how it works:

Wind, rain, and storm surge are all given a number from 1 to 5 based on the storm’s predicted severity. When taken as a whole, the final number can reach as high as category 6, to demonstrate the additional risk from multiple extreme hazards.

If one hazard is considered worse than others, the hurricane’s final category can be at least that high. For instance, if a storm is assumed to have wind and rainfall rated at Category 2 but a storm surge of Category 4, then the final category is 4.

In a situation where two hazards are rated Category 3 or higher, then the final number is increased by one. So, a story with a storm surge of 4 but a wind and rainfall of 5 would be considered a Category 6.

In addition to the storm’s overall category, the TCSS also lists the hazard-specific categories.

To test whether this alternative approach is more useful than the existing one, Collins and her team conducted an online experiment with 4,000 residents along the Gulf and East coast. This is a region that is frequently threatened by hurricanes from the Atlantic. Participant were given 10 fictional hurricane forecasts affecting their communities – half received warnings using the SSHWS and the other half received them using TCSS.

“We were interested in whether better identification of the main hazard would improve precautionary measures,” the team explained in their paper, “and whether the overall category difference would increase evacuation intent for more dangerous storms.”

Participants were asked questions, including:

1. What is the main danger of the storm – wind, rain or storm surge?
2. On a scale of 1–5, how risky would it be to stay in your home?
3. On a scale of 1–5, would you evacuate?

They found that those who received the TCSS were more likely to correctly identify the main hazard and significantly more likely to evacuate for non-wind-related 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.”

Having complete and satisfactory information about storms is important, as without it, people are more likely to underestimate personal risk or even to move towards inaction.

“Change is hard for any institution that’s been doing the same thing for years,” Collins explained. “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 that 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.”

The paper is published in Scientific Reports.