New Area To Watch In Atlantic

Do we need a better way to rate hurricanes, one that emphasizes the combination of wind speed, storm surge and rainfall, rather than wind alone?

We seem to think so, and so do many other meteorologists, including Jennifer Collins, a professor and hurricane researcher at USF.

“We know that wind only accounts for less than 10% of fatalities in general, but yet our current category system is based on wind alone,” Collins told weather.com. “Storm surge accounts for closer to 50% and rainfall accounts for closer to 30% so why not have a hurricane scale which takes into account all these hazards?”

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Using a scale that only takes into account one hazard associated with tropical systems is a disservice to the general public.

Terms To Know:

- Saffir-Simpson Hurricane Wind Scale: The current scale used by the National Hurricane Center to categorize hurricanes. This scale is only based on the hurricane’s maximum sustained wind speed and does not take into account storm surge, flooding and tornadoes.

- Tropical Cyclone Severity Scale: A proposed scale that incorporates not only wind, but also storm surge and rainfall.

Dr. Collins has studied hurricane evacuation over the last decade and concluded that people are relying too heavily on the category of a hurricane when making stay or go decisions.

“Many people say that they will only evacuate for a major hurricane, which is Cat 3 or higher on the current scale,” says Collins.

The reality is that large tropical storms and Category 1-2 hurricanes can produce as much damage and death potential as their stronger counterparts.

“Some underestimate the dangers of a lower category by simply focusing on the wind rather than the very deadly threat of flooding,” Collins says.

Collins cited 2018’s Hurricane Florence — which struck the Carolinas — as an example. ”On our scale, it would still have been a Cat 1 for wind, but it would have been a Cat 4 for surge and Cat 5 for rainfall.”

Of the 22 direct fatalities caused by Florence, more than 75% were caused by flooding.

The final category for Florence would have been a Category 5. “This is more aligned to the death and destruction caused,” says Collins.

The Proposed New Scale and How It Works

The new scale is called the Tropical Cyclone Severity Scale (TCSS) and it provides a different category for each hazard (wind, surge, rain) along with a combined final category for simplicity and to avoid confusion within the public.

According to Hans de Moel, a co-author of the study, calculating the category using the new scale would be simple. The wind category will exactly match that of the Saffir-Simpson Wind Scale and the other two scales are pictured below.

Then, a singular overall category is introduced for the storm that incorporates the three scales.

This overall category is equal to the highest individual threat category. Nadia Bloemendaal, another co-author of the study, hopes messaging of this type of storm would be something like this: “This hurricane will be a Category 5 at landfall, predominantly driven by its high storm surge.”

If two high-risk hazards (Category 3 or higher) have the same category, then you add one category to the highest category of the risks.

For instance, if a hurricane is rated Category 4 for both wind and storm surge, it will be rated overall as a Category 5. Similarly, if one threat is rated Category 4, but the other categories are both Category 3, then the final category would be 5.

A Category 6 Hurricane?

This boost in categories by tied threats opens the door to a Category 6 when any two threats are rated as major and at least one threat is Category 5.

De Moel wants to differentiate this possibility from the decades-old chat about adding a Category 6 to the SSHWS. “In the TCSS, a Category 6 signifies that there are more than one catastrophic hazard coming your way (i.e. both high winds and a storm surge together), making the storm more dangerous than if it was ‘just’ one catastrophic hazard.”

Previous hurricanes that would be rated Category 6 on the new scale: Dorian 2019, which was rated 5 for wind and surge; and Wilma 2005, which was rated Category 5 for surge and rainfall. Hurricane Ian in 2002 would also be rated Category 6 on the new scale because all three threats were major threats, so one category is added to the highest rated category, which was a Category 5 for surge.

Big Changes To Other Storms?

Some storms would see a major change in category under this proposed scale. Here are two examples:

Allison in 2001 was a tropical storm under the SSHWS. Nearly 37 inches of rain fell at the Port of Houston, surpassing the threshold for Category 5 under TCSS.

Similarly, Hurricane Sandy was rated as a Category 1 on the SSHWS, but on TCSS, it would have been a Category 5 due to its storm surge.

This new scale does elevate the number of major hurricanes in the history books based on impacts.

Most ratings on fish storms, those that do not produce storm surge or rainfall near land, will retain their SSHWS rating.

Where Do We Go From Here?

Collins told us she has contacted the National Hurricane Center very recently and is hoping to hear back from them soon to initiate further discussions.

She says, “We have heard from many others in the weather enterprise including emergency managers who are very excited about our scale.”

The most important takeaway: keep it simple for people to understand.

“I also feel that there is strength in simplicity,” says de Moel. “The Saffir Simpson Scale basically categorizes potential impacts (due to wind) in five classes. The proposed new scale does the same, but also for surge and rain.”

The authors’ survey of more than 4000 people showed no signs of confusion among the public.

“Actually, our survey shows that TCSS respondents can better identify the most dangerous hazard coming at them, allowing them to make better informed and more appropriate decisions,” says de Moel.