Did 'downgraded' Florence contribute to a false sense of security?

Doyle Rice, USA TODAY Published 6:00 a.m. ET Sept. 19, 2018 | Updated 3:20 p.m. ET Sept. 19, 2018

Several meteorologists and disaster experts say Florence highlights a problem with the way hurricane danger is rated. The current rating is based on wind strength, not rainfall or storm surge, which tend to be deadlier. (Sept. 18) AP

From 1 to 5, the numbers we use to categorize hurricanes are ingrained in the minds of millions of Americans from Texas to Maine.

But the 47-year-old Saffir-Simpson hurricane wind scale, which measures only wind speed, may not be the best way to gauge a storm's ferocity.

Last week, powerful Hurricane Florence was downgraded from a Category 4 to a Category 2 and eventually a Category 1 as it made landfall – and this may have contributed to a false sense of security.

“The concept of saying ‘downgraded’ or ’weakened' should be forever banished,” University of Georgia meteorology professor Marshall Shepherd said. “With Florence, I felt it was more dangerous after it was lowered to Category 2.”

This is because water – whether storm surge from the ocean or flooding from heavy rain – often ends up being much more deadly and destructive than the wind of a hurricane.

When Florence’s winds weakened and it dropped in storm category, “we made it very clear that in no way, shape or form that this is going to reduce the impacts in terms of flooding and surge,” said Bill Lapenta, director of the National Centers for Environmental Prediction.

The storm killed at least 37 people and will probably cost several billions in damage.

Other storms, such as Harvey, Ike and Sandy, have been far more destructive than their category might have suggested.

A study published last year in the British journal Nature Communications said barometric pressure might be a better gauge of a storm's ability to inflict damage.

"Sandy is the classic example," said Dan Chavas, an atmospheric scientist at Purdue University and the study's lead author. "It was a very big storm, but in terms of maximum wind speed, it was arguably not a hurricane."

The difference in pressure between the center of the storm and outside it is officially known as the "central pressure deficit."

"If you looked at the central pressure deficit, you would have expected Sandy to cause a lot of damage," Chavas said. "But if you used maximum wind speed, as people usually do, you wouldn't expect it to do the damage that it did."

Sandy killed more than 150 people and caused $70.2 billion in damage in the USA, the National Oceanic and Atmospheric Administration said.

Property damage is better predicted by variations in central pressure than by peak storm wind speed since the central pressure combines both wind speed and storm size, the study says.

The size of a storm is a critical factor in damage potential, particularly because of storm surge.

The limitations of the Saffir-Simpson scale have come under scrutiny in recent years. Wind speed is often only an estimate that depends on a speed sustained for a short time in one location. The scale is popular with the public and news media because of its simplicity.

Susan Cutter of the University of South Carolina said the National Hurricane Center and National Weather Service “have not done a good job at communicating the risks associated with tropical systems beyond winds.”

New ways of categorizing hurricanes have been proposed by many groups over the years, including the Hurricane Severity Index, the Cyclone Damage Potential Index and the Integrated Kinetic Energy Index.

All take into account factors other than wind speed, the idea being that more variables make a scale more valuable. None of those scales has caught on.

Contributing: The Associated Press

Two people use a neighborhood street as a boat ramp in floodwaters from the Waccamaw River caused by Hurricane Florence on Sept. 26, 2018 in Bucksport, S. C. Nearly two weeks after making landfall in North Carolina, river flooding continues after Florence in northeastern South Carolina. Sean Rayford, Getty Images

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