‘Downgraded’ storms pack dangerous punch
Weakening of Hurricane Florence may have led to false sense of security
From 1 to 5, the numbers used to categorize hurricanes are ingrained in the minds of millions of Americans.
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 has 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 categories 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.