Hurricane Scale No Longer Has Storm Surge, Inland Flooding Estimates

May 14, 2010

The five-category system that describes a hurricane’s strength and the havoc its winds could bring to the U.S. has been updated for the 2010 storm season.

Gone from the Saffir-Simpson Hurricane Wind Scale are estimates for storm surge and inland flooding from rainfall.

The scale was changed to alleviate confusion over storm surge and flooding predictions that didn’t match what actually happened as a hurricane made landfall, said Chris Landsea, science operations officer at the National Hurricane Center and leader of the team that made the changes.

The new scale still classifies hurricanes by maximum sustained wind speeds, starting at 74 mph with Category 1. Category 3 and above is considered a “major hurricane,” and the strongest — with winds greater than 155 mph — is Category 5.

More important, it tells people how strong a storm it will take to bring down the trees, fences, power lines and walls around them.

The damage descriptions to homes, shopping centers and industrial buildings are more specific than the previous version and updated to reflect more coastal development.

“In coastal areas of Florida, there are a lot more high-rises where the windows are susceptible to damage. That broken glass wasn’t covered at all in the old one,” Landsea said.

In a Category 1 hurricane, shingle or metal roof coverings could peel off mobile homes, stone chimneys can topple and large tree branches will snap.

By Category 3, most mobile homes, fences and unprotected windows face certain destruction, and people should expect to go without electricity or running water for days after the storm passes.

A Category 4 storm will cause severe structural damage even to well-built homes.

Category 5 damage is catastrophic: total roof failure, blown-out windows throughout high-rises, neighborhoods isolated by fallen trees and power poles, water shortages and other widespread suffering.

The damage described may be less severe in Florida and the Carolinas because those states have the best building codes in the country, Landsea said.

The revised Saffir-Simpson scale eliminates references to flooding caused by rain and estimates for storm surge, or the mound of seawater pushed ahead by a hurricane’s winds.

This simplifies the scale and shows that forecasters now have a better understanding of how storm surge works, said Florida’s state meteorologist, Amy Godsey.

The problem with the previous version was how inconsistent it proved to be when it came to storm surge, she said.

For example, a compact Hurricane Charley made landfall in 2004 in southwestern Florida with Category 4 winds, but it had the storm surge previously expected from a much weaker hurricane. A very large Hurricane Ike made landfall just outside Galveston, Texas, in 2008 as a Category 2 hurricane with a storm surge of 15 to 20 feet — a peak associated with a Category 4 or 5 hurricane on the old scale.

If Ike made landfall in Daytona Beach, it only would have produced a storm surge around 8 feet, Landsea said.

On the U.S. Atlantic coast, the deep waters offshore produce a smaller storm surge than the more shallow Gulf of Mexico.

“It’s like the difference between having a plate full of water and a bowl full of water,” Landsea says. “Put a fan next to them, and the water will be pushed off the plate, but the water will just swirl around in the bowl.”

The size of a hurricane and how fast it’s moving, the depth of the water along the coastline and the topography where the storm makes landfall also affect the surge produced — variables that Landsea said make developing a separate storm surge scale impractical.

Storm surge and flooding forecasts will remain in hurricane advisories and statements issued by the National Hurricane Center and local National Weather Service offices.

The hurricane center is considering adding a storm-surge warning to its list of watches and warnings during hurricane season. Sometimes places outside the cone of a hurricane warnings can still be vulnerable to storm surge, hurricane center director Bill Read said. No decision on adding storm-surge warnings will come for another two or three years.

The revision to the Saffir-Simpson scale took more than a year. Five wind engineers submitted updated descriptions of wind damage, and a draft was released for public comment last year.

It’s the first update to the scale since barometric pressure was removed from it about a decade ago, Landsea said.

A storm intensifies as the pressure drops, a phenomenon that once helped forecasters assign a Saffir-Simpson category.

“Back in the ’70s it was easy to measure the pressure but difficult to measure the winds,” Landsea said. “Nowadays we have much better aircraft that fly into the winds and can measure them.”

Herbert Saffir, a Miami-area structural engineer, created in 1969 a scale for the damage an approaching hurricane could bring. The five-category scale was expanded to include storm surge and flooding in the early 1970s by Robert Simpson, then director of the hurricane center.

Before the Saffir-Simpson scale, hurricanes were simply described as major or minor.

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