USA researcher discusses second eyewall in Hurricane Katrina

By Curt Chapman
Staff Writer
Posted 5/18/07

No one was prepared for the loss of life and property caused by Hurricane Katrina when it roared ashore in August 2005. But what especially took many by surprise was the damage the Category 3 storm (on the Saffir-Simpson scale) left in its wake, …

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USA researcher discusses second eyewall in Hurricane Katrina


No one was prepared for the loss of life and property caused by Hurricane Katrina when it roared ashore in August 2005. But what especially took many by surprise was the damage the Category 3 storm (on the Saffir-Simpson scale) left in its wake, more than 100 miles east of the point where it penetrated the Louisiana coast.

USA Coastal Weather Research Center hurricane expert Dr. Keith Blackwell, an assistant professor of meteorology in the University of South Alabama Department of Earth Sciences, released on Thursday the findings of a study in which he used the latest in microwave satellite technology to look inside Katrina’s storm clouds, leading to the discovery of a second, very potent eyewall, which he said extended severe hurricane winds far outward from the storm’s center.

His work explains why Katrina delivered catastrophic damage over such a wide area, creating the biggest natural disaster in American history.

“The first double eyewall in storms was found in the early 1950s when we developed radar,” Blackwell said. Research by Hugh Willoughby in the 1980s first confirmed that eyewall replacements occur naturally as a method of strengthening, he added.

Blackwell said, “Single eyewalls form feeder bands, which sometimes merge and become another eyewall. The outer eyewall begins to interfere with the inner eyewall, weakening it. As the outer eyewall strengthens, the inner eyewall weakens and collapses. Then the outer eyewall contracts, and the storm rapidly intensifies.”

Conventional satellite imagery hid Katrina’s dangerous outer eyewall that formed beneath the dense clouds swirling counter-clockwise around the hurricane’s eye as it moved toward the Mississippi-Louisiana line in the early morning hours of Aug. 29.

Katrina had been a Category 5 storm the day before it made landfall, Blackwell said, but when the outer band began to form, the process weakened the center.

“What the storm was doing was growing immensely in size,” he said. “The outer eyewall formed 30-50 miles off the coast and hit the coast much earlier than the inner eyewall.”

Blackwell said the winds initially swept across the Mississippi coast from the east, and the water stayed offshore because it was pushed parallel to the shore. As the hurricane moved closer, the winds shifted and created 28-foot waves that pounded structures into rubble.

“In Baldwin and Mobile counties, I was amazed at the intensity of the wind,” said the Summerdale resident. “We were much closer to severe hurricane conditions than we imagined.”

The discovery of Katrina’s outer eyewall through Blackwell’s research explains why Mobile Bay received the highest recorded storm surge in its history. He said New Orleans actually escaped the worst of the outer eyewall’s wrath, even though Katrina pounded the marshes of Plaquemines Parish and storm water surged up the Mississippi River Gulf Outlet and inundated the Ninth Ward. The levees began breaking around mid-morning.

Blackwell said the damage caused in southwest Alabama “speaks volumes about the way Mississippi Sound filled up and flowed north of Dauphin Island and into Mobile Bay. Mississippi Sound was so full of water, you had tremendous inflow from the Gulf and the Sound. Mobile Bay filled up like a bathtub.”

According to Blackwell, 70 percent of all major Atlantic basin hurricanes that formed between 1997 and 2005 had a double eyewall structure at some point in their lives. He noted other such storms have made landfall or come close with both eyewalls in place.

“Hurricane Ivan was a double eyewall storm several times and a Category 5 storm three times,” Blackwell said. Its center passed Grand Cayman 30 miles to the south on Sept. 12, 2004, but the double eyewall raked the island nation, wiping out 95 percent of the buildings there. Most of them were built using stringent building codes similar to those enacted in south Florida after Hurricane Andrew.

Ivan caused $1.85 billion in damage in Grand Cayman. Blackwell said the hurricane pushed 150 mph winds (gusting to 171) ashore there. He pointed out, “If you read the post-hurricane report, they mention the double eyewall as it hits the Alabama-Florida coast.”

Instruments dropped into Katrina’s outer eyewall clocked winds between 140 to 145 miles per hour inside the eyewall at an altitude of 1,500 feet. Blackwell estimated sustained winds near the ground of approximately 105 miles per hour with much higher gusts in the torrential rain.

Blackwell said the latest findings regarding Katrina should serve as a word of caution that coastal residents should not assume anything when it comes to Mother Nature’s fury.

“You can’t tell if a storm is going to have these outer eyewalls days in advance,” he said. “The public needs to revamp their idea of what a hurricane is. Don’t assume we’re going to miss the worst part. You may not.”

He continued, saying, “We need to be more vigilant about these storms than we’re used to. First responders need to take this very seriously. Are you going to be sending them to their death because you sent them out in the worst conditions? It’s a fundamental shift in the way residents, first responders and meteorologists view hurricanes. It needs to be ingrained in the public consciousness. The Saffir-Simpson Scale isn’t going to tell you a lot about the impact of a hurricane.”