Mobile, Ala.--University of South Alabama hurricane expert Dr. Keith Blackwell used the latest in microwave satellite technology to look inside Hurricane Katrina’s storm clouds, leading to the discovery of a second, or outer, very potent eyewall, which extended severe hurricane winds far outward from the storm’s center.

His findings explain for the first time why the Category 3 hurricane delivered catastrophic damage over such a wide area of land, creating the biggest natural disaster in America’s history.

Conventional satellite imagery hid the dangerous outer eyewall that lurked beneath the dense clouds swirling counter-clockwise around the hurricane’s eye as it roared toward the Mississippi-Louisiana line in the early morning hours of Aug. 29, 2005.

According to Blackwell, discovery of the outer eyewall explains the extremely heavy damage to the coastline, extending unusually far to the east through Jackson County, Miss., miles eastward from Katrina’s eye. The outer eyewall came ashore on the Mississippi coast shortly after daylight while the hurricane’s eye was still well offshore. High winds pounded the coast for hours before the eye finally made landfall. Blackwell said instruments dropped into the outer eyewall from aircraft recorded winds between 140 to 145 miles per hour inside the eyewall at an altitude of 1,500 feet. He estimated sustained winds near the ground of approximately 105 miles per hour with much higher gusts in the torrential rain. This outer eyewall swept across the entire Mississippi coast as far east as Pascagoula. It was then followed by the more intense inner eyewall, accompanied by a record-breaking 28-foot storm surge, over the western Mississippi coast later that morning.

Blackwell, an associate professor in the USA department of earth sciences and a hurricane forecaster and tropical weather research center specialist with USA’s Coastal Weather Research Center, said hurricane researchers have known about outer eyewalls for a long time. Previous research has shown that a double eyewalls often form in intense hurricanes, but only persist for a day or two, thus many strong storms that previously exhibited two eyewalls over the open ocean may not retain both until landfall. However, the development of microwave satellite imagery is helping researchers to learn more about the evolving internal structure of hurricanes and their potential to grow quickly in size as the original eyewall becomes surrounded by a much more expansive outer eyewall.

“If residents of eastern Jackson County near Pascagoula had known about this outer eyewall, or even the possibility that such a thing could occur in the hours before Katrina’s eye made landfall, then their focus may have shifted from the New Orleans and western Mississippi coastline areas to their own safety. In addition, many people more directly in the path of the hurricane were suddenly surprised by the early arrival of strong damaging winds on the coast well in advance of Katrina’s center or extremely high tides,” said Blackwell.

Blackwell said as the second eyewall approached the Mississippi coastline, the strong easterly winds were actually pushing water to the west, parallel to the coastline, rather than to the north onto land as the hurricane would do later once the eye made landfall. “Initially, high winds in the outer eyewall struck the Mississippi coast up to three to four hours before the highest water arrived. The problem with water created by the storm’s devastating tidal surge arrived later,” explained Blackwell.

For thousands of Alabama and Mississippi residents still fighting insurance companies over whether wind or water damaged or destroyed their structures, Blackwell’s findings could mean a difference in how some cases are settled.

Other storms with double eyewalls have delivered devastating wallops to coastal areas, including the outer eyewall of Hurricane Ivan on Sept. 12, 2004, as the storm’s eye and inner eyewall passed to the south of Grand Cayman. The outer eyewall packed sustained winds of 150 miles per hour with gusts to 171 miles per hour, destroying 95 percent of the island’s buildings and leaving damages of $1.85 billion. The building codes on Grand Cayman at that time were similar to the strict codes enacted in south Florida following Hurricane Andrew.

Blackwell said developing data on second eyewalls can help public safety officials determine wider evacuation areas and give first responders life-saving information as they respond to storm emergencies. It can also help residents reach a better decision on evacuation plans.

“Traditionally, people have looked at hurricane warning areas and thought they were relatively safe if their residence was not in the direct path of the eye or not near the center of the warning zone. What we are learning about outer eyewalls can change how they perceive the threat a hurricane may pose to areas closer to the edge of that zone,” said Blackwell.

The USA Coastal Weather Research Center, a self-supporting operation of the department of earth sciences, began operation on Jan. 1, 1988. Located in the Mitchell Center on the main USA campus, the weather center consists of a meteorological laboratory, information center and archive.

Hurricane Katrina Image at 6:45 a.m. CDT August 29, 2005
Background Paper: Hurricane Katrina and Double Eyewalls by Keith Blackwell
Fact Sheet: Double Eyewalls and Hurricane Katrina