Dr. Keith Blackwell, a meteorology professor at the University of South Alabama and a researcher at USA’s Coastal Weather Research Center, has been studying the phenomenon that occurs when heavy rain associated with a hurricane pushes the storm’s stronger winds downward. The result, he said, is often greater than expected damage in the area where it happens.

“These features are in most hurricanes,” Blackwell said. “Regions of elevated precipitation (beginning around 1,000 feet above the ground) first warms up in the clouds and falls to the surface. In regions of very heavy rainfall, there can be substantial downdraft.”

He pointed out that in the last 10 years, the strongest winds in hurricane eyewalls have averaged around 1,500 feet above the ground — about the same level of the base of high clouds on a summer afternoon.

“We know the hurricane already has strong winds in place at that altitude,” Blackwell said. “We believe when the suspended rain plunges, it directs these very strong winds to the ground.”

He added collapsing eyewalls were indicated by Doppler radar when Hurricane Danny made landfall in 1997, and Hurricane Ivan came ashore in 2004.

In fact, Danny, which stalled over the area for nearly a day, was the most prolific producer of the feature between 1994 and 2007, the study revealed.

Blackwell and colleagues from USA and the National Weather Service showed that Danny, only a Category 1 hurricane on the Saffir-Simpson Scale, was likely responsible for one of the two greatest precipitation events ever recorded in the United States.

The storm’s rainfall on Dauphin Island measured 37 inches, but when corrected for wind errors and equipment limitations, amounts likely exceeded 65 inches there.

During the period of heaviest rainfall on the island, winds gusted over 100 mph, associated with a collapsing core likely diverting very strong eyewall winds from 1,000 or 2,000 feet altitude downward toward the ground.

Blackwell said, “I found that Danny actually strengthened in the bay. For four hours — the morning and early afternoon of July 19, 1997 — it strengthened to 100 mph in the ship channel, not affecting land for most of that time.”

Ivan, which struck the Alabama and northwest Florida coasts, also contained prolific collapsing core activity, particularly in its eastern eyewall near the Baldwin County/Escambia County, Fla. line.

Severe wind damage extended from Perdido Key northward through Perdido Bay, the Perdido River, and many miles inland across I-10, through the Atmore area, across I-65 and northward to near Monroeville. Some homes and pine forests were devastated in those areas.

“We believe that type of destruction was caused by these collapsing cores of precipitation in what was left of Ivan’s eyewall,” Blackwell said.

Hurricane Katrina in 2005 also displayed significant collapsing core activity in its inner eyewall as it struck the Mississippi coast, although the weather feature was not as prolific as in Danny or Ivan.

Hurricane Dennis in 2005 and Umberto in 2007 were two other hurricanes that had numerous instances of collapsing eyewalls, he said. Dennis caused significant damage east of Pensacola and Umberto affected the Galveston, Texas area.

Blackwell said, “Some storms do not have these features. Hurricane Georges had few of these features. The central pressure was low, but damage was minimal.”

The research will benefit southwest Alabama residents and first responders because areas of heavy rain inside hurricane eyewalls as indicated on radar are early warning signs there is a potential for collapsing cores and rapidly increasing winds.

“Those are things people can look out for as the storm approaches, and ask themselves, ‘When do I go in the safest part of my house?’ They can make last-minute decisions where they should be in the house,” Blackwell said. “I really feel this helps explain what's going on in a hurricane.”

Blackwell was among the first researchers in the nation to discover why Hurricane Katrina, a Category 3 storm, was the most devastating in American history. Using the latest satellite technology, Blackwell found that Katrina had a double eyewall when it made landfall on the northern Gulf Coast.

The potent second eyewall formed near Pascagoula, Miss., and created a damaging storm surge that swamped parts of coastal Baldwin County.