At 10 a.m. on Saturday, as Dennis was beginning its journey across the Gulf, its strongest winds were 90 miles (145 kilometers) an hour. The weakened hurricane inflicted less damage than expected as its eye passed offshore of the Florida Keys.
Jeffrey Pinkus, mayor of the Keys city of Marathon, said, "Ninety-nine percent of the damage here is just landscape damage. I haven't heard anybody say there was a storm surge problem." Caused by strong winds, a storm surge is an abnormal rise in sea level that can lead to coastal flooding.
But more than 500 miles (800 kilometers of warm ocean water lay ahead of Dennis. Hurricanes draw their power from warm water, and the Gulf of Mexico was prime fuel for Dennis.
The hurricane steadily regained strength as it moved northward. By 6 p.m. Saturday its winds had increased to 115 miles (185 kilometers) an hour, making Dennis a major hurricane.
But between then and 4 a.m. Sunday, Dennis underwent a meteorological phenomenon often referred to as bombing out. In only ten hours the storm's strongest winds leaped to 145 miles (230 kilometers) an hour. By early Sunday morning city officials in Mobile were confronting a potential catastrophe.
At that time forecasters expected Dennis to charge into Mobile Bay. The bay would have acted as a giant funnel, channeling a storm surge of perhaps 17 feet (5 meters) into downtown Mobile. The surge would have flooded a part of the city where many residents hadn't evacuated, because they either had no place to go or were unable to leave.
Mobile officials called in the city's bus drivers early Sunday morning. All residents who wanted to leave were taken to a shelter on the west side of the city.
"It was a phenomenal effort," Mobile fire department captain Debbie Bryars said. "The emergency-management department got the drivers to come in and got the word out about the evacuation."
Hundreds of people were moved out of harm's way, Bryars said.
But as Dennis bore down on the narrow Alabama coast Sunday afternoon, several factors intervened to diminish its winds and drag it away from Mobile Bay.
How Dennis Was Defanged
Randy McKee, chief meteorologist at the National Weather Service's Mobile office, said Dennis's power was diminished when it went through an "eye wall replacement."
This process often happens to very intense hurricanes, such as Dennis, when a second wall of intense thunderstorms begins forming around the storm's existing eye. The eye wall acts as a giant noose, choking the hurricane's momentum and reducing its winds.
The thunderstorms forming and dissipating around Dennis's eye also pulled the storm off its Mobile-bound course.
The thunderstorms had the same effect as putting a small weight on a spinning top, McKee said. "If you had the top spinning perfectly and you put a little weight on one place, it would affect the top's path," he said.
"That little hiccup in there was enough to move the hurricane's landfall from Mobile Bay to the east," McKee said.
Hurricane Dennis ran into another impediment as its eye drew within 150 miles (240 kilometers) of the Alabama coast. At that point the hurricane began crossing over seas that had been churned up by Tropical Storm Cindy late last week. The churning had cooled the water, depriving Dennis of the warm seas it needed to maintain its fearsome winds.
So between 9 a.m. and 3 p.m. Sunday, Dennis became a different hurricane. It was still potent but not quite the monster it had been, and it was headed away from the heavily populated cities of Mobile and nearby Pensacola, Florida.
A Storming Trend?
Meteorologists have predicted that the 2005 hurricane season—which runs from June 1 to November 30—will continue a ten-year trend of unusually active seasons.
The hurricane season usually becomes busy in mid-August and reaches its peak around September 10. But so far this year, four named storms already have formed. That's the first time on record that this has happened, and the usual peak of the hurricane season is still two months away.
Willie Drye is the author of Storm of the Century: The Labor Day Hurricane of 1935, published by National Geographic Books.
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