Study Shows Gulf Coast Hurricanes Weaken Before Landfall

The U.S. Gulf Coast has been pounded by some fierce hurricanes in the past decade, but a recent study shows that cooler waters near the shore kept the storms from being much worse.

The study was conducted by the National Hurricane Center in Miami and Colorado State University.

Hurricanes draw their power from warm ocean water that has been heated to at least 80 degrees Fahrenheit to a depth of around 160 feet. During the summer, the water in the central and southern Gulf of Mexico often is much warmer than 80 degrees, and this water has fueled some of the most intense hurricanes in history.

In 2005, these warm waters allowed Hurricane Dennis to become one of the most powerful July hurricanes on record. On July 7, Dennis's winds reached 150 mph as it roared across the Gulf about 500 miles southeast of Mobile, Alabama. It looked like the eye of the powerful storm was going to go straight up Mobile Bay and into downtown Mobile as a devastating Category 4 hurricane.

But when Dennis got within about 150 miles of the coast, it encountered the cooler inshore water and that helped reduce its fearsome power. Still, the storm made landfall between Mobile and Pensacola as a Category 3 storm with winds of about 120 mph.

The cooler waters near shore also probably helped diminish two other very intense hurricanes just before they made landfall on the Gulf Coast in the summer of 2005 -- Hurricane Katrina in August and Hurricane Rita in September.

"It's something special about the Gulf of Mexico," said Mark DeMaria, one of the co-authors of the study.

The study noted that when storms are over the central and southern Gulf of Mexico, they're over warm water from the tropics. On average, storms' winds intensify by about 8 mph for every 12 hours they're over this water, the study said.

But the water in the northern Gulf hasn't been influenced by this tropical warming, and so the warm water inshore isn't as deep.

"When hurricanes move over that water, (the storm's) high surface winds tend to mix cooler water up to the surface, which can lessen a storm's intensity," DeMaria said.

There are, of course, always exceptions. Sometimes, ocean currents bring warmer water closer to the coast, and that can cause devastating consequences if a hurricane reaches that water. In 1969, Hurricane Camille may have crossed one of those freak currents of very warm water as it approached the Gulf Coast. The storm lost little, if any, intensity as it neared landfall, and on August 17, Camille's eye came ashore at Pass Christian, Mississippi with winds of about 190 mph.

The study was published in a recent edition of the Journal of Weather and Forecasting. Viewing the full study requires a paid subscription, but an abstract can be viewed here.

NOTE: The NOAA photo above shows Hurricane Dennis just before it made landfall on July 11, 2005.