Powerful Dean Bolsters Busy Hurricane Season Forecasts

Willie Drye
for National Geographic News

August 23, 2007

A record-setting heat wave kept Hurricane Dean from landing a devastating blow on the U.S. Gulf Coast, which has been pounded by a series of powerful storms since 2004.

But Dean's intensity bolsters forecasts of a busy 2007 Atlantic hurricane season that could see up to three more major storms.

And the high-pressure system that has baked the eastern and southern U.S. this month may weaken soon, which might mean bad news for the rest of the season, forecasters say.

Driven west by the high-pressure system, Dean came ashore early Tuesday morning at Mexico's Yucatán Peninsula with sustained winds of 165 miles (266 kilometers) an hour. It was the third-most powerful hurricane to make landfall in the Atlantic Basin since measurements began in the 1850s.

The storm, which has been blamed for 20 deaths, also lashed central Mexico with rain after a weaker second landfall and had earlier ravaged parts of the Caribbean. (Read the full story: "Dean Hits Mexico Again, With Hundred-Mile Winds" [August 22, 2007.])

Published reports say damage in Mexico could exceed 1.5 billion U.S. dollars. In Jamaica—where Dean's eye brushed the island's southern shore—destruction could be as much as three billion dollars. (See photos of the destruction.)

Protective Shield

Dean had made residents of Texas, Louisiana, and other coastal states nervous from the beginning and even prompted NASA officials to call home the space shuttle Endeavour a day early.

The storm roared into life with devastating potential, then encountered conditions perfect for strengthening into an intense Category 5 hurricane as it coursed through the Caribbean, said Lian Xie, a professor of meteorology at North Carolina State University.

"It did not encounter a major land mass on its way," Xie said. Dean also stayed over warm water, with low wind shear—high-strength winds that can prevent storms from developing.

In addition, since the storm was the first major hurricane this year, the deep, warm waters of the Caribbean had not been cooled by an earlier storm, Xie pointed out.

"There was plenty of energy available from the ocean."

Keith Blackwell, a hurricane researcher at the University of South Alabama's Coastal Research Center in Mobile, said the high-pressure system in the U.S. kept Dean from curving more to the north.

Most hurricanes in the Caribbean make that northward turn, Blackwell pointed out. But the so-called Bermuda high kept the storm's track "almost straight as an arrow" to the Yucatán.

"A hurricane can't move into a high-pressure system—it has to move around it," Blackwell said.

The high-pressure system is expected to weaken soon and bring a break from the intense heat. But that could be happening at a bad time. Without the hot weather, hurricanes could be more likely to curve into the U.S. coast, Blackwell said.

Harbinger of Busy Season?

Blackwell and Colorado State University forecaster Phil Klotzbach also think Hurricane Dean may be an indication that the rest of the hurricane season will be busy.

Klotzbach said Dean grew from a tropical wave—a moving region of low air pressure—that came off the west coast of Africa earlier this month.

"When you get a major hurricane from the easterly tropical waves in August, that tends to imply that the rest of the season will be fairly active," Klotzbach said.

Those waves roll off Africa during the late summer at or near the Cape Verde Islands (see a map of Cape Verde). As they move eastward across the Atlantic Ocean, they can develop into hurricanes.

Hurricanes that form in this manner often become very powerful, and these storms are referred to as Cape Verde hurricanes.

Hurricane Ivan, a devastating hurricane that made landfall as a Category 3 storm near Pensacola, Florida, in September 2004, was a Cape Verde storm.

"Dean was very much a Cape Verde system, along the lines of Ivan," Klotzbach said.

Klotzbach and CSU meteorologist William Gray predicted earlier this month that four intense hurricanes will form before the hurricane season ends on November 30.

No Clues on Warming

Hurricane Dean is the sixth Category 5 hurricane to form since 2003, and that fact has some meteorologists wondering if global warming is causing more frequent and more intense hurricanes. (Related: "Hurricanes Have Doubled Due to Global Warming, Study Says" [July 30, 2007].)

Some experts say the increase in hurricanes since 1995 is caused merely by a natural cyclical increase in the salt content of the Atlantic. The salt makes the water warmer and causes more storms to form.

But scientists on both sides of the debate don't think any conclusions can be drawn from just Hurricane Dean.

"It's impossible to talk about any individual weather event and its connection to the climate," said Kerry Emanuel, a meteorologist at the Massachusetts Institute of Technology who is a leading advocate of the theory that global warming is affecting hurricane formation.

"It's too much of a stretch to make a connection between global warming and one event," Emanuel said.

CSU's Klotzbach, who does not believe there's a link between global warming and hurricanes, agrees with Emanuel about not relying on Dean as an indicator.

"You can't take any one system and draw conclusions about climate change," Klotzbach said. "That's something we all can agree on."

Willie Drye is the author of Storm of the Century: The Labor Day hurricane of 1935, published by National Geographic.

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