"Active" Hurricane Season Predicted for U.S.

Willie Drye
for National Geographic News
June 1, 2005
Meteorologists think a decade-long trend of active Atlantic hurricane seasons will continue this summer. That's bad news for U.S. coastal residents who took a 45-billion-dollar (U.S.) pounding from the storms last year.

Forecaster William Gray of Colorado State University expects a busy summer in the Atlantic Basin, which includes the Gulf of Mexico and the Caribbean Sea. Gray, a pioneer in long-range hurricane forecasting, thinks eight hurricanes will form during the season, which officially began today and runs to November 30.

Gray said four of those storms will become major hurricanes, with winds exceeding 111 miles an hour (178 kilometers an hour).

The National Hurricane Center (NHC) in Miami, Florida, predicts seven to nine hurricanes in the Atlantic Basin, with three to five of them becoming major hurricanes.

Hurricanes begin as tropical storms. A tropical storm becomes a hurricane when its winds reach 74 miles an hour (119 kilometers an hour). Gray thinks 15 tropical storms will form, while the NHC expects 12 to 15.

"Many climate signals, such as warmer-than-average water in parts of the Atlantic Ocean, indicate the likelihood of an active 2005 Atlantic hurricane season," said meteorologist Stu Ostro of the Weather Channel. "That's why many of those who issue seasonal outlooks are predicting an above-average number of tropical storms and hurricanes."

More Salt, More Storms

Gray and NHC meteorologist Chris Landsea think the busy hurricane seasons in recent years are part of a well-established cycle of fluctuations in the temperatures of ocean waters.

The variances in seawater temperatures are related to periodic changes in ocean currents that affect the salt content of the water, Gray noted. Hurricanes draw their energy from warm ocean water.

When the salt content increases slightly in the Atlantic Basin—as it has now—the water is a little warmer, and that tends to produce more hurricanes, Gray said.

Another variable in the creation of hurricanes—decreased upper-level atmospheric winds—is also favorable to storm formation. During inactive seasons, these winds often tear apart hurricanes as they try to form. But for the past few years the winds have been minimal, and more hurricanes have formed.

Gray thinks there's a 77 percent probability that a major hurricane will make landfall somewhere on the U.S. coast this summer. He puts the chances of the Florida peninsula taking a hit at 59 percent, or 28 percentage points above the normal risk. The Gulf Coast has a 44 percent chance, compared to a 30 percent chance normally, Gray said.

During the 2004 season four hurricanes battered Florida in the space of about six weeks, and another hurricane lashed North Carolina's Outer Banks. The 45 billion dollars (U.S.) in damages done by these storms makes the 2004 season the most expensive on record. And that figure doesn't include millions of dollars in lost income for businesses forced to close because of the hurricanes.

Much of the damage was leveled by two of the season's most powerful storms, Hurricane Charley and Hurricane Ivan. On August 13, Hurricane Charley struck Punta Gorda on Florida's west coast with winds exceeding 140 miles an hour (225 kilometers an hour).

Hurricane Ivan, which formed in early September, was the monster hurricane of 2004. At one point, Ivan's winds reached 165 miles an hour (265 kilometers an hour), making it one of the most intense hurricanes in history. Ivan weakened considerably before making landfall on the Gulf Coast on September 16, but still inflicted massive damage to Pensacola, Florida.

Several factors contributed to the unprecedented destruction last summer.

Upper-level winds, which had been steering most powerful hurricanes away from U.S. shores during earlier active seasons, didn't fend off 2004's onslaught of storms.

Many hurricanes form each summer from the thunderstorms that roll off the west coast of Africa. Usually those thunderstorms, known as tropical waves, form around latitude 15° N.

But Gray noted that last summer the tropical waves formed farther south at around latitude 12° N. As those waves moved across the Atlantic, they didn't encounter the winds that might have shoved them away from the U.S. coast had the waves formed farther north.

Record Toll

The record toll of last year's hurricane season is linked to another phenomenon: More people and more wealth have concentrated on the U.S. coast than during the previous period of active hurricane seasons, which lasted from the late 1920s to the late 1960s.

During the period of relatively quiet hurricane seasons that followed from 1970 to 1994, the population along the Atlantic and Gulf coasts mushroomed.

"When the storms come ashore now, it's a lot different coastline than it used to be," Landsea, the NHC meteorologist, said. "There's been a huge increase in population as well as wealth on the coasts. Per capita, the coastal communities tend to be wealthier than the U.S. as a whole. There are vacation homes near the coast. That's the main reason why there was 45 billion dollars in damages last year."

Before the start of the busy 2004 hurricane season, Gray said the U.S. was overdue for a spate storms making landfall. Between 1995—the beginning of the present cycle of active seasons—and 2003, 32 major hurricanes formed. But only three of those made landfall. Usually, as many as ten of those storms would have struck the U.S., Gray said.

Forecasters don't know exactly how long the current cycle of active seasons will last, but they don't think it'll end anytime soon. During the past hundred years, the periods of increased activity have lasted about 25 to 40 years.

"We'll stay in an active period another 15 or 20 years or so," Gray said during a recent telephone interview. "We'll probably see hurricane damage like we've never seen it before. That's the only conclusion you can draw."

Gray does not think global warming has anything to do with the recent increase in hurricanes. "This is a natural thing," he said. "Even though the Atlantic has had increased hurricane activity, the activity in other global [ocean] basins has gone down a little."

"If global warming was causing more hurricanes in the Atlantic," he said, "the other tropical storm basins should have seen an increase also."

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

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