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MIAMI, July 8 (Reuters) - Hurricane Bertha weakened rapidly as it headed in the direction of Bermuda on Tuesday, just a day after suddenly burgeoning into the first major hurricane of the 2008 Atlantic storm season, forecasters said.

Bertha had begun a northerly turn that would likely spare the U.S. East Coast from a hurricane or tropical storm strike, the U.S. National Hurricane Center said.

The Gulf of Mexico, where the United States produces a third of its domestic crude oil and a large amount of its natural gas, was well out of the firing line.

Bertha, which on Monday became a powerful Category 3 hurricane on the five-step Saffir-Simpson scale, had dropped back to Category 1 by 5 p.m. EDT (2100 GMT) on Tuesday with top sustained winds of 85 mph (140 kph), the Miami-based center said in an advisory.

"Bertha's rapid intensification yesterday has been equaled by its rapid weakening today," it said. "Such intensity fluctuations remain a forecasting challenge and serve as an important reminder for the uncertainties associated with intensity forecasts."

The center said Bertha could weaken further in the next 24 hours as cooler waters, dry air and unfavorable atmospheric wind conditions affected the storm. But some computer models showed the hurricane regaining some strength after that.

The wealthy mid-Atlantic British colony of Bermuda would have to continue to keep an eye on the storm as it drifted toward the island, a major center for the world's reinsurance industry, the hurricane center said.

By 5 p.m. Bertha was about 900 miles (1,450 km) southeast of Bermuda and moving toward the northwest at 12 mph (19 kph), the center said.

Forecasters have predicted an average or above-average six-month hurricane season this year, although nothing like record-busting 2005 when 28 tropical storms formed, including Hurricane Katrina.

The Atlantic storm season, which begins on June 1, rarely gets into high gear before August. (Reporting by Jane Sutton and Michael Christie, Editing by John O'Callaghan)

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