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By Michael Christie

MIAMI, July 7 (Reuters) - Strengthening far more swiftly and vigorously than predicted, Hurricane Bertha became a "major" hurricane in the open Atlantic on Monday, with sustained winds of at least 115 miles per hour (185 kph), U.S. forecasters said.

The second named storm of the 2008 Atlantic hurricane season was heading west-northwest in the direction of Bermuda when it became a Category 3 storm on the five-step Saffir-Simpson scale of hurricane intensity, the National Hurricane Center said.

"A gradual turn toward the northwest with a decrease in forward speed is expected over the next couple of days," the Miami-based center said.

That could take Bertha near Bermuda, a wealthy mid-Atlantic British colony that is viewed as one of the more storm-proof islands in the region, with tough building codes and a storm-conscious population.

The National Hurricane Center said it did not expect Bertha to get much stronger due to unfavorable atmospheric conditions in its path. But the storm has already delivered surprises and forecasting long-range hurricane intensities and tracks is an uncertain science.

At 5 p.m. EDT (2100 GMT), the storm's center was located around 1,150 miles (1,855 km) southeast of Bermuda and it was moving at around 12 mph (19 kph) to the west-northwest.

Energy markets have paid close attention Atlantic storms since the devastating 2004 and 2005 hurricane seasons, during which a number of powerful hurricanes ripped through the Gulf of Mexico, toppling oil rigs and severing pipelines.

None of the computer models used to predict storm tracks indicated Bertha would steer south into the Caribbean or toward the Gulf.

Hurricane forecasters have predicted this six-month season, which began on June 1, will be average or above average. An average season has 10 tropical storms, of which six reach hurricane strength with winds of at least 74 mph (119 kph).

The record 2005 season, which included Katrina which swamped New Orleans and killed 1,500 people on the U.S. Gulf Coast, saw 28 storms.

Bertha formed last Thursday near the Cape Verde islands off Africa. It is unusual for storms to form so far east so early in the season. When it does happen, it is frequently a harbinger of heightened storm activity.

More information about the hurricane is available at the National Hurricane Center's Web site (http://www.nhc.noaa.gov/#BERTHA). (Editing by Alan Elsner and Jane Sutton)

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