(Updates with storm strengthening again)

By Michael Christie

MIAMI, July 9 (Reuters) - Hurricane Bertha strengthened again into a Category 2 storm on Wednesday as it inched closer to Bermuda, but it remained uncertain whether the hurricane would actually strike the British mid-Atlantic colony, U.S. forecasters said.

The first hurricane of the 2008 Atlantic storm season surprised forecasters with the speed and vigor at which it strengthened into a "major" Category 3 hurricane on Monday, only to almost fizzle back into a tropical storm on Tuesday.

But warm waters and more favorable atmospheric wind conditions allowed the storm to once again gain traction and reach the second level on the five-step Saffir-Simpson scale of hurricane intensity, the U.S. National Hurricane Center said.

"Additional strengthening is forecast during the next 24 hours and Bertha could again become a major hurricane," the Miami-based center said in an advisory.

Hurricanes of Category 3 and above are called "major" hurricanes and are the strongest and most destructive. Hurricane Katrina, which devastated New Orleans in 2005, was a monstrous Category 5 storm in the Gulf of Mexico before coming ashore as a Category 3.

Hurricane Bertha's top sustained winds had reached 105 miles per hour (165 km per hour) by 5 p.m. EDT (2100 GMT), the hurricane center said.

The storm was around 600 miles (1,060 km) southeast of Bermuda, a wealthy finance center whose 66,000 people are regarded as among the most storm-conscious and whose building codes rank among the strictest in the region.

It was moving northwest near 12 mph (19 kph) and was expected to slow down and turn north on a course that would take it well to the east of Bermuda. Bermuda, though, still needed to keep an eye on Bertha, the hurricane center said.

It was highly unlikely that the storm would target the U.S. East Coast, hurricane experts said, and the Gulf of Mexico, where the United States produces a third of its domestic crude oil, has been out of the firing line for days.

Bertha developed last week near the Cape Verde islands off Africa.

Its formation so far east so early in the season that began on June 1 and its explosive growth from a tropical storm into a major hurricane could be seen as harbingers of a busy summer.

Hurricane experts have predicted the six-month season, which rarely gets into high gear before August, would see an average or above-average number of storms, though nothing like record-busting 2005, when 28 formed. (Reporting by Michael Christie, Editing by Jane Sutton and Jackie Frank)

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