(Updates storm strength, position throughout) MIAMI, July 10 (Reuters) - Hurricane Bertha weakened back into a Category 1 storm on Thursday as it churned its way toward the British colony of Bermuda, U.S. hurricane forecasters said. The first hurricane of the 2008 Atlantic storm season saw its top sustained winds decrease to near 85 miles per hour (140 km per hour) and it was about 425 miles (685 km) southeast of the mid-Atlantic offshore financial center by 5 p.m. EDT (2100 GMT), the U.S. National Hurricane Center said. Bertha was moving northwest near 10 mph (16 kph) and was expected to turn north and slow its forward speed as it approached Bermuda on Friday and Saturday. Computer forecasting models suggested that atmospheric conditions could trap the storm near Bermuda for several days, while some models showed it jogging toward the island. "The new track is closer to Bermuda than the previous track," the forecasters said. It was too soon to determine how much the storm would affect Bermuda but residents were urged to pay close attention. "Large swells and high surf have started to affect Bermuda. These conditions are expected to persist for the next few days," the center said. Bertha 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 on Wednesday allowed the storm to gain traction again and reach the second level on the five-step Saffir-Simpson scale of hurricane intensity, the Miami-based Hurricane Center said. It weakened again on Thursday but the hurricane center warned it could re-intensify in the next 24 hours. 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 moving ashore as a Category 3. Both the U.S. East Coast, and the Gulf of Mexico, where the United States produces a third of its domestic crude oil, are out of Bertha's firing line. 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 Tom Brown and Jane Sutton)
