MIAMI — Hurricane Bertha strengthened to a Category 3 storm yesterday as it swirled in the central Atlantic, but it posed no immediate threat to land.

Forecasters expect the Atlantic season’s first hurricane to head toward Bermuda. It was too early to tell whether the storm would hit the island, but residents were urged to monitor Bertha’s progress.

At 5 p.m. yesterday, the hurricane was located 730 miles east-northeast of the Northern Leeward Islands and about 1,150 miles southeast of Bermuda. It was moving west-northwest at about 12 mph. Maximum sustained winds were measured at 115 mph.

A forecaster at the National Hurricane Center, Daniel Brown, said Bertha was expected to stay at roughly the same intensity.

“It looks like it will remain over warm enough waters for the next couple of days,” Mr. Brown said. “Conditions aren’t going to change too much over the next couple days, so it will likely remain a strong hurricane.”

The hurricane center has projected 12 to 16 named storms in the Atlantic this season, with six to nine of them hurricanes.

America has not been hit by a major hurricane since 2005, the busiest season on record, which brought a destructive triple-punch of Katrina, Wilma, and Rita ashore.

The Atlantic hurricane season runs through November 30.