WASHINGTON, Sept. 6 (Xinhua) -- Florence has weakened to a tropical storm, but it is expected to restrengthen to become a hurricane again over the weekend, the U.S. National Hurricane Center (NHC) said on Friday.

The storm is located about 1,590 km east-southeast of Bermuda, 1,490 km east-northeast of northern Leeward Islands, with maximum sustained winds of 100 km per hour, moving west at about 11 km per hour, according to the Miami-based weather forecaster.

"This general motion will likely continue for the next couple of days," the NHC said. "Swells generated by Florence will begin to affect Bermuda later today and reach portions of the U.S. East Coast over the weekend."

There are no coastal watches or warnings in effect, it added.

Florence, the first major hurricane of the Atlantic season, was upgraded to a Category 4 hurricane Thursday, packing maximum sustained winds of 215 km per hour.

Florence is the basin's sixth named storm of the year.

Forecasters have said the Atlantic hurricane season is now expected to be less active as it enters its peak months due to a combination of conditions in the ocean and atmosphere that helps to limit storm development.

They are expecting nine to 13 named storms with winds of 60 km per hour or higher, of which four to seven will strengthen into hurricanes.

Of those storms, there may be up to two major hurricanes, which are classified as Category 3, 4 and 5 with possible winds in excess of 170 km per hour.

After Florence, at least two other storms are forecast to form in the Atlantic. Either could threaten North America by mid-month. The next two named storms would be Helene and Isaac.