At 5 a.m. EDT, Isabel maintained maximum sustained wind speed of 145 mph with higher gusts, making it a strong Category 4 storm. It would become a Category 5, the top of the scale, if its winds reach 156 mph.
"We're not really expecting an increase to a Category 5," said Martin Nelson, lead forecaster at the National Hurricane Center in Miami. Forecasters said little change in overall strength was expected for the hurricane but there could be fluctuations in the storm's intensity.
Isabel was 650 miles east-northeast of the northern Leeward Islands — in the southeastern Caribbean — and was moving west at 10 mph. Forecasters said the storm could spawn large ocean swells, creating hazardous surf conditions in the Leeward Islands.
Still, Nelson said it's too early to say whether the storm will directly threaten other islands or, eventually, the United States.
The Atlantic hurricane season began June 1 and ends Nov. 30. Although the exact midpoint of the season was Sept. 1, forecasters say Wednesday is historically the midpoint of seasonal storm formation.
Meanwhile, a tropical depression in the Atlantic was dissipating. The storm was packed 30 mph winds and was moving northward of the Cape Verde Islands. Some heavy rains over portions of the islands were still possible.