A tropical depression about 1,000 miles east-northeast of the Leeward Islands developed further on Thursday and is expected to become a named storm by sometime this weekend.
Subtropical Storm Oscar formed Friday evening in the open waters of the Central Atlantic Ocean.
Maximum sustained winds have increased to near 50 miles per hour with higher gusts.
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Oscar strengthened into a tropical storm late on October 27 over open waters and is centered 815 miles east-southeast of Bermuda and 780 miles northeast of the northern Leeward Islands, the NHC said.
There were no coastal watches or warnings in effect. Hurricane season runs through November 30. The storm is continuing to transition from a hybrid storm to a full-fledged tropical system on Saturday night.
The system "is gradually becoming better defined and the associated shower and thunderstorm activity is showing signs of organization", the Miami-based Hurricane Center said in an 8 a.m. advisory. The potential Oscar is then expected to turn westward "well to the north or northeast of the Lesser Antilles through early next week", the center said.