(RNN) – Tropical Storm Isaac formed late Saturday afternoon in the Atlantic, between Tropical Storm Florence and Tropical Storm Helene.
The National Hurricane Center said the storm is about 1,640 miles east of the Windward Islands, which are in the West Indies.
The storm is heading west at 7 mph and packing maximum-sustained winds of 40 mph.
It's expected to speed up in the next few days as it continues moving westward. It's also expected to strengthen, possibly becoming a hurricane by Monday.
Isaac is the ninth named storm of the Atlantic hurricane season.
Tropical Storm Florence, the sixth named storm, is expected to become a hurricane Saturday night, the NHC said.
The storm could intensify rapidly by Sunday, becoming a "major hurricane" by Monday.
The storm is moving westward at 5 mph. It's about 810 miles southeast of Bermuda, with maximum-sustained winds of 70 mph.
"Interests along the U.S. East Coast, particularly from north Florida through North Carolina, should closely monitor the progress of Florence, ensure they have their hurricane plan in place, and follow any advice given by local officials," the NHC said.
Swells from the storm, which are likely to cause life-threatening conditions, are beginning to affect the East Coast.
South Carolina Gov. Henry McMaster declared a state of emergency Saturday because of Florence's potential impact on the state.
Virginia and North Carolina also declared states of emergency in preparation for Florence.
Helene, the eighth named storm of the season, is 155 miles southeast of Cabo Verde, an archipelago nation off the African coast.
It's moving westward at 13 mph, with maximum-sustained winds of 60 mph.
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