Rafael has reached hurricane strength in the Atlantic Ocean south of Bermuda, forecasters said.
The Hurricane Centre said Rafael's top sustained winds had risen to near 75 mph, making it the ninth hurricane of the Atlantic season.
Rafael is centered about 560 miles south of Bermuda. It is moving north at 10 mph and is forecast to pass near or to the east of Bermuda.
Tropical Storm Isaac could become a hurricane, with sustained winds of 45 miles per hour (75kph), US forecasters have warned.
It was centred about 265 miles (425 km) southeast of San Juan, Puerto Rico, earlier today, the US National Hurricane Centre said.
Computer forecast models showed the storm moving west-north west across Haiti tomorrow, posing risk of mudslides in the country.
Isaac was projected to weaken over Cuba and Haiti, before strengthening into a hurricane in the Florida Straits between Cuba and Florida.
A potential hurricane is posing a threat to next week's US Republican National Convention in Florida, which culminates in the nomination of Mitt Romney as presidential candidate.
The US National Hurricane Centre said Tropical Storm Isaac was expected to strengthen and could become a hurricane by later today.
Convention organisers in Tampa knew it was a possibility during the peak of the hurricane season with 70,000 people expected to attend.
A tropical storm in the Caribbean could become a hurricane, US forecasters have warned as it moves on the track that could hit the coasts of Florida on Monday.
Tropical Storm Isaac has been predicted to move onto Haiti and the Dominican Republic, the US National Hurricane Centre said.
It coincides with next week's Republican National Convention in Florida, which culminates in the nomination of Mitt Romney for presidential candidate.