* Could become hurricane by Friday night or Saturday
* May affect Azores as extratropical cyclone in 4-5 days
MIAMI, Oct 7 (Reuters) - Tropical Storm Otto, the 15th named storm of an active 2010 Atlantic hurricane season, could become a hurricane by Friday night or Saturday as it drifts northeastwards over the ocean, the U.S. National Hurricane Center said on Thursday.
Otto, which formed initially as a sub-tropical storm in the western Atlantic, posed no threat to land or energy assets but was expected to bring heavy rain to the north Leeward Islands, the Virgin Islands and Puerto Rico in the next day or so.
So-called sub-tropical storms are weather hybrids that have characteristics of both tropical and extra-tropical storms, but they must transition to fully tropical, as Otto has done, to be able to form eventually into hurricanes.
At 11 a.m. EDT (1500 GMT), the center of Otto was located about 255 miles (410 km) northeast of Grand Turk Island in the Turks and Caicos, a British overseas territory.
Otto was carrying top sustained winds of 60 miles per hour (95 kph) as it meandered slowly northeastwards into the open Atlantic, heading well away from the U.S. East Coast.
The Miami-based hurricane center said that on its forecast track Otto could end up affecting the Portuguese Azores archipelago in the Atlantic as a "significant extratropical cyclone" in four to five days.
The six-month Atlantic hurricane season that ends on Nov. 30 has seen seven hurricanes so far, five of which achieved major hurricane status, but the United States has so far escaped a significant landfall impact. (Reporting by Pascal Fletcher; Editing by Eric Beech)