MIAMI, Sept 8 (Reuters) - Subtropical Storm Gabrielle formed on Friday off the U.S. mid-Atlantic Coast with top sustained winds of 45 mph (72 kph), but was not expected to strengthen in the next 24 hours, the U.S. National Hurricane Center said.

Gabrielle, the seventh named storm of the Atlantic hurricane season, was about 385 miles (625 km) southeast of Cape Lookout, North Carolina, the Miami-based hurricane center said in its first advisory on the storm at 11 p.m. (0300 GMT on Saturday).

"Little change in strength is forecast during the next 24 hours," the center said. Top winds must reach at least 74 mph (119 kph) for a tropical or subtropical storm to become a hurricane.

A tropical storm watch was issued for parts of the North Carolina and South Carolina coasts. Gabrielle was moving west-northwest at around 10 mph (16 kph).

"Coastal residents from Onslow County north along the Outer Banks are urged to take action now to protect any property that may be threatened by tropical storm conditions," the National Weather Service said in a statement late on Friday from Newport, North Carolina.

"Winds will start to reach tropical storm force along the coast on Sunday morning. At this point winds are expected to reach 45 to 55 mph (72 to 89 km)," it said.

Increasing swells and high tides would combine to produce dangerous rip currents along area beaches this weekend, it said.

Computer models indicated the weather system would most likely head northwestward toward the coast of North Carolina before looping around to the northeast and cooler waters.

It was very unlikely, however, the system could reach the top-rank strength of Hurricanes Dean and Felix, which slammed into Mexico's Yucatan Peninsula in August and Central America this week respectively as Category 5 hurricanes on the five-step Saffir-Simpson scale of hurricane intensity.

Sea surface temperatures and atmospheric conditions off the U.S. Southeast were nowhere near as favorable for tropical cyclones as in the western Caribbean, where Dean and Felix grew into monster storms, the hurricane center said earlier.

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