MIAMI, Sept 9 (Reuters) - Tropical Storm Gabrielle strengthened slightly as it neared the North Carolina coast on Sunday but it was not expected to reach hurricane strength as it passed over the Outer Banks and curved back out into the Atlantic, U.S. forecasters said.

With top sustained winds of 50 mph (85 km per hour), Gabrielle, the seventh named storm of the 2007 Atlantic hurricane season, was about 30 miles (50 km) south-southeast of Cape Lookout, North Carolina, at 8 a.m. (1200 GMT).

The system was moving toward the north-northwest at 10 mph (17 kph) and was expected to turn to the north later in the day and then to the northeast, the U.S. National Hurricane Center said.

Gabrielle developed on Saturday as the six-month hurricane season neared its historical peak on Sept. 10, having already produced two of the most ferocious hurricanes to ever tear through the Caribbean, Dean and Felix.

Both those storms became maximum-strength Category 5 hurricanes before slamming into Mexico's Yucatan Peninsula and Central America, respectively.

But conditions off the U.S. East Coast, where Gabrielle formed, are not as favorable for tropical cyclones as the deep warm water of the western Caribbean where Dean and Felix found the fuel they needed to grow into monster storms.

Gabrielle's maximum sustained winds were expected to top out at 63 mph (102 kph), the Miami-based hurricane center said. Tropical storms become hurricanes when their top winds reach 74 mph (119 kph).

The storm was expected, however, to dump between 1 and 3 inches (2.5 cm to 7.6 cm) of rain across coastal North Carolina and southeastern Virginia, with some areas receiving up to 5 inches (12.7 cm).

Tropical storm warnings were issued from Surf City in North Carolina to Cape Charles Light in Virginia and a tropical storm watch was in effect for the lower Chesapeake Bay, south of New Point Comfort, in Virginia.

A warning means tropical storm conditions are likely within 24 hours and a watch means tropical storm conditions can be expected within 36 hours.

The National Weather Service predicted seas would build to 10 to 12 feet (3 to 3.7 metres) on Sunday, resulting in rough surf and minor beach erosion.

The agency urged residents in affected areas to stock up on provisions and prepare for a loss of power.

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