(Updates speed and location, adds details)

By Michael Peltier

TALLAHASSEE, Fla., Sept 8 (Reuters) - Tropical Storm Gabrielle swirled toward the North Carolina coast on Saturday but was unlikely to become a hurricane over the relatively cool Atlantic waters.

Upgraded from a subtropical storm by the National Hurricane Center, Gabrielle, the seventh named storm of the 2007 Atlantic hurricane season, was about 185 miles (300 km) southeast of Cape Lookout, North Carolina, at 5 p.m. (2100 GMT).

Subtropical storms typically lack the warm core and organized thunderstorm activity of tropical systems.

During the day, Gabrielle weakened somewhat with sustained winds slowing to about 40 mph (65 kph) but forecasters expect the storm to strengthen gradually in the next 24 hours as it nears landfall, the Miami-based agency said in an advisory.

"The center of Gabrielle will be near or over the coast of North Carolina tomorrow afternoon," the center said. "But outer rain bands will likely reach the coast tonight."

Given the region's cooler waters, forecasters said earlier they did not expect Gabrielle to become a hurricane. Top winds must reach at least 74 mph (119 kph) for a tropical storm to become a hurricane. A storm reaches tropical storm status when its sustained winds exceed 39 mph (63 kph) and its energy is generated at or near its center.

Tropical storm warnings were issued in parts of coastal North Carolina up to the Virginia border as Gabrielle moved northwest at about 8 mph (13 kph). Two to 6 inches (5 to 15 cm) of rain were forecast for areas of the North Carolina coast, the center said. A 2-to-3-foot (60-to-90 cm) storm surge was also projected.

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 provisions and prepare for a loss of power.

Computer models show the weather system looping around to northeast and cooler waters after reaching the coast sometime late on Sunday afternoon or evening. Depending on its track, the storm will either make landfall or, if it takes a more easterly track, skirt the coast near Cape Hatteras, North Carolina, and head back out to sea.

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 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 ferocious storms, the hurricane center said earlier.

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