(Updates position, wind speed)

MIAMI, Aug 15 (Reuters) - The sixth tropical storm of the 2008 Atlantic hurricane season formed on Friday over the Dominican Republic and was expected to track westward in the direction of Florida and the Gulf of Mexico, the U.S. National Hurricane Center said.

Computer models used to predict storm tracks indicated Tropical Storm Fay was likely to cross the mountainous center of the island of Hispaniola, shared by Haiti and the Dominican Republic, then hit Cuba and emerge somewhere near south Florida by Monday.

The storm was not projected by the Miami-based hurricane center to strengthen into a hurricane, with winds of at least 74 miles per hour (119 km per hour). The high mountains of Hispaniola and the amount of time it seemed destined to spend over land in Cuba would most likely drain it of energy.

At 8 p.m. EDT (2400 GMT), Fay was located around 35 miles (55 km) west-northwest of Santo Domingo and was moving toward the west near 14 mph (22 kph).

Top sustained winds had reached near 45 mph (75 kph) but the storm's potential rainfall of 4 to 8 inches (10-20 cm) was of greater concern than its winds, particularly over the denuded hillsides of Haiti, where thousands of people have been killed by mudslides and floods during the past few hurricane seasons.

In some areas torrential rainfall of 12 inches (30 cm) could be expected, the hurricane center said.

Tropical storm warnings were in effect for the northern coasts of the Dominican Republic and Haiti, for parts of eastern Cuba and for the southeastern islands of the Bahamas.

A tropical storm warning means storm conditions with gusty winds and rain can be expected within 24 hours.

Energy markets have been watching the storm system closely as several computer models projected it could enter the Gulf of Mexico, home to U.S. oil and gas production rigs.

The 2008 Atlantic hurricane season is on track to be more active than average, due to warm sea waters and favorable atmospheric conditions.

Two of the tropical storms so far, Bertha and Dolly, reached hurricane strength before fading out over the open Atlantic and washing up on the shores of south Texas, respectively. (Reporting by Michael Christie; editing by Jim Loney and Mohammad Zargham)

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