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By Michael Christie

MIAMI, Aug 25 (Reuters) - The Atlantic hurricane season's seventh tropical storm formed in the Caribbean on Monday and was likely to strengthen into a hurricane before striking southwest Haiti soon, the U.S. National Hurricane Center said.

Tropical Storm Gustav threatened Haiti with as much as 25 inches (64 cm) of rain in isolated areas, which could trigger deadly floods and mudslides in the impoverished nation of some 9 million people.

Hurricane warnings were issued for the southern coasts of the Dominican Republic and Haiti west of Santo Domingo shortly after Gustav formed 225 miles (365 km) south-southeast of Port-au-Prince, the Haitian capital, the Miami-based hurricane center said.

The storm's top sustained winds were already at 60 mph (95 kph) by early Monday afternoon, and the center's official forecast saw them reaching around 80 mph (130 kph) when it reached southwest Haiti within 48 hours.

Haiti was still recovering from the passage of Tropical Storm Fay, the remnants of which were causing flooding across the U.S. southeastern states on Monday.

Fay may have killed more than 50 people in Haiti last week, including dozens missing after floodwaters swept a bus down a river in the south.

Forecasters said Gustav could produce rainfall of 5 to 7 inches (13-18 cm) over Hispaniola, the island shared by Haiti and the Dominican Republic, with the possibility of 15 to 25 inches (38-64 cm) some areas, the hurricane center said.

"These intense rains may produce life-threatening flash floods and mudslides," the center said.

Haiti is vulnerable to devastating floods because hillsides have been stripped of trees by people desperate for charcoal to be used as cooking fuel. In 2004, Tropical Storm Jeanne was blamed for flooding that killed some 3,000 people.

The official forecast calls for Gustav to move to the northwest across Haiti and then over the southeastern tip of Cuba toward the Bahamas and Florida.

But the computer models used to predict the future path of hurricanes disagreed significantly. Some had it moving more toward the west, south of Cuba in the direction of Jamaica and then the Yucatan peninsula.

Another showed Gustav moving over a significant portion of Cuba and into the Gulf of Mexico. Yet another had it crossing Hispaniola and heading toward the southern Bahamas. (Editing by Jim Loney and Eric Walsh)

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