Tropical Storm Ernesto threatens Windward Islands

* Islanders could feel its effects by Thursday night

* Storm expected to cross into Caribbean on Friday

* Ernesto could strengthen into hurricane early next week

By Jane Sutton

MIAMI, Aug 2 (Reuters) - Tropical Storm Ernesto formed in the Atlantic Ocean near the Windward Islands on Thursday and could strengthen into a hurricane as it races westward across the Caribbean Sea, forecasters said.

The storm was expected to hit the southern Windward Islands and cross into the Caribbean early on Friday, the U.S. National Hurricane Center said.

Island governments issued storm warnings for Barbados, St. Vincent and the Grenadines, Dominica, St. Lucia, Martinique and Guadeloupe, alerting residents to expect storm conditions as early as Thursday night.

Ernesto formed 295 miles (475 km) east of the Windward Islands and was moving rapidly west at 22 mph (35 kph). It was expected to turn more to the northwest early next week and strengthen into a hurricane southeast of Jamaica.

It had top winds of 50 miles per hour (85 kph) on Thursday and would become a hurricane if those swirling winds reach 74 mph (119 kph).

Forecasters warned island residents to expect large waves and 2 to 3 inches (5 to 8 cm) of rain, with 5 inches (nearly 13 cm) in isolated areas.

Several computer forecasting models showed it moving through or near the Yucatan Channel into the southern Gulf of Mexico in the middle of next week. It was too early to know whether Ernesto could disrupt oil and gas operations clustered in the Gulf.

August and September are usually the most active months of the Atlantic-Caribbean hurricane season, which runs from June 1 to November 30.

"It's our first system coming out of the deep tropics this year, so maybe it's a good time for people to review their preparedness plans as we're getting into the part of the season where things normally begin to get a little more active," said David Zelinsky, a meteorologist at the National Hurricane Center. (Reporting by Jane Sutton)