MIAMI | Tropical Storm Matthew will likely become a hurricane by Friday as it pushes across the Caribbean Sea, posing a serious threat to vulnerable islands in a region prone to deadly flooding and mudslides.

At 8 p.m. Wednesday, National Hurricane Center forecasters said Matthew was located about 90 miles west of St. Lucia and had slowed to about 15 mph as it continued moving west. Sustained winds increased to 65 mph.

The storm was expected to move away from the Windward Islands Wednesday night and keep slowing down. Tropical storm force winds, which extend about 205 miles from the center of Matthew, should continue pummeling the islands as well as the southern end of the Leeward chain. The islands could also see between 4 and 8 inches of rain.

The storm’s worst winds were blowing on its northeast side, which could spell trouble for Haiti as the storm rolls west, where rains are expected to begin Saturday. The country has been hit with devastating floods this year after an unusually wet winter. In February, flooding killed at least one person and damaged 10,000 homes.

Computer models keep Matthew rolling west for the next two to three days, then begin to turn as the storm nears the edge of a steering ridge. Where that turn happens depends on how intense the storm becomes. Forecasters warned that most models can be off by as much as 240 miles five days out.

Over the day Wednesday, the storm became better defined, forecasters said. But a hurricane hunter plane reported finding no drop in pressure, leading forecasters to project little strengthening as the storm lingers over the Windward Islands. The plane was scheduled to return Wednesday evening.

Forecasters have issued a tropical storm warning for Guadeloupe and Martinique, St. Lucia, Dominica, Barbados, St. Vincent, and the Grenadine Islands. A watch is in effect for Bonaire, Curacao, and Aruba.