MIAMI (AP) — Tropical Storm Tammy formed Wednesday in the central tropical Atlantic, putting a group of islands on alert for its expected approach.

The Miami-based National Hurricane Center said Tammy emerged in the afternoon and was centered at 5 p.m. EDT about 625 miles (1,005 kilometers) east of the Windward Islands. It was moving briskly to the west at 23 mph (37 kph) and expected to gradually strengthen in coming days.

Tropical storm watches have been issued for Barbados, Dominica, and Martinique and Guadeloupe, forecasters said. They noted that tropical storm conditions were possible in the region within 48 hours and that further watches and warnings will likely be required Wednesday or Thursday.

Tammy is expected to initialy spread heavy rains over the northern Windward and Leeward Islands through Friday and then across the U.S. and British Virgin Islands and Puerto Rico over the weekend, forecasters said.

Total rainfall totals of between 3 and 6 inches (7-14 centimeters) with maximum amounts of 10 inches (25 centimeters) in some spots has been forecasted for the northern Windward Islands and Leeward Islands.

Lesser rainfall amounts are expected in the British and U.S. Virgin Islands into eastern Puerto Rico, the hurricane center said, adding the storm raises threats of scattered flash flooding, urban flooding and isolated mudslides in higher terrain as it nears the islands.