SAN JUAN, Puerto Rico (AP) — Heavy rains pelted the northern Caribbean on Tuesday night as forecasters warned that a disturbance could soon become Tropical Storm Fred following a quiet month of no named storms in the region.

Power outages were already reported in Puerto Rico, where Luma, the company in charge of the U.S. territory's transmission and distribution system, warned those who depend on electricity for life-saving medical devices to activate emergency plans.

“Puerto Rico's system ... continues to be very fragile,” the company said, referring to a power grid that was razed by Hurricane Maria in September 2017.

The disturbance labeled Potential Tropical Cyclone Six was centered 40 miles (60 kilometers) southeast of Ponce, Puerto Rico, according to the National Hurricane Center in Miami.

Its maximum sustained winds increased to 40 mph (65 kph) by Tuesday night, but it still wasn't considered a tropical storm.

Dennis Feltgen, spokesman for the U.S. National Hurricane Center, said the system did not yet have a closed, low level circulation. “Until that happens, we basically have a strong tropical wave,” he said.

However, tropical storm warnings were in effect for the U.S. Virgin Islands and Puerto Rico, with forecasters expecting the system to become the sixth named storm of the Atlantic hurricane season late Tuesday.

“The most important thing today is preparation,” said Puerto Rico Gov. Pedro Pierluisi. “I am not going to minimize the potential impact of this event...we expect a lot of rain.”

Pierluisi said government agencies would close by noon Tuesday and officials noted that some gas stations had closed after running out of fuel. The heaviest rain was expected to fall overnight, forecasters said.

The Dominican Republic, Haiti and the southern Bahamas could get hit Wednesday, and people in Florida were urged to monitor updates, but forecasters said it remained uncertain where the storm would move later in the week.

Heavy rains hit parts of the eastern Caribbean during Monday night, including the islands of Guadeloupe and Dominica.

More than a month had passed since the last Atlantic storm, Hurricane Elsa, but this time of summer usually marks the start of the peak of hurricane season.

The hurricane center issued warnings for Puerto Rico, the U.S. Virgin Islands and the Dominican Republic on the south coast from Punta Palenque eastward and on the north coast from Cabo Frances Viejo eastward. A watch was in effect for Hispaniola's north coast, from Cabo Frances in the Dominican Republic to Gonaives, Haiti.

The Turks and Caicos Islands and the southeastern Bahamas were also under a tropical storm watch, including the Acklins, Crooked Island, Long Cay, the Inaguas, Mayaguana, and the Ragged Islands.

The storm was expected to produce rainfall of 2 to 4 inches (5 to 10 centimeters) over the Leeward Islands, Virgin Islands, and Puerto Rico, with up to 6 inches (15 centimeters) in some areas, and could lead to flooding and mudslides.

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