Tropical Storm Fiona threaded between Guadeloupe and Montserrat while entering the eastern Caribbean and dropping heavy rains over the northern Leeward Islands.
Forecasters said the storm would slowly make its way toward Puerto Rico on Saturday and would pass near or over the US territory by Sunday morning, with the potential for dangerously heavy rain in isolated spots.
Fiona was then expected to cross over the Dominican Republic on Monday with the same threat of extreme rains in places that could cause flash floods and mudslides.
The US National Hurricane Centre in Miami said Fiona was then likely to strengthen into a hurricane near the Bahamas by Tuesday night.
Meanwhile, Tropical Storm Lester in the eastern Pacific was on a projected path that could bring landfall near the Acapulco area on Mexico’s southwestern coast Saturday night.
Fiona was predicted to bring 4 to 8 inches (10 to 20 centimetres) of rain in eastern and southern Puerto Rico, with as much as a foot (30 centimetres) in isolated spots.
Rains of 6 to 10 inches (15 to 25 centimetres) were forecast for the Dominican Republic, with up to 16 inches (41 centimetres) in places.
Life-threatening surf also was possible from Fiona’s winds, forecasters said.
Fiona, which is the Atlantic hurricane season’s sixth named storm, had maximum sustained winds of about 60 mph (95 kph) late Friday, the centre said.
It was moving westward at 14 mph (22 kph), and was centred about 55 miles (90 kilometres) west-northwest of Guadeloupe.
In the Pacific, Lester was expected to remain a tropical storm until hitting the Mexican coast, but forecasters warned of potential dangers from heavy rains.
The storm had maximum sustained winds of 40 mph (65 kph) late Friday. It was centred 200 miles (325 kilometres) southeast of Acapulco and moving to the northwest at 12 mph (19 kph).
A tropical storm warning was up from Puerto Escondido to Zihuatanejo. The hurricane centre said Lester could drop from 8 to 12 inches (20 to 30 centimetres) of rain on the coasts of upper Guerrero state and Michoacan state, with isolated areas getting 16 inches (41 centimetres).