Hurricane Dorian posed an increasing menace to Florida Thursday as it pushed over open waters after doing limited damage in Puerto Rico and the Virgin Islands.

The U.S. National Hurricane Center said Dorian was expected to grow into a potentially devastating Category 3 hurricane before hitting the U.S. mainland late Sunday or early Monday somewhere between the Florida Keys and southern Georgia.

"Hurricane Dorian looks like it will be hitting Florida late Sunday night," President Donald Trump tweeted. "Be prepared and please follow State and Federal instructions, it will be a very big Hurricane, perhaps one of the biggest!"

Dorian blew through the Virgin Islands as a Category 1 hurricane on Wednesday while raking nearby Puerto Rico with high winds and rains.

The storm caused an islandwide blackout in St. Thomas and St. John in the U.S. Virgin Islands, and scattered power outages in St. Croix, government spokesman Richard Motta said. The storm also downed trees and at least one electric pole in St. Thomas, he said.

And there were no reports of serious damage in the British Virgin Islands, where Gov. Augustus Jaspert said crews were already clearing roads and inspecting infrastructure by late Wednesday afternoon.

Early Thursday, Dorian was centered about 150 miles (240 kilometers) north-northwest of San Juan, Puerto Rico. The U.S. National Hurricane Center said its top winds were blowing at 85 mph (140 kph) as the storm moved northwest at 13 mph (20 kph).

The Hurricane Center projected the storm could have winds of 125 mph (200 kph) by the time it reaches the mainland. Also imperiled were the Bahamas, with Dorian's forecast track running just to the north of Great Abaco and Grand Bahama islands.

In Florida, Gov. Ron DeSantis declared a state of emergency for counties that could be in the storm's path, and he urged people to have a week's worth of supplies on hand.

County governments along the state's central east coast distributed sandbags and many residents rushed to warehouse retailers to load up on water, canned food and emergency supplies.

Puerto Rico seemed to be spared any heavy wind and rain, a huge relief on an island where blue tarps still cover some 30,000 homes nearly two years after Hurricane Maria. The island's 3.2 million inhabitants also depend on an unstable power grid that remains prone to outages since it was destroyed by Maria, a Category 4 storm.

Several hundred customers were without power across Puerto Rico, said Ángel Figueroa, president of a union that represents power workers.

Police said an 80-year-old man in the northern town of Bayamón died Wednesday after he fell trying to climb up to his roof to clear it of debris ahead of the storm.