Hurricane Erin exploded in strength to a Category 5 storm in the Caribbean on Saturday, rapidly powering up from a tropical storm in a single day, the U.S. National Hurricane Center (NHC) said.

While the compact hurricane's centre isn't expected to strike land, it threatens to dump flooding rains on the northeast Caribbean as it continues to grow larger.

The first Atlantic hurricane of 2025, Erin ramped up from a tropical storm to a Category 5 hurricane in a mere 24 hours. By late Saturday morning, its maximum sustained winds more than doubled to 255 km/h.

Mike Brennan, director of the NHC in Miami, said Erin had grown into a "very powerful hurricane." He said its winds gained 96 km/h in intensity within about nine hours on Saturday.

"We expect to see Erin peak here in intensity relatively soon," Brennan said in an online briefing.

The NHC said Erin should weaken somewhat late Saturday or early Sunday as the storm encounters increased wind shear and possibly takes in more dry air. However, forecasters predict it will remain a major hurricane until midweek.

The hurricane was located 180 kilometres north of Anguilla at 2 p.m. Saturday, moving west at 26 km/h. The storm's centre was forecast to remain at sea, passing 233 kilometres north of Puerto Rico, according to the U.S. National Weather Service.

Erin was close enough to affect nearby islands. Tropical storm watches were issued for St. Martin, St. Barts and St. Maarten. The NHC warned that heavy rain in some areas could trigger flash flooding, landslides and mudslides.

Tropical-storm force wind gusts are possible in the Turks and Caicos Islands and southeast Bahamas.

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Though compact in size, with hurricane-force winds extending 45 kilometres from its centre, the NHC said Erin was expected to double or even triple in size in the coming days.

Erin could create powerful rip currents off the U.S. east coast from Florida to the mid-Atlantic next week, even with its eye forecast to remain far offshore, Brennan said.

'Incredible for any time of year'

Hurricane specialist and storm-surge expert Michael Lowry said Erin gained strength at a pace that was "incredible for any time of year, let alone August 16th."

Lowry said only four other Category 5 hurricanes have been recorded in the Atlantic on or before Aug. 16.

The most powerful storms tend to form later in the year, with the hurricane season typically peaking in mid-September.

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In October 2005, Hurricane Wilma rocketed from a tropical storm to a Category 5 storm in less than 24 hours, according to NHC advisories from that time. Wilma weakened to a Category 3 hurricane before striking Florida.

Including Erin, there have been 43 hurricanes that have reached Category 5 status on record in the Atlantic, said Dan Pydynowski, senior meteorologist at AccuWeather, a private forecasting company.

"They're certainly rare, although this would mark the fourth year in a row that we've had one in the Atlantic basin," Pydynowski said about Category 5 hurricanes. Conditions needed for hurricanes to reach this strength include very warm ocean water, little to no wind shear and being far from land, he said.

Scientists have linked rapid intensification of hurricanes in the Atlantic Ocean to climate change. Global warming is causing the atmosphere to hold more water vapour and is spiking ocean temperatures. The warmer waters give hurricanes fuel to unleash more rain and strengthen more quickly.

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Storms that ramp up so quickly complicate forecasting for meteorologists and make it harder for government agencies to plan for emergencies. Hurricane Erick, a Pacific storm that made landfall on June 19 in Oaxaca, Mexico, also strengthened rapidly, doubling in intensity in less than a day.

Erin is the fifth named storm of the Atlantic hurricane season, which runs from June 1 to Nov. 30. It's the first to become a hurricane.

The 2025 hurricane season is expected to be unusually busy. The forecast calls for six to 10 hurricanes, with three to five reaching major status with winds of more than 177 km/h.

People urged to track storm

In San Juan, the capital of Puerto Rico, locals and tourists walked, exercised and shopped on Saturday as usual. Restaurants were busy, and despite warnings to avoid the beaches, people could be seen in the water at Ultimo Trolley and Ocean Park beaches. Parents, however, kept their children from swimming.

Sarahi Torres and Joanna Cornejo, visiting San Juan from California for a Bad Bunny concert, said they decided to go to the beach and wade into the water because the skies appeared calm.

"The weather looked fine, so we came out," Torres said.

The U.S. government deployed more than 200 employees from the Federal Emergency Management Agency and other agencies to Puerto Rico as a precaution. Puerto Rico Housing Secretary Ciary Perez Pena said 367 shelters were inspected and ready to open if needed.

Meanwhile, officials in the Bahamas said they prepared some public shelters as a precaution as they urged people to track the hurricane.

"These storms are very volatile and can make sudden shifts in movement," said Aarone Sargent, managing director for the Bahamas' disaster risk management authority.