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The storm’s center was located about 150 miles (235 kilometers) northeast of San Juan, Puerto Rico, and it was heading toward the west at 15 mph (24 kph).

While the hurricane's center was not expected to strike land, it threatened to dump flooding rains on islands in the region.

Mike Brennen, director of the ��ɫtv Hurricane Center in Miami, said Erin grew into a “very powerful hurricane,” with its winds gaining 60 mph (96 kph) in about nine hours.

The Hurricane Center said Erin should begin to slowly weaken as it increased wind shear. However forecasters predicted that it will remain a major hurricane until late next week.

Erin close enough to land to trigger flooding, landslides

The storm's center was forecast to remain at sea, passing north of Puerto Rico, according to the ��ɫtv Hurricane Center.

But tropical storm watches were issued for St. Martin, St. Barts and the Turks and Caicos Islands.

The Hurricane Center warned that heavy rain in some areas could trigger flash flooding, landslides and mudslides.

In San Juan, locals and tourists walked, exercised and shopped as usual Saturday. Restaurants were busy, and despite warnings to avoid beaches, people could be seen in the coastal waters. Parents kept their children from swimming, however.

Sarahí Torres and Joanna Cornejo, who were visiting 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 Pérez Peña said 367 shelters were inspected and ready to open if needed.

Meanwhile officials in the Bahamas prepared some public shelters as a precaution as they urged people to monitor 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.

Though compact, with hurricane-force winds extending 30 miles (45 km) from its center, Erin was expected to double or even triple in size in the coming days.

Powerful rip currents could affect the U.S. East Coast from Florida to the mid-Atlantic next week, despite the eye of the storm forecast to remain far offshore, Brennan said.

An ‘incredible’ race from tropical storm to Category 5

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 Aug. 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.

In October 2005, Hurricane Wilma rocketed from a tropical storm to a Category 5 in less than 24 hours, according to ��ɫtv Hurricane Center advisories from that time. Wilma weakened to a Category 3 hurricane before striking Florida. And in October 2007, Hurricane Felix took just over a day to go from a tropical storm to Category 5.

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. Conditions needed for hurricanes to reach such strength include very warm ocean water, little to no wind shear and being far from land, he said.

Scientists say warming climate linked to storms strengthening faster

Scientists have linked in the Atlantic to climate change. Global warming is causing the and is spiking ocean temperatures, and warmer waters give hurricanes fuel to unleash more rain and strengthen more quickly.

Storms that ramp up so quickly complicate forecasting for meteorologists and make it harder for government agencies to plan for emergencies. , a Pacific storm that made landfall 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 and is . Six to 10 hurricanes are predicted for the season, including three to five reaching major status with winds of more than 110 mph (177 kph).

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Bynum reported from Savannah. Georgia. Associated Press writers Isabella O'Malley in Philadelphia and Ivelisse Rivera in San Juan contributed.

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