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"We didn't anticipate it gaining this much strength this early," said center meteorologist Chris Landsea, adding that the ocean's warm temperatures and the current atmosphere is "very conducive" to energizing storms.
Projections for U.S. coast
Most computer forecast models show Irene threatening Florida and South Carolina by the end of the week.
Forecasters said a low pressure trough over the eastern United States was expected to shift Irene's track to the east, reducing the risk of a direct landfall in densely populated South Florida but raising the risk in the Carolinas.
"The storm is tracking north of all the big islands in the Caribbean now," said meteorologist Brad Panovich of WCNC-TV in Raleigh, N.C. "This is significant because now the storm will have little interference as it moves towards the U.S. This also means a shift in the track east squarely puts the Carolinas in the strike zone."
"I would prepare now along the entire South and North Carolina coasts," he added. "Preparing for the worst and hoping for the best is the goal here. Get your supplies and plan together today through Wednesday. Thursday we’ll know who needs to activate that plan. If you wait you’ll be fighting crowds for supplies late week."
Earlier Monday, the storm slashed directly across Puerto Rico, tearing up trees and knocking out power to more than a million people, then headed out to sea north of the Dominican Republic, where the powerful storm's outer bands were buffeting the north coast with dangerous sea surge and downpours.
Late Monday, the storm's downpours forced more than 1,000 Dominicans to evacuate their homes, with some families in low-lying areas fleeing to churches and public buildings. Others hunkered down inside their homes as the winds howled outside and heavy waves pounded the piers and washed onto coastal boulevards.
"We are going to see if the zinc roof resists" the storm, Fidelina Magdaleno, 60, said in her house in Nagua while a chicken dinner was prepared inside without electricity.
Residents earlier had jammed supermarkets and gas stations to get supplies for the storm. Schools were closed and emergency services were placed on alert. At least 33 flights were canceled at Santo Domingo's international airport.
The first hurricane of the Atlantic season was a large system that could cause dangerous mudslides and floods in Dominican Republic, the hurricane center said. It was not expected to make a direct hit on neighboring Haiti, though that country could still see heavy rain from the storm.
Dominican officials said the government had emergency food available for 1.5 million people if needed and the country's military and public safety brigades were on alert.
"We have taken all precautions," presidential spokesman Rafael Nunez said.
Irene is forecast to grow into a Category 3 hurricane late Tuesday as it moves over the warm waters of the Turks and Caicos Islands and the southeastern Bahamas, and could maintain that strength as it nears the U.S. coast.