Powerful Hurricane Irma barreled towards the Caribbean and the southern United States on Tuesday as warnings of the Category 4 storm took effect in Leeward Islands, the British and US Virgin Islands and Puerto Rico. The storm intensified as it surged forward with 150 mph (240 kph) winds, the US National Hurricane Center (NHC) said. "Dangerous Hurricane Irma heading for the Leeward Islands," the hurricane center said. "Preparations should be rushed to completion as tropical storm-force winds are expected to arrive in the hurricane warning area by late Tuesday." Officials across the northeastern Caribbean canceled airline flights, shuttered schools and urged people to hunker down indoors as Hurricane Irma barreled toward the region. People on various Caribbean islands boarded up homes and rushed to find last-minute supplies, forming long lines outside supermarkets and gas stations. A Category 4 hurricane on the five-step Saffir-Simpson scale means sustained winds of 130-156 mph (209-251 kph) with "catastrophic" outcomes. They range from uprooted trees and downed power lines to water and electricity outages and enough damage to leave property uninhabitable, according to the Miami-based hurricane center. In preparation for the storm, the government of economically struggling Puerto Rico has declared a state of emergency and activated the National Guard. The US territory, home to about 3.4 million people, has 456 emergency shelters prepared to house up to 62,100 people. Puerto Rico also froze prices on basic necessities, including food and water, medicines, power generators and batteries, to help residents prepare. Telemundo TV station WIPR in Puerto Rico showed long lines of shoppers stocking up on bottled water, flashlights, batteries, generators, food and other items. The executive director of the state power authority, Ricardo Ramos, told the station that the power grid was so vulnerable from lack of investment that parts of the US territory could be without power for three to four months. "Were preparing for the worst-case scenario," he said. A hurricane warning was posted for Antigua and Barbuda, Anguilla, Montserrat, St. Kitts and Nevis, St. Martin, Saba, St. Eustatius, St. Maarten and St. Barts, Puerto Rico, and the US and British Virgin islands. A tropical storm warning was in effect for Guadeloupe and Dominica. Irma also threatens the US East Coast and Florida, which has declared a state of emergency. The hurricane center expects Irma to reach southern Florida on Saturday. Florida Governor Rick Scott said on Twitter late on Monday he had spoken to US President Donald Trump, who he said "offered the full resources of the federal government as Floridians prepare for Hurricane Irma." Authorities warned that the storm could dump up to 10 inches (25 centimeters) of rain, cause landslides and dangerous flash floods and generate waves of up to 23 feet (7 meters). "This is not an opportunity to go outside and try to have fun with a hurricane," US Virgin Islands Governor Kenneth Mapp warned. "Its not time to get on a surfboard." The NHC cautioned that it was too early to forecast the storms exact path or what effects it might have on the continental United States, but warned of likely effects to hit some areas by later this week. "There is an increasing chance of seeing some impacts from Irma in the Florida Peninsula and the Florida Keys later this week and this weekend. In addition, rough surf and dangerous marine conditions will begin to affect the southeastern US coast by later this week," the center said. Residents on the US East Coast were urged to monitor the storms progress in case it should turn northward toward Florida, Georgia or the Carolinas. "This hurricane has the potential to be a major event for the East Coast. It also has the potential to significantly strain FEMA and other governmental resources occurring so quickly on the heels of (Hurricane) Harvey," Evan Myers, chief operating officer of AccuWeather, said in a statement. Irma will be the second powerful hurricane to thrash the United States and its territories in as many weeks. Residents of Texas and Louisiana are still reeling from the catastrophic effects of Hurricane Harvey, which struck Texas as a Category 4 hurricane on August 25 and dumped several feet of rain, destroying thousands of homes and businesses.