Tropical Storm Isaac barreled towards the Dominican Republic and Haiti on Thursday, as forecasters warned it could turn into a hurricane that hits Hispaniola and Guantanamo Bay by the weekend. Hurricane-force winds and heavy rain could hit parts of Hispaniola, the Caribbean island divided between Haiti and the Dominican Republic, as early as Friday, the Miami-based National Hurricane Center said. \"Strengthening is forecast during the next 48 hours and Isaac could become a hurricane on Friday before it reaches Hispaniola,\" the NHC said. \"Preparations to protect life and property should be rushed to completion.\" At 1500 GMT, Isaac\'s center was about 320 kilometers (200 miles) south-east of San Juan, Puerto Rico. It should pass south of the Virgin Islands and Puerto Rico on Thursday, and approach the Dominican Republic late Thursday and Friday. In southwest Puerto Rico, locals stocked up on water, fuel and supplies ahead of the storm, and restaurants, hotels, and homes tied down outdoor chairs and tables. Heavy rain could be seen far out to sea. Haiti is particularly vulnerable because thousands of people are still living in makeshift homes, two years after an earthquake devastated the capital Port-au-Prince and killed an estimated 250,000 people. The storm was on track to make landfall in Cuba on Saturday at Guantanamo, the site of the US naval base and prison that houses the alleged September 11 plotters and other detainees from the so-called \"War on Terror.\"Several hundred people were evacuated from the base on Thursday, and legal proceedings against the alleged masterminds of the 2001 attacks on New York and Washington were postponed as the base buckled down for the storm. \"We sandbag anything where we know there is a flooding risk,\" Sergeant Jessica Brown told AFP, as she led a team of roughly 10 young American servicemen in the stifling tropical heat. Donning a white construction helmet and shielding his eyes with sunglasses, Lieutenant David Moore monitored the hoisting of a landing craft by an enormous crane at the base port as smaller craft were backed for safety. The storm was packing maximum sustained winds of 65 kilometers (40 miles) an hour, and moving at a speed of 24 kilometers (15 miles) and hour. Isaac could cross Hispaniola on Saturday, reaching Cuba between Saturday and Sunday, then reaching Florida on Monday and Tuesday, just in time for the US Pepublican party\'s National Convention in Florida. Republican delegates from around the country will be in Tampa, Florida for four days to formally nominate former Massachusetts governor Mitt Romney to challenge President Barack Obama in the November 6 election. But meteorologists caution that it is too early to accurately predict Isaac\'s path. Isaac is forecast to dump up to 20 inches (51 centimeters) of rain over Haiti, possibly spelling disaster for the 400,000 Haitians still huddled in makeshift after the nation\'s devastating 2010 earthquake. \"These rains could cause life-threatening flash floods and mudslides,\" the NHC warned. Meanwhile a second tropical depression gained strength in the eastern Atlantic and became Tropical Storm Joyce, the NHC said, though there was no immediate threat to land. Joyce was some 2,100 kilometers (1,305 miles) east of the Leeward Islands, with sustained winds of 65 kilometers (40 miles) per hour and moving towards the northwest at around 28 kilometers (17 miles) per hour. Joyce was far from land and was not forecast to change in strength over the next 48 hours, the NHC said.