Tropical Storm Isaac strengthened over the Caribbean on Wednesday, threatening to slam into impoverished Haiti -- still reeling from a devastating 2010 earthquake -- and Cuba, US forecasters said. The Miami-based National Hurricane Center projected that Isaac would become a hurricane on Thursday and make landfall early Saturday near the US naval base at Guantanamo Bay in southern Cuba. A hurricane watch has been issued for all of Haiti, often hit by disastrous floods and mudslides, as well as for Puerto Rico, the Puerto Rican islands of Vieques and Culebra, and the US and British Virgin Islands. The storm, packing gusts of up to 45 miles (72 kilometers) per hour, was nearing the coast of Hispaniola, the island shared by the Dominican Republic and Haiti, the NHC said in its 0000 GMT bulletin. Isaac was expected to dump up to 20 inches (51 centimeters) of rain over Hispaniola, spelling trouble for the 400,000 Haitians still huddled in makeshift slums more than two years after the nation\'s disastrous earthquake. \"These rains could cause life-threatening flash floods and mudslides,\" the NHC warned, noting that Isaac was about 65 miles southwest of Guadeloupe and moving westward at 21 miles per hour. The storm forced the US military at Guantanamo to delay hearings for the five alleged plotters of the September 11, 2001 attacks. Commanders at the base have made hurricane preparations. The NHC said the storm could strike Florida as early as Monday, when the US Republican National Convention opens in Tampa. Meteorologists, however, have cautioned that it is too early to accurately predict Isaac\'s path and said it is unlikely to disrupt the convention. Thousands of Republicans will be in Tampa for four days during which former Massachusetts governor Mitt Romney will be formally nominated to challenge President Barack Obama in the November 6 election. On Wednesday, meanwhile, much of the southern coast of the Dominican Republic was under a hurricane warning. A tropical storm warning was in effect throughout the Leeward Islands, including Martinique, Dominica, Guadeloupe and the surrounding islands, Saint Martin, Sint Maarten, Saint Kitts, Nevis, Antigua, Barbuda, Montserrat and Anguilla, Saba and Saint Eustatius. \"For residents in Puerto Rico and the US Virgin Islands who may be affected... it\'s critical that you take this storm seriously and take steps now to prepare your families, homes and businesses,\" US Federal Emergency Management Agency Administrator Craig Fugate said in a statement. FEMA has deployed teams to Puerto Rico and the US Virgin Islands to work with local officials on how to manage their response to any potential disaster. In Guadeloupe, companies closed their doors ahead of a storm with the entire island under a red alert. Coming on the heels of Isaac, was another tropical depression -- to be named Joyce if gains tropical storm strength -- that formed over the eastern tropical Atlantic on Wednesday, the NHC said.