A warning flag flies on the beach as people swim in Condado, Puerto Rico, as Hurricane Erin approaches on Friday. (Alejandro Granadillo | AP)SAN JUAN, Puerto Rico — Erin strengthened into a powerful Category 4 hurricane in the Caribbean on Saturday and continues to intensify, the National Hurricane Center said. Erin is the fifth named storm of the Atlantic hurricane season, which runs from June 1 to Nov. 30, but the first to reach hurricane status. “Erin is forecast to explode into a powerful Category 4 hurricane as it moves across very warm waters in the open Atlantic. Puerto Rico Housing Secretary Ciary Pérez Peña said 367 shelters have been inspected and could be opened if needed.