SAN JUAN, Puerto Rico—Tropical Storm Fred swirled just south of Puerto Rico early Wednesday heading for the Dominican Republic and Haiti, with forecasters warning that its heavy rains could cause dangerous flooding and mudslides. After a quiet month of no named storms in the region, Fred became the sixth of the Atlantic hurricane season as expected late Tuesday and tropical storm warnings were already in effect for the U.S. Virgin Islands and Puerto Rico. “The most important thing today is preparation,” said Puerto Rico Gov. Pedro Pierluisi. “I am not going to minimize the potential impact of this event … we expect a lot of rain.” Rains pelted the northern Caribbean and power outages were reported in Puerto Rico, where Luma, the company in charge of the U.S. territory’s transmission and distribution system, warned those who depend on electricity for life-saving medical devices to activate emergency plans. “Puerto Rico’s system …