SAN JUAN, Puerto Rico—Tropical Storm Fred made landfall in the Dominican Republic on Wednesday, with forecasters warning that heavy rains could cause dangerous flooding and mudslides there and in the neighboring country of Haiti. After a quiet month of no named storms in the region, Fred became the sixth of the Atlantic hurricane season late Tuesday as it moved past the U.S. Virgin Islands and Puerto Rico on a forecast track that would carry it toward Florida over the weekend. Government crews with megaphones walked through impoverished neighborhoods in the Dominican capital of Santo Domingo ahead of the storm urging those in low-lying areas to evacuate. Tropical storm warnings were discontinued in the U.S. territories after pelting the islands with rain, leaving some 13,000 customers without power in Puerto Rico. Fred was centered 30 miles (50 kilometers) west of Santo Domingo, Dominican Republic, on Wednesday morning and moving west-northwest at …