NEW : Omar becomes Category 3 hurricane as it approaches Leeward Islands . U.S. Coast Guard shuts down Caribbean , where refinery is . Parts of Leeward Islands could see 74 mph of rain , mudslides , flash floods . Category is 15th named storm of hurricane season , which ends hurricane season .
NEW : Omar becomes Category 3 hurricane as it approaches Leeward Islands . U.S. Coast Guard shuts down Christiansted Harbor , where refinery is . Parts of Puerto Rico could see 20 inches of rain , mudslides , flash floods . Omar is 15th named storm of hurricane season , which ends November 30 .