The National Weather Service said that the first tornado to ever hit Block Island touched down there during a storm earlier this month. The announcement was made in a public information statement released Monday by the weather service’s office in Taunton, Mass. The statement said that at about 4 p.m. Aug. 10, a ‘‘tornadic waterspout’’ over waters south of Block Island moved northward and touched down on land. The service said that the tornado’s wind speed reached 70 miles per hour and that it covered about 4 miles in a path that was at most 20 yards wide.