Severe thunderstorms unleashed heavy rain and strong winds across parts of the Midwest and Northeast on Thursday, grounding hundreds of flights and leaving tens of thousands of people without power. The storms spawned a tornado that touched down in Elmira, New York, damaging a mall and a local country club, the National Weather Service said. There were no immediate reports of injuries. Meteorologists said 70 mile-an-hour winds were reported in parts of Ohio, Kentucky, Pennsylvania and Oklahoma.