According to the pilot, during his approach to land he observed thunderstorms in the vicinity of the airport and observed moisture on the windscreen of the airplane. He elected to continue the approach and encountered strong turbulence during the base and final legs of the approach. During the turbulent encounter on short final, the airplane was turned 30 degrees to the right of runway heading and the nose was forced upward. The pilot regained control of the airplane and landed firmly on the runway surface. The airplane departed the left side of the runway, the left wing "dug" into the grass surface, cartwheeled, and came to rest inverted. The outboard four feet of the left wing and the firewall of the airplane were bent. According to the pilot and a Federal Aviation Administration inspector, there were no mechanical anomalies with the airplane. Witnesses observing the flight indicated that the wind shifted from a right quartering head wind to a right quartering tail wind and increased in velocity while the airplane was on short final.