THE PILOT REPORTED THAT WHEN HE RETURNED TO THE AIRPORT TO LAND, A LINE OF DARK CLOUDS WAS NEXT TO THE DOWNWIND LEG FOR RUNWAY 31. DURING THE FINAL APPROACH SEGMENT, THE AIRCRAFT ENCOUNTERED HEAVY RAIN AND TURBULENCE. INITIALLY, THE PILOT ATTEMPTED A GO-AROUND, THEN DECIDED TO CONTINUE THE LANDING. DURING THE LANDING, STRONG/GUSTY WINDS WERE ENCOUNTERED AND THE AIRCRAFT TOUCHED DOWN ON THE ADJACENT GRASS AREA. SUBSEQUENTLY, THE AIRCRAFT WAS DAMAGED WHEN IT CROSSED A DITCH AND CAME TO REST ON A PUBLIC ROAD.