The airplane veered off of runway 34 in a sharp turn to the right while back taxiing for takeoff. The pilot reported that there were light to moderate rain showers located 15-20 miles west/southwest of the airport prior to his preflighting the airplane. He also reported that the wind velocity changed in magnitude and direction during the taxi. The pilot also reported that following the accident, the winds increased from the east/northeast to 12-15 mph and that there was convective activity in the area.