While enroute to his destination airport, the pilot of the tail-wheel equipped airplane, observed weather developing to the West and diverted to a nearby airport. The pilot stated that the approach for landing was normal, but after landing a wind gust forced the airplane to depart the runway and contact a runway distance remaining sign with the left wing. Post accident examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the wing spar was substantially damaged. No evidence of any preimpact malfunctions or failures of the airplane or engine were discovered. A review of automated weather observing system data for the diversion airport revealed that the peak wind speed never exceeded 6 knots and no wind gusts were recorded before, during, or after, the time of the accident. A review of the Pilot/Operator Aircraft Accident/Incident Report (NTSB Form 6120.1) revealed that the pilot had accrued only 10.5 hours of flight experience in the airplane.