The commercial pilot/owner was using his high-wing, tailwheel-equipped airplane to participate in an Experimental Aircraft Association Young Eagles event at a non-towered airport. The airport was situated in high desert terrain of the west, at an elevation of about 6,000 feet. About 1300, when the pilot had already completed 8 such flights, and the other airplanes and pilots had also been actively flying, the pilot boarded three young passengers. According to the pilot, there were no clouds in the immediate vicinity, calm winds, and an automated weather broadcast for an airport about 17 miles to the northeast reported similar conditions. He started the airplane and taxied out for departure on runway 8. The airport windsock was "motionless" as he began the takeoff roll. A few hundred feet down the runway, while the airplane was still below liftoff speed, the left wing rose up, and the airplane weather-vaned nose left about 60 degrees. The airplane exited the left (north) side of the runway, and then rotated about 120 degrees nose right before it came to rest. The right wing and fuselage sustained substantial damage, and the pilot and passengers were uninjured. About the same time, a flight instructor in an airplane on the downwind leg for runway 8 encountered "turbulence," which resulted in a 500-foot altitude loss before she could stop the descent by "climbing" at her best angle-of climb speed. Numerous persons at the airport for the event reported that a sudden, strong, and unexpected "wind gust" swept across the airport, and disturbed many objects on the ground, including some airplanes. They reported that it traveled approximately northwest to southeast, which meant that it approached the accident airplane from its left rear quarter. One pilot on the ground reported that there was a "storm brewing" near the mountains, about 6 miles northwest of the airport, at the time of the accident. According to Federal Aviation Administration Advisory Circular 00-24 (Thunderstorms), "Gust fronts often move far ahead (up to 15 miles) of associated precipitation. The gust front causes a rapid and sometimes drastic change in surface wind ahead of an approaching storm."