The pilot reported he taxied to the end of the runway for a full length departure and during the initial climb the wind shifted from a headwind to a tailwind. The pilot initially reported that during the initial climb, there were no aerodynamic stall indications, but during a follow-up interview he reported that "the airplane definitely stalled." Subsequently, the airplane impacted terrain in a vacant lot north of the runway and nosed over, resulting in substantial damage to the wings, fuselage, and empennage. The pilot reported there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. A review of recorded data from the automated weather observation station located about 4 miles to the northeast, revealed that, about 3 minutes before the accident the wind was 260 degrees true at 13 knots, with gust 17 knots, and wind variable direction from 210 to 280. The airplane departed runway 4. According to a witness on the ground, who was an airline transport pilot with about 22,000 hours of flight time, the airplane taxied onto runway 4 at the midfield point and applied takeoff power. The witness further reported that the he heard the power reduced and then re-applied during the takeoff roll. Subsequently, during the initial climb he witnessed the left wing stall, impact the ground, and nose over after the nose wheel collapsed.