While en route to the destination airport, the pilot made a stop to fuel the helicopter and check the weather. According to the pilot, no precipitation was predicted for his route of flight, the winds were expected from the northwest at 15 knots, and a cloud ceiling of 2,500 feet broken was forecasted. He stated that no specific windshear or turbulence was forecasted. The pilot continued the flight and noted that the wind became stronger over the mountains. He decided he would land the helicopter after he passed over the second ridge; however, as he continued, "strong windshear" lifted the helicopter and then "dropped it to the ground." The pilot reported no pre-impact mechanical anomalies with the helicopter. He also reported approximately 600 hours of total flight experience, all of which were in make and model of the accident helicopter. Examination of recorded weather information revealed AIRMET TANGO for turbulence was in effect for the pilot's route of flight.