Prior to departure, the pilot received two weather briefings which contained an advisory of a current AIRMET and SIGMET. The SIGMET forecast occasionally severe turbulence below 15,000 feet, with low level wind shear potential due to a strong low level westerly air flow. The AIRMET advised that mountain obscurement and moderate turbulence were possible throughout the area below 15,000 feet. In addition, a pilot report was passed along to the pilot from a Boeing 737 which had encountered moderate turbulence in the vicinity of the accident site. The pilot filed an IFR flight plan and departed along a route over mountainous terrain at an assigned altitude of 8,000 feet. The flight encountered a strong down draft, which the pilot said he could not counter despite applying full power and maintaining best angle of climb airspeed. According to the pilot, the down draft at times showed a descent rate of 1,500 feet per minute on the VSI. The flight entered the clouds at 6,000 feet, and, shortly thereafter, collided with trees and mountainous terrain at about the 5,500 foot level.