The pilot's decision to depart into known adverse weather conditions over mountainous terrain, which required operation near the limits of the airplane's performance capability and which resulted in a loss of airplane control and subsequent ground impact. Contributing to the accident was an improper clearance issued by the air traffic controller and the pilot's acceptance of that clearance. Also contributing to the accident was the extended loss of radar data from the Rock Springs Air Route Surveillance Radar, which caused loss of radar contact and consequent loss of minimum safe altitude warning protection for the flight.