BEFORE DEPARTING TUCSON, THE PLT WAS ADVISED OF 2 MARGINAL WX SYSTEMS ALONG HIS ROUTE. HE TOOK OFF AT 1038 MST. AT 1130,HE CONTACTED ATC & REPORTED AT 13,800'. FROM THEN UNTIL 1214, HIS ALT VARIED BETWEEN 10,000' & 16,000'. RADAR CONTACT WAS LOST AT 1228 WHILE HE WAS BELOW 10,000'. AT 1240, HE REPORTED AT 12,5000', THEN CLIMBED TO 14,500' TO REESTABLISH RADAR CONTACT WITH ATC. SHORTLY THEREAFTER, RADAR & RADIO CONTACT WERE LOST. SCHOOL CHILDREN SAW THE ACFT BEFORE IT CRASHED. ACCORDING TO THEM, IT WAS IN LEVEL FLT AT APRX 3000' AGL (7000' MSL), THEN PITCHED NOSE DOWN, BEGAN SPINNING & CRASHED. THEY DID NOT SEE ANYTHING SEPARATE FROM THE ACFT. AN EXAM REVEALED THE LEFT OUTBOARD WING PANEL & PART OF THE LEFT AILERON HAD SEPARATED IN FLT. THE OUTER PORTION OF THE LEFT HORIZONTAL STABILATOR HAD FOLDED UPWARD. THE WING PANELWAS FOUND 3.5 MI AWAY WITH EVIDENCE IT HAD FAILED UP & AFT. THERE WAS EVIDENCE OF MODERATE TO SEVERE TURBULENCE IN THE VICINITY OF MTNS. THE PLT HAD BEEN BRIEFED ON AREAS OF TURBULENCE, ICING, PRECPITAITON & DETERIORATING CEILINGS.