THE ACFT DEPARTED WINSLOW, AZ ON A FLT TO LONG BEACH, CA WITH AN EN ROUTE STOP SCHEDULED FOR PRESCOT, AZ. BEFORE DEPARTING WINSLOW, THE PLT WAS WARNED BY ANOTHER PLT THAT HE HAD ENCOUNTERED EXTREME TURBULENCE IN THE VICINITY OF MORMON MTN. THE PLT AND PAX'S REMAINED OVERNIGHT AT WINSLOW. THE NEXT MORNING THE PLT WAS ADVISED BY THE OTHER PLT THAT THE WX CONDS AT MORMON & MINGUS MTNS WERE STILL HAZARDOUS, HE RECOMMENDED THAT THEY FLY A SOUTHERLY ROUTE TOWARD PHOENIX, AZ TO CIRCUMNAVIGATE THE HIGH TERRAIN. THE PLT TOOK OFF & CONTD WESTBOUND. HE REPORTED THAT AS THEY PASSED OVER MORMON LAKE, THE PLANE BEGAN TO LOSE ALTITUDE. REPORTEDLY, HE STARTED TO CIRCLE OVER THE LAKE TO GAIN ALTITUDE, BUT ENCOUNTERED A DOWNDRAFT AND THE ACFT COLLIDED WITH TREES & CRASHED (7600' MSL). DURING AN EXAM OF THE WRECKAGE, NO PREIMPACT, MECHANICAL DISCREPANCIES WERE FOUND THAT WOULD HAVE CONTRIBUTED TO THE ACCIDENT. A WEIGHT & BALANCE COMPUTATION REVEALED THE GROSS WEIGHT WAS APRX 2474 LBS. THE MAX AUTHORIZE GROSS WEIGHT WAS 2300 LBS.