BEFORE TAKEOFF, THE PLT WAS ADZD THAT STRONG HEAD WINDS, MOD TO SVR TURBC, MTN OBSCUREMENT, MULTIPLE LAYERS OF ICING & DETERIORATING WX WERE FORECAST ALONG HIS ROUTE OF FLT. HE FILED AN IFR CLNC & TOOK OFF AT 1115 PST. WHILE EN ROUTE, HE REQD THE MEDFORD WX & WAS ADZD OF STANDING LENTICULAR CLOUDS. AT 1328 PST, HE CANCELED HIS IFR CLNC. LATER, AT 1535, HE OBTAINED ANOTHER IFR CLNC & WAS ASSIGNED AN ALT OF 11,000'. AT 1546 PST, THE PLT RPRTD HE WAS ENCTRG DOWNDRAFTS; 8 MIN LATER, HE SAID HIS ALT WAS 9300'. SHORTLY, THEREAFTER, RADAR & RADIO CTC WERE LOST AFTER THE ACFT DSCNDD THRU 8900'. SUBSEQUENTLY, THE ACFT CRASHED ON THE LEE SIDE OF A 9025' MTN AT APRX THE 6200' LVL. IT WAS FOUND IN A NEAR VERTICAL NOSE DOWN ATTITUDE WITH ITS BELLY AGAINST AN IMPACT DAMAGED TREE. THE AFT FUSELAGE WAS BENT & TWISTED ABOUT 90 DEG, BUT THE EMPENNAGE WAS ESSENTIALLY UNDAMAGED. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS EVIDENT. APRX 7 MI NORTH AT WEED ARPT (ELEV 2938'), THE WIND GUSTS WERE ESTD AT 70 TO 75 MPH.