A BEECH 58P DESCENDED AND COLLIDED WITH MOUNTAINOUS TERRAIN WHILE IN INSTRUMENT METEOROLOGICAL CONDITIONS. THE PILOT WAS ISSUED A 20 MINUTE WEATHER BRIEFING FROM A FLIGHT SERVICE STATION. THE WEATHER BRIEFING FORECASTED IFR CEILINGS ALONG ENTIRE FLIGHT, OCCASIONAL MODERATE TURBULENCE OVER THE ENTIRE ROUTE, A SIGMET FOR SEVERE TURBULENCE AND LOW LEVEL WIND SHEAR, STRONG UP AND DOWN DRAFTS IN THE VICINITY OF MOUNTAINS AND PASSES, AND OCCASIONAL MODERATE MIXED ICING FROM THE FREEZING LEVEL TO FLIGHT LEVEL 200 OVER ENTIRE ROUTE OF FLIGHT. AFTER RECEIVING THE BRIEFING, THE PILOT DECIDED TO INITIATE THE FLIGHT AND FILED AN IFR FLIGHT PLAN. THE AIRPLANE WAS OBSERVED ON RADAR CLIMBING THROUGH 11,400 FEET MSL AT A POSITION THAT OVERLIES AN 8,000 FOOT MSL MOUNTAIN RANGE. RADIO AND RADAR CONTACT WAS THEN LOST. NO MECHANICAL PROBLEMS WERE FOUND.