THE PLT GOT AN EXTENDED WX FCST THE DAY PRIOR TO DEPARTURE WHICH INDICATED BKN TO OVC CLOUDS BASED AT ABOUT 8,000 FT FOR THE FIRST PART OF THE TRIP & SCT TO BKN CLOUDS FOR THE LAST HALF. MOUNTAIN AREAS WERE PREDICTED TO BE OBSCURED. PLT WAS ENCOURAGED TO CALL IN THE MORNING FOR A DETAILED UPDATE, HOWEVER, NO RECORD OF ANY CONTACT WAS FOUND. NO FLT PLAN WAS FILED & THE ACFT DEPARTED AT ABOUT 0900 HRS ON AN UNKNOWN ROUTE FOR THE DESTINATION. THE ACFT WRECKAGE WAS LOCATED BY ELT SIGNAL & SEARCH ACFT AT 6,200 FT MSL ON A MOUNTAIN SIDE AT THE END OF A BOX CANYON. A WITNESS AT THE BOTTOM OF THE CANYON (ABOUT 4,000 FT MSL) REPORTED VISIBILITY 100 FT IN FOG & RAIN AT THE TIME OF THE ACCIDENT. WX CONDITIONS ALONG THE GENERAL ROUTE OF FLT WERE WORSE THAN THE FCST PLT RECEIVED THE DAY PRIOR, WITH CEILINGS IN 3,000 FT RANGE & TOPS TO 22,000. DESTINATION APRT WAS ONE-QUARTER MILE IN SNOW AT THE TIME OF THE ACCIDENT. THE WX CONDITIONS ON THE DAY OF THE ACCIDENT WERE ACCURATELY REPORTED IN WX INFO AVAILABLE TO THE PLT