DRG A PREFLT WX BRIEFING, THE PLT WAS ADZD THE FORECAST FOR OAKLAND (19 MI NNW OF HIS DESTN) WAS FOR A 3000' BROKEN CEILING WITH OCCASIONAL 3000' SCATTERED, 5000' BROKEN & A CHANCE OF LIGHT RAIN SHOWERS. HE FILED A VFR FLT PLAN & DEPD. AT 1347 PDT, THE PLT OPENED HIS FLT PLAN & PROCEEDED WITHOUT FURTHER COMMUNICATION UNTIL HE WAS ABOUT 45 MI SE OF HIS DESTN. AT 1615 PDT, HE CONTACTED BAY TRACON & ASKED FOR & RCVD TRAFFIC ADZYS. ABOUT 12 TO 14 MIN LATER, RADIO & RADAR CONTACT WERE LOST. THE ACFT CRASHED ABOUT 22 MI FROM THE DESTN WHEN IT IMPACTED JUST BELOW THE TOP OF A MTN PEAK AT AN ELEV OF APRX 3350'. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FOUND. PSNL IN THE VCNTY OF THE ACDNT SITE SAID THERE WAS FOG WITH A VISIBILITY OF 50' TO 100'. THE CLOUD BASES IN THAT AREA WERE ESTIMATED TO BE ABOUT 800' BELOW THE ACDNT SITE. SVRL PEOPLE, WHO KNEW THE NON-INSTRUMENT RATED PLT, SAID HE OFTEN FLEW IN CLOUDS.