ON DECEMBER 6, 1992 AT 1054 HOURS, THE PILOT TELEPHONED RIVERSIDE, CALIFORNIA FAA FLIGHT SERVICE STATION (FSS) AND REQUESTED A WEATHER BRIEFING FOR HIS INTENDED FLIGHT FROM HIS DEPARTURE AIRFIELD TO HIS DESTINATION. THE FSS BRIEFER TOLD THE PILOT THAT VISUAL FLIGHT RULES FLIGHTS WERE NOT RECOMMENDED FROM HIS INTENDED DEPARTURE LOCATION TO HIS DESTINATION. THE PILOT ASKED IF THE WEATHER WAS GOING TO IMPROVE OR GET WORSE. THE FSS BRIEFER SAID THE WEATHER WOULD CONTINUALLY GET WORSE THROUGHOUT THE DAY. THE PILOT DECIDED TO GO AHEAD WITH HIS PLANNED FLIGHT. THE PILOT, WITH HIS 3 PASSENGERS ABOARD, WAS CLEARED FOR TAKEOFF AT 1203 HOURS. THE PILOT CONTACTED FAA DEPARTURE CONTROL AND RECEIVED ADVISORIES UNTIL RADAR SERVICE WAS TERMINATED AT 1214 HOURS. NO FURTHER TRANSMISSIONS WERE RECORDED FROM THE PILOT. WHEN THE AIRPLANE AND ITS OCCUPANTS DID NOT ARRIVE AT ITS DESTINATION FAMILY MEMBERS NOTIFIED LOCAL AUTHORITIES. A SEARCH WAS UNSUCCESSFUL. THE DAMAGE AND INJURY CLASSIFICATION ARE PRESUMED.