THE PLT STATED THAT HE WAS OPERATING VFR OUT OF FT. LAUDERDALE, FLORIDA, WHEN HE ENCOUNTERED IMC WEATHER CLOSE TO VALDOSTA, GEORGIA. AT THIS POINT, THE PLT FURTHER STATED, HE AIR-FILED AN IFR FLIGHT PLAN BUT ELECTED NOT TO LAND AND REFUEL. ACCORDING TO ATC INFORMATION, THE AIRCRAFT DISAPPEARED FROM RADAR 44NM SHORT OF STUTTGART, ARKANSAS, HIS DESTINATION AIRPORT. REPORTS BY THE NATIONAL WEATHER SERVICE AT THE TIME OF THE ACCIDENT INDICATED THAT A LOW PRESSURE TROUGH WITH LOW IFR CONDITIONS EXISTED FROM THE MIDWESTERN PLAIN STATES TO THE FLORIDA PANHANDLE AND ON THROUGH THE OHIO VALLEY REGION. POST-ACCIDENT CALCULATIONS REVEALED THAT THE AIRCRAFT HAD BEEN OPERATED OVER THE MAXIMUM ALLOWABLE GROSS WEIGHT DURING THE INITIAL PORTION OF THE FLIGHT AND THAT FUEL RESERVES FOR IFR OPERATIONS WERE NONEXISTENT. NO USABLE FUEL WAS FOUND IN THE AIRCRAFT'S TANKS, AND NO MECHANICAL MALFUNCTIONS COULD BE DETECTED DURING AN OPERATIONAL CHECK OF BOTH POWERPLANTS.