THE PLT ENCOUNTERED FORECAST TSTMS EN ROUTE. AT 1430 CDT, THE PLT HAD RPTD 2400 FT MSL, CLIMBING TO HIS CRUISE ALTITUDE AT 8000 FT. AT 1440, THE PLT REQUESTED, AND DEVIATION AROUND WEATHER WAS APPROVED. THE PLT DID NOT REPORT FURTHER DIFFICULTY, BUT LOSS OF CONTROL AND AN IN-FLIGHT BREAKUP OF THE ACFT OCCURRED SHORTLY THEREAFTER. WEATHER AND RADAR DATA INDICATE THAT THE AIRPLANE BROKE UP NEAR THE EDGE OF A LEVEL TWO THUNDERSTORM. HEAVY RAIN AND LIGHTNING WERE REPORTED BY LOCAL RESIDENTS. THE OUTBOARD END OF THE LEFT WING AND THE STABILATOR HAD SEPARATED IN-FLIGHT. THE LEFT WING FAILED IN POSITIVE OVERLOAD. ALTHOUGH THE RIGHT WING HAD NOT SEPARATED, IT ALSO REVEALED DAMAGE FM HIGH POSITIVE LOADING. THE STABILATOR FRACTURES INDICATED TORSIONAL OVERLOAD. BOTH PLTS HELD PVT PILOT CERTIFICATES AND WERE INSTRUMENT-RATED.