BEFORE TAKEOFF, THE PLT RCVD A WX BRIEFING & WAS ADZD OF TSTMS & HVY RAIN ALONG THE ROUTE. THE BRIEFING INCLUDED A WARNING OF A SEVERE TSTM WATCH WITH HAIL, WIND GUSTS TO 65 KTS & A SIGMET (31C). THE PLT FILED AN IFR FLT PLAN & TOOK OFF AT 1148 CST. WHILE EN ROUTE AT 15,000 MSL, HE REQD & RCVD CLNC TO DIVERT 10 DEG LEFT TO AVOID 'SOME CLOUDS.' SOON THEREAFTER, THE ATC CTLR NOTICED THE ACFT WAS ABOUT 500' HI & REMINDED THE PLT TO MAINT 15,000'. THE PLT ACKNOWLEDGED, THEN THERE WAS NO FURTHER COMMUNICATION WITH THE ACFT. SUBSEQUENTLY, AN IN-FLT BREAKUP OF THE ACFT OCCURRED & THE PLANE CRASHED. THE LEFT OUTBOARD WING PANEL SEPD FM POSITIVE OVERLOAD FAILURE & WAS FOUND ABOUT 1 MI FM THE MAIN WRECKAGE. NO PREIMPACT FATIGUE WAS FND. A WITNESS, WHO SAW THE ACFT DSCNDG OUT OF THE CLOUDS, SAW A FUNNEL CLOUD IN THE AREA BEFORE THE ACDNT.