A COMPANY PLT RECEIVED TWO WX BRIEFINGS FROM THE SAN ANTONIO FSS, ONE WAS AT ABOUT 1700 THE DAY BEFORE THE ACCIDENT & THE OTHER ON 11/2. AFTER EACH WX BRIEFING THE COMPANY PLT BRIEFED THE PLT OF THE ACCIDENT ACFT. THE ACFT DEPARTED AT ABOUT 0835. AT 0953 THE PLT CONTACTED FLIGHT WATCH & WAS ADVISED OF A CONVECTIVE SIGMET OF EMBEDDED THUNDERSTORMS. AT 1001 THE PLT REQUESTED & WAS CLEARED TO FL190. THIS WAS THE LAST RADIO CONTACT WITH THE ACFT. IN FLIGHT SEPARATION OF THE WINGS, VERTICAL & HORIZONTAL STABILIZERS OCCURRED. THE ACFT PENETRATED A DENSE AREA OF THUNDERSTORMS WITH ACTIVITY UP TO INTENSE (LEVEL 5) & PROBABLY ENCOUNTERED SEVERE TURBULENCE. IN ADDITION THERE WOULD PROBABLY BEEN MODERATE TO SEVERE ICING IN RAIN SHAFTS WITHIN THE THUNDERSTORMS AT THE ACFT'S FLIGHT ALTITUDE. THE 0817 WX BRIEFING WAS INADEQUATE IN THAT THE AREA FORECAST WHICH WARNED OF THUNDERSTORM ACTIVITY ENROUTE WAS NOT UTILIZED.