THE PLT RCVD A FULL WX BRIEFING AT 0540 CDT & A WX UPDATE AT 0737 WHEN HE FILED AN IFR FLT PLAN. THE FLT WAS WITHOUT INCIDENT WITH A FINAL CRUISE ALT OF 16,000' UNTIL 1055:43, WHEN THE PLT REQD A 'LEFT TURN FOR DEVIATION.' ABOUT 3 MIN LATER, THE CTLR NOTICED AN ALT ALERT WHEN THE ACFT DEVIATED BLO THE ASSIGNED ALT. THE CTLR TRIED TO CONTACT THE PLT, BUTLOST RADIO & RADAR CONTACT. SHORTLY THEREAFTER, WITNESSES SAW THE ACFT COME OUT OF CLDS IN A STEEP RGT WING LOW, NOSE DOWN ATTITUDE. IT CRASHED WITH A HI RATE OF SPD & DISINTEGRATED DRG IMPACT; PARTS OF THE WRECKAGE WENT INTO A NRBY LAKE.NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND. AN INV REVEALED THERE WAS A LINE OF TSTMS MVG THRU THE AREA AT 35 KTS WITH TOPS TO 34,000'. THE PLT RCVD A DETAILED BRIEFING OF TSTMS AT 0540, BUT THE UPDATE BRIEFING AT 0737 DIDN'T INCLUDE PERTINENT INFO IN SEVERE TSTM WATCH #516 (ISSUED AT 0611) OR CONVECTIVE SIGMET 18C. PLT DIDN'T ASK FOR UPDATE OF WX HE WAS APCHG, BUT THE ACFT WAS RADAR EQUIPPED. THE 1102 AUSTIN WX WAS IN PART: TSTMS, WIND 010 DEG AT 18 GUSTING 32 KTS.