THE ACFT DISAPPEARED FROM RADAR DURING DESCENT FROM 4000 FT MSL TO 2000 FT MSL. INVESTIGATION REVEALED SIGMET BRAVO 3 IN EFFECT IN THE AREA OF THE ACCIDENT AT THE TIME OF THE CRASH. THE TRANSCRIPT OF RADIO COMMUNICATIONS WITH THE FLT SHOWS NO IN-FLT WEATHER BRIEFINGS. THE SIGMET BRAVO 3 WARNED THAT THERE WAS MODERATE TO SEVERE TURBULENCE IN THE AREA DUE TO STRONG LOW LEVEL WINDS; A LOW LEVEL WINDSHEAR POTENTIAL EXISTED BELOW 4000 FT MSL. THE PLT HAD BEEN CLEARED TO DESCEND TO 2000 FT MSL AT 2312. THE PLT ACKNOWLEDGED THE TRANSMISSION. AT 2312+41 THE PLT WAS ADVISED TO CHANGE RADIOFREQUENCIES AND THE PLT ACKNOWLEDGED. AT 2313 THE APPROACH CONTROLLER LOST THE FLTS RADAR CONTACT AND CALLED THE FLT. THERE WERE NO FURTHER RADIO CONTACT WITH THE FLT. AT 2318 RESIDENTS IN THE AREA OF BROOKSVILLE ADVISED THE SHERIFF THAT A SMALL ACFT HAD CRASHED. THE PA-32-301 PLTS HANDBOOK SHOWS A VA SPEED OF 110 KTS AS THE LIMITATION FOR TURBULENT AIR PENETRATION. THE ATC RADAR ITAP READOUT SHOWS THE ACFT SPEED BEFORE RADAD CONTACT WAS LOST AT ABOUT 168 KTS.