PREFLT WX BRIEF INCLUDED ADVISORY FOR SCATTERED SHOWERS/THUNDERSTORMS ALONG ROUTE OF FLT. DEPARTED IN VFR CONDITIONS BUT ENCOUNTERED DETERIORATING WX ENROUTE, RECEIVED INFLT WX ADVISORY TO EXPECT LOW CLNGS AND REDUCED VISIBILITY AHEAD. PLT STATED HE 'WOULD KEEP AN EYE ON THE WEATHER. 'ABOUT 50 MINS LATER, PLT REQUESTED RADAR VECTORS TO ASSIST IN CIRCUMNAVIGATING RAIN SHOWERS. VECOTR GIVEN BUT ACFT TGT OBSERVED NOT FOLLOWING VECTOR. PLT NEXT REQUEDTED A HEADING TO 'GET HIM OUT OF WEATHER.' STATING HE WAS IN RAIN AND 'HAVING A HARD TIME.' TGT DISAPPEARED FROM RADAR SHORTLY THEREAFTER. INVESTIGATION REVEALED ACFT IMPACTED IN HEAVILY WOODED AREA IN NEAR-VERTICAL ATTITUDE AT HIGH RATE OF SPEED. WITNESSES HEARD SOUNDS OF HIGH ENGINE POWER PRIOR TO IMPACT; CITED WX CONDS AS DRIVING RAIN, GUSTY WINDS AND REDUCED VISIBILITY IN RAIN AND OVERCAST-INDUCED DARKNESS.