WHILE EN ROUTE, THE PLT WAS CLEARED TO DEVIATE 20 DEG LEFT OF COURSE TO AVOID THUNDERSTORM (TSTM) ACTIVITY THAT ARTCC RADAR SHOWED WAS RGT OF COURSE. THE PLT RPRTD TURBC, BUT DEMURRED DESCENDING TO A LOWER ALT DESPITE A PIREP OF CLEAR WX BELOW. HE THEN RPRTD 'IN A CELL' WITH TURBC. RADAR & RADIO CONTACT WITH THE ACFT WERE LOST. ANOTHER PLT, WHO FLEW APRX 15 MI FURTHER SOUTH, RPRTD NO WX, BUT SAID THERE WAS EXTREME WX TO THE NORTH WITH NEAR CONSTANT LIGHTNING. SUBSEQUENTLY,N340MV IMPACTED THE GROUND IN A STEEP NOSE-DOWN, LEFT WING LOW ATTITUDE, AFTER A 14,000 FT DSCNT. THE LANDING GEAR WAS EXTENDED WHEN THE PLANE CRASHED. AN EXAM OF THE WRECKAGE REVEALED NO PREIMPACT STRUCTURAL FAILURE OR ACFT COMPONENT MALFUNCTION. ALSO, THE WRECKAGE SHOWED NO EVIDENCE OF WEATHER DAMAGE FROM HAIL OR LIGHTNING. HOWEVER, WX INFO SHOWED THERE WAS HIGH ALT TSTM ACTIVITY WITH TURBC IN THE AREA AT THE TIME OF THE MISHAP.