TB21 TRINIDAD PLT ENCOUNTERS SEVERE TURB; HAS SLIGHT INJURY; HAS ALTDEV AND DIVERTS TO ANOTHER ARPT.

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I WAS ENRTE TO BAF WITH PLANS TO MOVE A REFUELING STOP AT NEW CASTLE; IN (UWL). THE WX BRIEFING HAD INDICATED A THREAT OF TSTMS AND AS I APCHED UWL THE SKY BECAME INCREASINGLY OVCST. I SAW SOME LIGHTNING AT MY 10 O'CLOCK POS BUT COULD NOT JUDGE HOW FAR AWAY IT WAS. IT WAS RAINING BUT I WAS VMC UNDER THE OVCST. THE CTLR BEGAN GIVING ME VECTORS FOR THE PLANNED GPS APCH. HE INSTRUCTED ME TO TURN ABOUT 30 DEGS L AND DSND TO 4000 FT. AS I WAS DSNDING; STILL VMC; I FELT A TREMENDOUS KICK IN THE SEAT AND THE NEXT THING I KNEW MY HEAD BANGED AGAINST THE ROOF OF THE AIRPLANE. THIS HAPPENED TWICE; DESPITE THE FACT THAT MY SEATBELT WAS FASTENED TIGHT (IT IS AN INERTIA SEATBELT). I WAS STUNNED FOR A MOMENT AND THE NEXT THING I REMEMBER WAS THE APCH CTLR CALLING ME ASKING FOR MY ALT. I LOOKED AND AT THAT TIME I WAS AT 3300 FT. I EXPLAINED TO THE CTLR THAT THE TURB WAS EXTREME AND HE OFFERED ME THE NEAREST ARPT WHICH WAS GEZ. IT WAS RAINING HEAVILY BUT I WAS ABLE TO MAKE AN UNEVENTFUL LNDG. I COMPLETED MY TRIP THE NEXT DAY AS THE WX STEADILY GOT WORSE WITH TSTMS AND HAIL. I BELIEVE THIS HAPPENED BECAUSE I GOT TOO CLOSE TO A CELL. I WAS VMC BUT I HAD SEEN LIGHTNING AND SINCE I COULD NOT ACCURATELY JUDGE HOW FAR AWAY IT WAS; I SHOULD NOT HAVE ACCEPTED ANY VECTOR IN THAT DIRECTION. I ALSO BELIEVE THAT EFFECTS OF TURB ARE ALSO UNDERAPPRECIATED IN GA. WE ALL LEARN 'FIRST; FLY THE ACFT' BUT IT CAN BE DIFFICULT TO DO SO WHEN ONE HAS BEEN STRUCK IN THE HEAD TWICE. I AM THANKFUL THAT THE ALERT CTLR REALIZED SOMETHING WAS WRONG AND CALLED MY ATTN TO IT.