PA34 PLT ENCOUNTERED TSTMS AND LANDED AT THE NEAREST SUITABLE ARPT.

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WHILE ON AN IFR FLT PLAN FROM HOUSTON TO CHICAGO; I ENCOUNTERED MODERATE TURB; DIFFICULTY IN MAINTAINING ALT AND SAW; DURING THE OCCASIONAL BREAKS IN THE CLOUDS; LIGHTNING AT A DISTANCE TO MY R. I CONTACTED THE CTR AND REQUESTED A 180 DEG TURN TO FURTHER MYSELF FROM THE LIGHTNING AND TURB. IT HAD BEEN LIGHTER 5-10 MI BEHIND MY POS. I RECEIVED CLRNC TO TURN; RPTED DOING SO AND WAS STILL IN MODERATE TURB. I REQUESTED VECTORS TO NEAREST ARPT; GOT CARBONDALE TWR; WAS CLRED FOR APCH (ILS) TO RWY 18L. I TOLD THE TWR I DID NOT HAVE THE APCH PLATES FOR CARBONDALE. WHILE HEADING TO CARBONDALE; I SAW THE FIELD UNDERNEATH MY POS; ALERTED TWR AND PROCEEDED WITH AN APCH IN AND OUT OF IMC. THE FIELD WAS IN FRONT OF ME ON TURN TO FINAL; WHICH WAS MADE AT 700 FT AGL. I LANDED WITHOUT INCIDENT. I BELIEVE MY WX PLANNING WAS FINE; WITH THE ORIGINAL APPROVED COURSE KEEPING ME MORE THAN 20 MI TO THE W OF THE STORMS THAT WERE SHOWN ON RADAR. I WAS REROUTED 2 TIMES; THE SECOND TO THE E; WHICH PUT ME IN A CLOSER PROX TO THE STORMS. MY DECISION TO LAND AT THE NEAREST WAS BASED ON THE CONTINUING TURB AND STORM PROX. MY FAILURE TO HAVE THE NEEDED APCH PLATES WAS THE ROOT OF MY PROB; AS WAS NOT TAKING INTO CONSIDERATION AN EVENT WARRANTING AN UNSCHEDULED LNDG SHORT OF MY DEST. MY LACK OF PREPAREDNESS COULD HAVE BEEN PREVENTED BY PURCHASING MY ENTIRE RTE'S PLATES; WHICH WERE FOR 4 STATES; WHICH CAN BE EXPENSIVE; BUT NOT SO AT THE COST OF SAFETY. I BELIEVE MY JUDGEMENT IN TURNING AROUND WAS SOUND; AS WAS MY CTL OF THE PLANE. THIS SIT COULD HAVE; AND WILL BE; PREVENTED BY THE ACQUISITION OF ALL NECESSARY PLATES.