A SINGLE PLT WHILE FLYING VFR; ENCOUNTERS A TSTM AT NIGHT AND INADVERTENTLY ENTERS IFR CONDITIONS WHILE TRYING TO AVOID THE TSTM.

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I FILED A VFR FLT PLAN FROM ADG (ADRIAN; MI) DIRECT TO OWB (OWENSBORO; KY). I OBTAINED WX INFO FROM A COMPUTER BASED SYS WHICH PROVIDED ME WITH ALL THE FOLLOWING WX RPTS FOR MY RTE: METARS; TAF; AF; NOTAMS; AIRMETS; SIGMETS AND CONVECTIVE SIGMETS. I ALSO CALLED FSS FOR A STANDARD WX BRIEFING. BASED ON THIS INFO IT APPEARED AS THOUGH I WOULD BE ABLE TO MAKE A VFR FLT. MY ESTIMATED TIME EN RTE WAS 2 HRS; 45 MINS. I DEPARTED ADG AND I OBTAINED WX RPTS SEVERAL TIMES DURING MY FLT FROM ATIS; ASOS AND FROM CTR. ABOUT 50 MI N OF EVANSVILLE; IN; I ASKED LOUISVILLE RADAR IF THEY HAD A WX RPT FOR EVANSVILLE; IN. EVANSVILLE IS THE CLOSEST LOCATION (ABOUT 40 NM) TO MY DEST ARPT; OWB. AS INDICATED IN THE PREFLT RPTS; EVANSVILLE'S AUTOMATIC RPT SAID VARIABLE WINDS AT 3 KTS AND THE CEILING WAS BROKEN AT 6000 FT. I ASSUMED THAT OWENSBORO (ONLY 40 MI AWAY) WOULD ALSO BE VFR. WHEN I GOT OVER OWENSBORO THE TSTM SEEMED TO COME OUT OF NOWHERE. SINCE IT WAS AT NIGHT I DIDN'T SEE ANY SIGNS OF A STORM. ALL OF A SUDDEN THERE WAS THUNDER AND RAIN AND I SAW SOME DISTANT LIGHTNING. I IMMEDIATELY MADE A 180 DEG TURN AND STARTED TO CLB. AT THIS POINT I WENT INTO CLOUDS AND HAD TO FLY ON INSTS. AT FIRST I WAS IN AN UNUSUAL ATTITUDE BUT WAS ABLE TO GET WINGS LEVEL AND MAINTAIN A 500 FPM CLB. I MADE SURE THE PLANE WAS LEVEL AND CLBING. WHEN I REACHED ABOUT 5000 FT I TURNED ON MY AUTOPLT HEADING AND ALT HOLD. I CALLED ZID AND ASKED FOR HELP TO FIND THE NEAREST ARPT IN VFR CONDITIONS. THEY IMMEDIATELY STARTED GIVING ME VECTORS TO HUNTINGBURG; IN. I LANDED SAFELY AND CALLED LOUISVILLE FSS AND ASKED THEM TO CALL ZID TO LET THEM KNOW I LANDED SAFELY. LOUISVILLE FSS CLOSED MY VFR FLT PLAN. MY TRIP TO ADG WAS FOR IFR TRAINING. I CREDIT THE IFR TRAINING I RECEIVED AND THE HELP I GOT FROM ZID FOR SAVING MY LIFE.