PA32 PLT ENCOUNTERED IMC WHILE ON A VFR FLT PLAN.

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DURING THE FIRST LEG OF THE FLT TO UMP; I CALLED FLT WATCH TO GET THE CURRENT WX FOR GYY. THEY EXPLAINED THAT VFR CONDITIONS WERE FORECAST AS WELL AS PRESENT AT GYY AND THAT HE SAW NO PROBS FOR MY RTE FROM IND TO GYY. AT XA10Z I LANDED AT UMP; WENT INTO THE FBO AND CALLED FSS AND WAS AGAIN ASSURED THAT CONDITIONS WERE CLR AND VFR; WITH NO PIREPS TO THE CONTRARY. XC20Z: DEPARTED UMP; CLBED TO 6500 FT IN CLR SKIES N OF INDIANAPOLIS AREA. REQUESTED FLT FOLLOWING AND RECEIVED. XC50Z: BEGAN MONITORING ATIS (IN GYY; AND THE RPT WAS 1600 FT SCATTERED; 2400 FT BROKEN). XD15Z: 12 MI FROM GYY; I RPTED TO CHICAGO THAT SOME CLOUDS WERE BUILDING BELOW AND THAT I WAS DSNDING TO REMAIN VFR. RPTED TO CHICAGO AT 2500 FT; BELOW CLOUDS; VFR. XD20Z: CHICAGO CANCELED FLT FOLLOWING; APPROVED FREQ CHANGE TO GYY TWR. ATIS AT THAT TIME STILL RPTED 1600 FT SCATTERED; 2400 FT BROKEN...'PRESSURE RISING RAPIDLY.' XD21Z. AS SOON AS I SWITCHED FREQ TO GYY; I WAS ENVELOPED IN FOG. I CALLED GYY AND ADVISED OF MY POS; 5 MI S; NO VISIBILITY AT 2500 FT AND VFR. I ASKED IF THEY COULD CONFIRM SAFE MINIMUMS BELOW MY RTE AND THEY REFERRED ME TO ORD APCH. I IMMEDIATELY CALLED APCH; ADVISED I WAS VFR AND HAD ENCOUNTERED FOG; AND HAD NO VISIBILITY. I REQUESTED VECTORS AND SAFE ALT TO GYY. HE ASKED IF I WAS REQUESTING AN IFR CLRNC; AND I TOLD HIM I WAS NOT; THAT I WAS VFR AND NEEDED HELP OUT OF THE FOG BY EITHER DSNDING TO A SAFE ALT BELOW THE FOG OR ELSE CLBING AND VECTORING AWAY FROM THE FOG. HE ASKED ME IF I COULD HOLD PRESENT ALT (2500 FT) AND I RESPONDED AFFIRMATIVE; AND HE TOLD ME TO MAINTAIN PRESENT HDG AND ALT. XD23Z: APCH INSTRUCTED ME TO 'DSND AND MAINTAIN 2100 FT; ARPT IS 3 MI AHEAD.' XD25Z: UPON REACHING 2100 FT; I FLEW OUT OF THE FOG; ADVISED APCH THAT I WAS AGAIN VFR AND HAD THE ARPT IN SIGHT. APCH APPROVED FREQ CHANGE. I CONTACTED GYY TWR; RPTED L BASE; AND REQUESTED TO LAND RWY 30. I WAS CLRED TO LAND. XD30Z: AFTER LNDG; I HAD DIALOGUE WITH GYY TWR AND ASKED WHERE THE FOG CAME FROM AS IT WAS NOT ON ATIS. SHE REMARKED THAT IT HAD JUST ROLLED IN. I TOLD HER THAT THE INFO I HAD RECEIVED FROM ATIS AND THAT THE WX I ENCOUNTERED WAS NOTHING LIKE WHAT WAS RPTED. SHE AGAIN CONFIRMED THAT THE FOG HAD JUST ROLLED IN. SHE ALSO GAVE ME A TELEPHONE NUMBER TO CALL BECAUSE CHICAGO APCH HAD REQUESTED THAT I CALL AFTER I LANDED. XD50Z: I CALLED CHICAGO APCH AND WAS ASKED ABOUT THE CIRCUMSTANCES THAT LED UP TO MY BEING IN THE FOG. I RPTED WHAT I HAVE WRITTEN ABOVE. REFLECTION: I DO NOT KNOW WHAT I COULD HAVE DONE DIFFERENTLY TO AVOID FOG THAT I DID NOT KNOW WAS THERE. I HAD BEEN MONITORING ATIS FROM 50 MI OUT; THE CONDITIONS RPTED WERE VFR AND THERE WAS A SPREAD BTWN DEWPOINT AND TEMP. ONCE IN THE FOG; MY FIRST REACTION WAS TO CLB AND REVERSE COURSE; BUT I KNEW THAT I HAD CLOUDS ABOVE AT 3000 FT AND THAT I WAS VERY NEAR (5 MI) CLASS B AIRSPACE. I DID NOT WANT TO POP OUT OF A CLOUD AND BE IN THE LINE OF FLT OF ANOTHER ACFT DEPARTING OR ARRIVING GARY; MIDWAY; OR O'HARE. I FELT FAIRLY CONFIDENT THAT LOWER WAS BETTER; BECAUSE OCCASIONALLY I WOULD GET A BRIEF GLIMPSE OF GND LIGHTS. I HAD NO INTENTION OF LNDG IN IMC; AND SIMPLY WANTED TO GET TO A SAFE MINIMUM WHERE VISUAL MIGHT BE RE-ESTABLISHED. IF I HAD GOTTEN TO THAT SAFE MINIMUM AND STILL NO VISIBILITY; I WOULD HAVE ASKED TO BE VECTORED OUT OF THE AREA.