PA28 PLT PUSHES ON DESPITE DETERIORATING CONDITIONS. ULTIMATELY ENDS UP VFR IN IMC.

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FLT ENRTE TO LVN. FIRST 35-40 MINS WX 3000-4000 FT. CEILINGS +6 VISIBILITY LIGHT RAIN. ABOUT 1/2 WAY THROUGH FLT; CHKED WX WITH FSS PRINCETON; MN. RPTED AS LIGHT RAIN FOR REMAINDER OF FLT WITH CEILINGS VARIABLE 2000-4000 FT IN AND UNDER MINNEAPOLIS CLASS B AIRSPACE. THIS WAS DIFFERENT FROM ORIGINAL BRIEF AT XA00. CALLED ATC FOR FLT FOLLOWING AND REMAINED CLR OF CLASS B AIRSPACE AND VFR. RTE OF FLT REMAINED 20 MI W OF CLASS B IN CLASS E AIRSPACE. BTWN 1500-2000 FT WHILE MANEUVERING S AND THEN E TO DEST; CLOUDS WERE 2000 FT VARIABLE; LIGHT RAIN. VISIBILITY 1-3 MI. AT THIS POINT THERE WAS NO CLOSER ARPT THAN FCM; WHICH HAD GONE IFR ON LVN WHICH RPTED 2300 FT SCATTERED TO BROKEN. STAYED S OF FCM CLASS D AIRSPACE AND CONTINUED 10 MI TO LVN. UNABLE TO MAINTAIN VFR DURING APCH AND LNDG TO LVN. VISIBILITY DROPPED TO LESS THAN 1 MI AND UNABLE TO MAINTAIN ALT OF 500 FT BELOW CLOUDS OR 2000 FT SEPARATION FROM CLOUDS. IN REVIEWING CHAIN OF EVENTS AND CONTRIBUTING FACTORS I SHOULD HAVE DONE THE FOLLOWING: 1) CALLED FOR AN UPDATED BRIEFING JUST BEFORE DEP RATHER THAN RELY ON TAF BRIEF FROM THE MORNING; 2) SHOULD HAVE REALIZED THAT THE AWOS AND ASOS I LISTENED TO WERE SHOWING A TREND OF DETERIORATING WX AND EITHER FILED IN THE AIR FOR IFR CLRNC (NOT IFR CURRENT) OR; 3) REALIZED WHILE LISTENING TO CTR THAT ALL FLTS IN THE METRO AREA WERE NOW IFR; 4) RETURNED TO THE LAST POINT WHERE GOOD VFR WAS PRESENT AND LANDED. I HAD AT LEAST 60 MINS OF FUEL ON BOARD AT FINAL DEST SO FUEL WOULD NOT HAVE BEEN A FACTOR.