C172 PILOT REPORTS ENTERING IMC ON VFR FLIGHT IN ORDER TO AVOID TERRAIN. IFR CLEARANCE IS OBTAINED AFTER THE FACT.

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PLT WANTED TO GET TO IMPORTANT FAMILY GATHERING. (GET-THERE-IT IS) DIRECT AIRWAY RTE HAD WX THAT WAS IMC WITH LOW CEILINGS. PLT TENDS TO AVOID IMC CONDITIONS WHEN POSSIBLE AND CHOOSE TO TRY AND CIRCUMNAV IMC WX BY FIRST FLYING VFR WBOUND FOR A PLANNED FUEL STOP. THERE WAS AN AIRMET FOR MOUNTAIN OBSCURATION FOR THIS RTE OF FLT. FLT PROCEEDED WELL AT A LEVEL OF 4500 FT UNTIL ACFT APCHED THE HIGHEST MOUNTAINS ON THE RTE (4900 FT). AT THIS POINT CEILINGS STARTED TO CONVERGE TOWARD TERRAIN AND VISIBILITY DECREASED. VISIBILITY WAS APPROX 5 MI BUT PLT COULD NOT STAY 500 FT BELOW CLOUDS. PLT CONCLUDED THE SITUATION WAS BECOMING QUITE HAZARDOUS AND COMMENCED A CLBING 180 DEG TURN. PLT INTENDED TO CLB INTO CLOUDS WITHOUT A CLRNC. CONCERNED ABOUT HAZARD OF COLLIDING WITH ANOTHER ACFT IN CLOUDS PLT THEREFORE SET XPONDER TO 7700 TO SIGNAL A HAZARD TO ATC. PLT IMMEDIATELY CALLED ON 121.5 FREQ ASKING FOR A FREQ TO CONTACT ATC IN ORDER TO OBTAIN AN IMMEDIATE IFR CLRNC. ATC PROMPTLY GAVE PLT IFR CLRNC AND ALT ASSIGNMENT OF 6500 FT THEN 7000 FT. FLT WAS THEN STABILIZED UNDER IMC CONDITIONS. PLT THEN NOTICED THAT AT NEW ALT GND SPD WAS ONLY 75 KTS. PLT BECAME NERVOUS THAT HE MIGHT NOT HAVE ENOUGH FUEL AND REQUIRED RESERVES FOR DEST. (IN ACTUALITY ACFT DID HAVE ENOUGH FUEL AND RESERVE BUT CALCULATION WAS BORDERLINE.) PLT ASKED FOR DEV TO NEARBY ARPT. ATC PROVIDED VECTOR. BAL OF FLT WAS UNEVENTFUL. PLT DSNDED UNDER RADAR VECTORS FOR VISUAL APCH. CAUSE OF INCIDENT: PLT EXPRESSED BAD JUDGMENT BY ASSUMING MOUNTAIN OBSCURATION MENTIONED IN AIRMET WOULD CLR OR BE NAVIGABLE. PLT PUT OTHER USERS OF AIRSPACE SYS AT RISK (EVEN IF JUST FOR A COUPLE OF MINS) BY CLBING INTO THE CLOUDS WITHOUT FIRST OBTAINING A CLRNC TO DO SO. PLT NEEDS TO RESTUDY WX FORECASTS FOR ENRTE CEILINGS VISIBILITY AND ALSO UNDERSTAND THEIR LIMITATIONS IN MOUNTAINOUS REGIONS. PLT SHOULD ALSO REFRAIN FROM FLYING VFR IN MOUNTAINOUS AREAS WHEN AIRMETS FOR MOUNTAIN OBSCURATION ARE IN EFFECT.