IMC IN VFR FLT. ERROR ADMITTED INFLT IN PROX OF CLOUDS TERRAIN.

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ENRTE WX THE SAME; WITH FORECAST CALLING FOR OCCASIONAL MVFR DUE TO RAIN AND CEILING BELOW 1000 FT. DROVE THIS RTE 2 HRS PRIOR TO DEP AND IT WAS VFR. BEFORE LEAVING; MADE AN ADDITIONAL WX CHK AND TALKED VIA TELEPHONE TO A PLT WHO JUST FLEW INTO APC. BOTH CHKS CONFIRMED WX SHOULD NOT BE A PROB. IT WAS COPLT'S LEG AND FLT WAS ROUTINE AND AS PLANNED AS WE PROCEEDED VFR AT 1200 FT MSL IN CONTACT WITH ATC FOR FLT FOLLOWING AND TA'S. APCHING CONCORD FROM THE NE; WE COULD SEE THE CEILING WAS LOWER AND SINCE I KNEW THE AREA WELL; I SUGGESTED THAT THE COPLT GAR THE WX TO THE E; OVER THE HILLS WHERE THE CEILING WAS HIGHER (2500-3000 FT MSL). CCR ATIS WAS RPTING VFR AND EVERYTHING LOOKED GOOD UNTIL WE GOT CLOSER TO CCR AND A CALL TO THE TWR ADVISED THEY JUST WENT BELOW VFR DUE TO RAIN SHOWERS. BECAUSE OF RISING TERRAIN BEHIND US; A 180 DEG TURN WAS IMPRACTICAL AND WE CONTINUED OUR DSCNT TO REMAIN VFR. AS WE APCHED THE RIVER AND TURNED TOWARDS APC WE WERE DOWN TO 800 FT. NEEDLESS TO SAY; THERE WAS REAL CONCERN AS CONDITIONS WERE CHANGING RAPIDLY. AT THIS TIME I TOOK CTL OF THE ACFT AND INSTRUCTED THE COPLT TO CONTACT ATC IN CASE WE NEEDED VECTORS FOR AN APCH OR TO VFR CONDITIONS. VISIBILITY WAS DETERIORATING AND THE COPLT WAS HAVING A LOT OF TROUBLE RAISING ATC (LATER DETERMINED TO BE SWITCH POS AND VOLUME CTL; WE THINK). AS VISIBILITY DETERIORATED; I DETERMINED AND THE CREW AGREED (WE HAD AN ACM) THAT THE SAFEST COURSE OF ACTION WAS TO CLB TO INSURE TERRAIN CLRNC AND HEAD FOR SCK WHERE THE WX WAS BETTER; EVEN THOUGH WE MIGHT HAVE TO ENTER IMC. I MADE A 720 DEG CLBING TURN AND HEADED E AS CONTACT WAS MADE WITH ATC. WE WERE ADVISED THAT THERE WAS IFR TFC AT 3000 FT AND AS I HAD GND CONTACT; I DSNDED TO 2000 FT WHERE WE PROCEEDED TO SCK VFR AND LANDED UNEVENTFULLY. HAD I NOT KNOWN THE AREA I WOULD HAVE MADE A 180 DEG TURN AT THE FIRST SIGN OF DETERIORATING WX RATHER THAN DEVIATING AROUND THE HILLS. BUT; BECAUSE I KNEW THE AREA WELL; I ALLOWED MYSELF TO BE DRAWN INTO A VERY UNSATISFACTORY SIT. FAMILIARITY ALLOWED ME TO LOWER MY GUARD -- NOT GOOD. I SHOULD HAVE INSURED THAT THE COPLT WAS READY TO TALK TO ATC. THE INABILITY TO CONTACT ATC WAS DISTRACTING AND THE TRANSFER OF OUR DUTIES DID NOT GO SMOOTHLY. AN EARLY DECISION TO AVOID BAD WX (WHEN YOU ARE STILL IN GOOD WX) IS THE BEST ADVICE WHEREVER YOU ARE!