A C172 ENCOUNTERED IMC WITH LCL TERRAIN FACTORS AFFECTING CLOUD FORMATION.

Narrative

I WAS FOLLOWING THE VALLEY BTWN KLAMATH FALLS AND REDMOND; IN MVFR (AS FORECAST); HAVING NOTED SOME POTENTIAL ARPTS FOR LNDG ON THE WAY IF THE WX SHOULD DETERIORATE. HAVING BEEN GIVEN A THOROUGH BRIEFING; I WAS REASONABLY CONFIDENT THAT THE CEILINGS AND VISIBILITY WOULD ALLOW ME TO COMPLETE THE RTE. I WAS EASILY ABLE TO MAINTAIN TERRAIN CLRNC; AND THE REQUIRED CLOUD DISTANCES; HOWEVER; WHEN I GOT TO THE SUNRIVER AREA; VISIBILITY AND CLOUD BASES DROPPED SLOWLY AT FIRST (SO THAT I THOUGHT THIS WAS JUST TEMPORARY); BUT THEY REMAINED ABOVE VFR MINIMUMS FOR THE CLASS G AIRSPACE. I WAS CONCENTRATING ON MAINTAINING VFR RULES; AND DID NOT IMMEDIATELY REALIZE THAT I HAD ENTERED CLASS E AIRSPACE; AND AROUND THAT POINT IN TIME; VISIBILITY AND CLOUD DISTANCES SUDDENLY DROPPED FURTHER AND WERE NOW BELOW VFR MINIMUMS. I HAD ALREADY STARTED MY 180 DEG TURN; TO RETRACE MY RTE (AND LAND AT THE NEXT ARPT TO THE S); WHEN I DECIDED THAT I WOULD MAKE A PRECAUTIONARY LNDG. LNDG WAS UNPROBLEMATIC; AND I MADE CALLS ON THE CTAF TO ANNOUNCE MY ARR; ALTHOUGH I ABBREVIATED MY NORMAL PROCS FOR AVOIDING NOISE-SENSITIVE AREAS; AS WELL AS PREPARING FOR LNDG AT AN UNCTLED ARPT; IN ORDER TO REMAIN CLOSE TO THE ARPT AND AVOID ANY POSSIBILITY OF LOSS OF SITUATIONAL AWARENESS. AFTER LNDG; I CLOSED MY FLT PLAN; AND DISCUSSED THE LCL WX PECULIARITIES WITH THE FBO; AS WELL AS ONWARD TRAVEL POSSIBILITIES BY RENTAL CAR. I HAD LET MY NORMAL VIGILANCE AND STANDARDS SLIP; WAS LULLED INTO A FALSE SENSE OF SECURITY BY THE FORECAST; AND HAD A MENTAL MODEL OF A CLR PATH TO MY DEST. I LEARNED FROM THE FBO THAT FORECASTS IN THIS AREA ARE OFTEN MISLEADING (MY FIRST TIME FLYING HERE); AS THE TERRAIN IN THIS LOCATION IS HIGHER THAN THAT AT THE WX RPTING STATIONS AT EITHER END OF THE VALLEY (KLAMATH FALLS AND REDMOND); AND THAT THERE IS JUST A SHORT STRETCH OF SEVERAL MI; WHERE THE WX FREQUENTLY IS AS I HAD EXPERIENCED. I NEED TO CHK THE VFR CHARTS MORE CAREFULLY FOR GRADUAL RISING TERRAIN -- PARTICULARLY IF I EXCEPT TO BE FLYING WITHIN A FEW THOUSAND FT OF THE GND. WHILE THE GND MAY RISE; THE CLOUD BASES CAN STAY AT CONSTANT ALT. IT STRUCK ME THAT IT COULD BE HELPFUL ON VFR CHARTS TO INDICATE HIGH POINTS IN VALLEYS; SIMILAR TO THE PASSES THAT USED TO BE SHOWN ON THE CHARTS. IN THAT PARTICULAR AREA; THE COLORATION BAND FOR THE CONTOURS IS THE SAME FOR ALTS BTWN 3000-5000 FT; BUT I COULD CHK FOR HIGH POINTS IN SUCH A VALLEY BASED ON WHERE THE NARROW POINTS ARE.