Narrative:

I was following the valley between klamath falls and redmond, in MVFR (as forecast), having noted some potential airports for landing 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 route. I was easily able to maintain terrain clearance, 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 east 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 degree turn, to retrace my route (and land at the next airport to the south), when I decided that I would make a precautionary landing. Landing was unproblematic, and I made calls on the CTAF to announce my arrival, although I abbreviated my normal procedures for avoiding noise-sensitive areas, as well as preparing for landing at an uncontrolled airport, in order to remain close to the airport and avoid any possibility of loss of situational awareness. After landing, I closed my flight plan, and discussed the local 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 clear path to my destination. 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 reporting 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 check the VFR charts more carefully for gradual rising terrain -- particularly if I except to be flying within a few thousand ft of the ground. While the ground may rise, the cloud bases can stay at constant altitude. 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 altitudes between 3000-5000 ft, but I could check for high points in such a valley based on where the narrow points are.

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Original NASA ASRS Text

Title: A C172 ENCOUNTERED IMC WITH LCL TERRAIN FACTORS EFFECTING 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.