SMA FLYING FORMATION ENCOUNTERS LOWERING CEILINGS; ENTERS IMC ATTEMPTING TO TURN AROUND.

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I DEPARTED INDEPENDENCE; OR; WITH 2 OTHER ACFT; BOUND FOR THE DALLES; OR; ON A FIRST LEG OF A PLANNED FLT TO INDEPENDENCE; MO. THE FLT HAD BEEN PLANNED FOR SEVERAL WKS AND WE WERE INTENT ON REACHING OUR ULTIMATE DEST TO JOIN SEVERAL OTHER ACFT ON A SCHEDULED COMMEMORATIVE FLT ALONG THE OREGON TRAIL BACK TO INDEPENDENCE; OR. THE WX AT INDEPENDENCE WAS ESTIMATED TO BE 3000 FT; OVCST WITH VISIBILITY OF 25 MI. MC MINORVILLE FSS RPTED THE WX IN THE COLUMBIA RIVER GORGE TO BE AT 2400 OVCST AND CLRING AT THE DALLES; OR. THE OTHER 2 ACFT TO MY R WERE IN SIGHT AT ALL TIMES UNTIL APPROX 20 MI SW OF CASCADE LOCKS. THERE WAS GOOD GND CONTACT AND WE WERE CLR OF CLOUDS AT OUR CRUISING ALT OF 3000 FT BUT WE WERE NOW FLYING IN LIGHT RAIN AND THE CONDITIONS AHEAD APPEARED MARGINAL. I LOST SIGHT OF THE OTHER 2 ACFT AS THEY DISAPPEARED BEHIND A LOWER HANGING CLOUD TO MY R. AT THAT TIME I DECIDED TO ABORT THE FLT; MAKE A 180 DEG TURN AND RETURN TO INDEPENDENCE; OR. I DID NOT KNOW THE POS OF THE OTHER 2 ACFT; OR ON MY R; I COULD NOT TURN IN THAT DIRECTION WITHOUT RISKING A MIDAIR COLLISION. THERE WAS ANOTHER LOW HANGING CLOUD TO MY L AT ABOUT 9 O'CLOCK. MY PLAN WAS TO MAKE A L TEARDROP TURN TO REVERSE MY COURSE AS SOON AS I HAD CLRED THE CLOUD ON MY L. AT THAT PARTICULAR POINT; I COULD NOT TURN R; I COULD NOT TURN L OR CLB WITHOUT ENTERING CLOUDS; AND I COULD NOT DSND DUE TO RISING TERRAIN NOR COULD I CONTINUE AHEAD DUE TO DETERIORATING WX. MY BEST CHOICE WAS TO ATTEMPT A L TURN AS I PASSED THE CLOUD ON MY L. IN A MATTER OF SECONDS LOW WISPY CLOUDS REDUCED VISIBILITY IN ALL DIRECTIONS AND I INADVERTENTLY ENTERED IFR CONDITIONS. AS FACTORS AFFECTING THE QUALITY OF HUMAN PERFORMANCE; I HAVE TO MENTION THAT MY ACFT WAS THE FASTEST OF THE 3 AND THAT I HAD TO ADJUST MY PWR CONTINUOUSLY TO FLY AT THEIR SPD. PERCEPTIONS; JUDGEMENTS AND DECISIONS WERE AFFECTED AND SLOWED BY OUR FLT OF 3 WHERE NOBODY WANTED TO BE THE FIRST ONE TO CALL THE FLT OFF.