PRIVATE PLT OF AN SMA SEL ACFT CLBED THROUGH AN OVCST AFTER BECOMING CAUGHT UNDER THE OVCST TOWARD HIGHER RISING TERRAIN WITHOUT AN IFR CLRNC.

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I HAD DEPARTED SHELL LAKE; WI (SSQ) ON MORNING OF JUL/SAT/93 ENRTE TO MONTROSE; CO (MTJ). FIRST LEG - SS1 TO ABERDEEN; SD; WAS FILED AND FLOWN IFR. PIERRE FSS IN PHONE WX BRIEFING RECEIVED AT ABERDEEN HAD RPTED SCATTERED THROUGH BROKEN CEILINGS IN W; NE AND SE; WY FOR MIDDAY AND GOOD WX AT DEST; SO SECOND LEG WAS FILED VFR. AFTER PASSING SCOTTSBLUFF AT 6500 FT MSL; THE CEILING BECAME OVCST; BUT I CONTINUED SW ON COURSE EXPECTING THE FORECAST WX OF SCATTERED TO BROKEN CLOUDS TO RECUR. THE OVCST CONDITION CONTINUED AND BECAME DARKER AS I PROCEEDED SW; WHILE THE TERRAIN CONTINUED TO RISE. AT A POINT THEN DETERMINED TO BE AT ABOUT 33 NM FROM BFF VOR ON THE 232 DEG RADIAL AND UNDERSTOOD TO REPRESENT UNCTLED AIRSPACE; I DETERMINED THAT IT WOULD BE SAFER TO CLB TO VFR CONDITIONS ON TOP RATHER THAN TO CONTINUE IN UNFORECAST LOW CEILINGS OVER RISING TERRAIN AND EXECUTED A CLB ON COURSE; BREAKING OUT AT APPROX 10500 FT MSL. THE REST OF THE FLT WAS CARRIED OUT IN GOOD VFR. I AM COMPLETING THIS RPT BECAUSE IF MY DETERMINATIONS AS TO MY LOCATION WERE NOT ACCURATE; IT IS POSSIBLE THAT MY BRIEF CLB IN IMC INTRUDED UNINTENTIONALLY INTO CTLED AIRSPACE. FORTUNATELY; NO OTHER ACFT WERE OBSERVED AND NO PROB OF ANY KIND WAS ENCOUNTERED.