SMA ENCOUNTERS WX; FILES IFR; DIVERTS TO LAND.

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PRIOR TO DEPARTING SAC FOR LAS; I SECURED A VERY EXTENSIVE WX BRIEFING IN THE AUX FSS AT SAC. MY INITIAL AND PRIMARY CONCERN WAS ABOUT THE POTENTIAL TURB ENRTE OVER THE MOUNTAINS. AS IT TURNED OUT; THAT PHASE OF THE FLT WAS SILKY SMOOTH. LAS WAS RPTING 2000 E50 OVCST WITH 35; LIGHT RAIN SHOWERS. THIS FORECAST TO IMPROVE PRIOR TO MY ARR TO 3000 SCATTERED; CEILING 6000 BROKEN; OCCASIONAL CEILING 3000 BROKEN LIGHT RAINSHOWERS; AND AFTER XAZ 6000 SCATTERED; CEILING 1000 BROKEN OCCASIONAL CEILING 6000 BROKEN; AND AFTER XCZ VFR. ENRTE I HAD ENCOUNTERED SOME SCATTERED CLOUDS AND AT TIMES BREAKS BELOW MY ALT OF 11500. AS I APCHED BEATTY I OBSERVED SOME CLOUDS AHEAD AT MY ALT AND REQUESTED WX FROM CTR. IT WAS RPTED TO BE 2000 SCATTERED; 3000 BROKEN; 5000 OCCASIONAL. SINCE IT HAD NOT IMPROVED AS EXPECTED; I STILL ANTICIPATED PRIMARILY BROKEN TO SCATTERED CONDITIONS UNTIL REACHING LOWER ALTS BELOW THE FREEZING LEVEL SO I REQUESTED AND OBTAINED AN IFR CLRNC AT 12000. SOON AFTER ENTERING THE CLOUDS I ENCOUNTERED SOME LIGHT MIXED ICING SO IMMEDIATELY REQUESTED 130 EXPECTING TO CLB ABOVE THE CLOUDS. AT THAT TIME; I INTENDED TO SEE IF FURTHER FLT WOULD BE POSSIBLE SINCE I WOULD NEED TO GET BACK DOWN TO 12000 RIGHT AWAY. WHEN I DID BREAK OUT OF THE CLOUDS I REQUESTED AN IMMEDIATE 180. AS SOON AS I WAS CLR OF THE CLOUDS I CANCELLED IFR AND PROCEEDED TO A LOWER ALT TO GET RID OF THE ICE AND PROCEEDED VFR TO RENO AND LANDED THERE. THE LESSON I LEARNED FROM THIS EXPERIENCE IS DO NOT LET FORECAST WX INFLUENCE MY OPINION OF CURRENT CONDITIONS. I LEARNED LATER AS I SECURED A NUMBER OF WX UPDATES; THAT THE LAS WX WAS UP AND DOWN ALL DAY; BUT NEVER IMPROVED AS EXPECTED UNTIL MUCH LATER. WE REMAINED IN RENO OVERNIGHT; (GLAD TO BE ON THE GND) AND ENJOYED A BEAUTIFUL CAVU SMOOTH FLT TO LAS THE FOLLOWING MORNING.