A VFR-ON-TOP PILOT CLBED TO FL190 ATTEMPTING TO STAY CLEAR OF CLOUDS BEFORE CALLING ATC FOR A CLRNC.

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FLYING AT 17500 FT MSL W OF BUILDUPS AND W OF THE CONTINENTAL DIVIDE; ON A VFR FLT TO ZZZ DEVIATED FROM DIRECT TO ZZZ TO AVOID SOME BUILDUPS OVER THE STATES FRONT RANGE. EQUIP ON BOARD INCLUDED A GARMIN 430 AND 496 GPS (WITH WX). VFR ON TOP AT 17500 FT UNTIL XING DENVER FRONT RANGE MOUNTAINS. CLOUD LAYER BEGAN TO RISE AND TO STAY VFR I CLBED TO 18000 FT; THEN 19000 FT TO STAY ON TOP OUT OF CLOUD DECK. CONTACTED APCH IMMEDIATELY; BUT WAS AT 19000 FT BEFORE THEY HAD A CHANCE TO IDENT. YES I PENETRATED CLASS A WITHOUT AUTH. ATC GRACIOUSLY PROVIDED A XPONDER CODE AND I DSNDED IFR WITHOUT INCIDENT TO ZZZ. WHAT SHOULD I HAVE DONE DIFFERENTLY: DONE A 180 DEG TURN AND CONTACT ATC BEFORE ENTERING CLASS A. WHAT WILL I DO DIFFERENTLY IN THE FUTURE: WX IS SUBJECT TO DIFFERENT ACTUAL CONDITIONS VERSUS FORECAST ALONG THE FRONT RANGE ROCKIES. I PURPOSELY SKIRTED ALL WX TO THE W OF THE INTENDED RTE AND FORECAST CALLED FOR VFR. I UPDATED WX CONSISTENTLY ALONG THE RTE WITH METARS; RADAR AND CLOUD COVER. I DID NOT CHOOSE AN EASTERN PLAINS RTE ALONG V81 PUB-BRK-BJC BECAUSE WINDS ALOFT WERE FROM THE W AND I FELT ANY PERCIP WOULD BUILD TO TSTMS IN THE EARLY AFTERNOON NOT MAKING FOR A CHOICE RTE. YELLOW NEXRAD DISPLAY WAS IN PUEBLO FRONT RANGE AREA. I THOUGHT I WOULD HAVE A LOWERING CLOUD DECK APCHING THE FRONT RANGE AT FORT COLLINS; NO MATTER; IN FUTURE CONTACT ATC EARLY.