AN ATP PA32 PLT; DURING WX AVOIDANCE MANEUVERING; PENETRATES THE CLASS D AIRSPACE OF SQL; CA.

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I OBTAINED A WX BRIEFING FROM OAK FSS PRIOR TO DEP TO FLY A FRIEND TO PAO IN HER AIRPLANE FOR A DOCTOR'S APPOINTMENT. I WAS TOLD PAO WAS CLR. SANTA ROSA WAS ALSO CLR AND WE DEPARTED AT XA20. APCHING THE BAY AREA VFR VIA THE PYE VOR 140 DEG RADIAL AT 2900 FT TO STAY BELOW CLASS B AIRSPACE; I COULD SEE CUMULUS BUILDUPS AHEAD OF ME EXTENDING FROM ABOUT SFO ARPT OVER HALF MOON BAY AND SOUTHWARD. A PRETTY SOLID LINE WITH TOPS TO PERHAPS 12000 FT. IT LOOKED LIKE I COULD GET BELOW THEM SO I BEGAN A DSCNT TURNING NORTHWARD FROM A DIRECT LINE TO OSI VOR DUE TO 2400 FT TERRAIN THERE. MY ATTN WAS ON AVOIDING CLOUD AND TERRAIN CLRNC AND WHEN I LEVELED OFF AT 800 FT I SAW A RWY A FEW MI AHEAD OF ME WHICH DID NOT LOOK FAMILIAR. THE ARPTS ON THE SHORELINE ARE HARD TO FIND IN THE GND CLUTTER BUT I KNEW IT WASN'T PALO ALTO. THE GPS INDICATED I WAS 6 MI AWAY FROM PAO SO I TURNED S TO SEARCH FOR IT. AFTER LNDG THERE; GND ASKED ME IF I KNEW I WAS IN SAN CARLOS CLASS D AIRSPACE WITHOUT A CLRNC. I TOLD HIM I HAD FIGURED THAT OUT AND THE REASON WAS DISTR CAUSED BY WX AVOIDANCE AND CONCERN FOR TERRAIN CLRNC. HE SAID IT WAS OK; THEY HAD CLRED IT WITH SAN CARLOS. MY FEELING IS A MORE COMPREHENSIVE WX BRIEF WOULD HAVE PREPARED ME BETTER.