A NON INST RATED PA38 PLT GOT CAUGHT ABOVE A SOLID LAYER WHEN IN NCT'S AIRSPACE AND LANDED AT STS WITH 4.2 GALS OF FUEL REMAINING.

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I ARRIVED IN THE VICINITY OF THE SAN FRANCISCO BAY AREA IN THE AFTERNOON TO DISCOVER THAT ALL ARPTS WERE UNDER A SOLID OVCST. I AM NOT IFR RATED; SO I DECLARED AN EMER. I WAS GIVEN AN ALTERNATE OF SANTA ROSA ARPT (STS) WHERE I MADE AN UNEVENTFUL LNDG APPROX 1 HR LATER. FUEL REMAINING AT SANTA ROSA WAS 4.2 GALLONS USABLE; APPROX 1/2 HRS WORTH. I RECEIVED A WX BRIEFING THAT MORNING FOR VISIBILITY GREATER THAN 6 MI; FEW CLOUDS AT 12000 FT; BEGINNING AT XA00 AND ENDING LATE NIGHT AT THE OAKLAND ARPT; CLOSE TO MY DEST AT HAYWARD (HWD). I DEPARTED L08 (BORREGO SPRINGS) IN THE AFTERNOON FLYING TO LANCASTER (WJF) WHERE I TOPPED OFF WITH FUEL. FLYING OVER THE TEHACHAPI'S I SAW A SOLID OVCST OVER THE VALLEY; WHICH I EXPECTED. I FLEW ON; OVER PANOCHE VOR (PXN); WHEN I PICKED UP THE SAN JOSE ATIS THAT ANNOUNCED OVCST AT 1200 FT. I CALLED FLT WATCH AND GOT THE WX FOR HAYWARD ALSO ANNOUNCING OVCST. AT THAT POINT (ABOUT 60 MI FROM HWD); I DECLARED AN EMER STATING 2 HRS OF FUEL REMAINING. AFTER SEVERAL FREQ CHANGES; I WAS GIVEN A DISCRETE XPONDER CODE AND DIRECTED TO AN ALTERNATE AT SANTA ROSA. I MADE A MISTAKE IN NOT CONFIRMING THE WX EARLIER IN THE FLT (PERHAPS AT BAKERSFIELD). I ALSO WAS MISTAKEN IN FLYING VFR ON TOP BEYOND THE POINT AT WHICH I COULD HAVE REACHED VFR TO LAND IN CASE OF A MECHANICAL FAILURE. FINALLY; I MISTAKENLY RELIED ON MY 'KNOWLEDGE' OF STANDARD WX PATTERNS IN THE AREA -- IE; OVCST CENTRAL VALLEY; BUT HAZE OVER THE OUTER VALLEYS FROM PASO ROBLES TO HOLLISTER; TO SAN JOSE AND INTO HAYWARD. THUS; I DID NOT CHK WX EARLY ENOUGH TO KNOW THAT I WOULD HAVE HAD TO HOLD IN PASO ROBLES; WHICH WAS CLOSE TO MY PATH OF FLT AND THE LAST VFR ARPT BEFORE MY DEST.