A PVT PLT DEPARTED ON A XCOUNTRY IN VFR CONDITIONS. THE WX RPTS THAT HE RECEIVED INDICATED THAT THE STORM HAD PASSED AND THE FORECAST WAS FOR IMPROVING CONDITIONS. THE PLT WAS FORCED TO TURN BACK WHEN THE WX INSTEAD DETERIORATED. WITH ATC ASSISTANCE; THE PLT DIVERTED TO A MAJOR ARPT.

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SJC SA 5500 FT SCATTERED; GREATER THAN 10000 FT BROKEN; VISIBILITY GREATER THAN OR EQUAL TO 15 MI. FORECAST INDICATED STORM HAD PASSED AND WX CONDITIONS WERE IMPROVING. DEPARTED SJC INTENDING DIRECT WVI DIRECT HAF DIRECT PAO TO TAKE ADVANTAGE OF FORECAST IMPROVING WX. 14 MI OUT OF SJC; NOTICED THAT MY ALT WOULD PUT ME BTWN LAYERS INTO WVI. CLOUDS TO S MUCH LOWER THAN PREDICTED. PERFORMED 180 DEG TURN TO PROCEED DIRECT PAO. OBSERVED WALL OF PRECIP JUST SE OF NUQ. CTLR CALLED TO ADVISE PAO BELOW MINIMUMS. DECIDED TO LAND SJC. WALL CONTINUED TO MOVE TOWARDS SJC AND DETERIORATED TO 900 FT. 1/2 TO 1 MI FROM SJC I WAS UNABLE TO POSITIVELY IDENT SJC. I DECIDED TO PROCEED TO 800 FT AND 1/4 MI. I HAD DECIDED IF I DID NOT SEE THE RWY ENVIRONMENT BY THEN; I WOULD DECLARE AN EMER AND CLB INTO CLOUDS; REQUESTING ILS RWY 12R AT SJC. FORTUNATELY; I DID SEE SJC; AND LANDED VFR JUST BEFORE THE WALL OF PRECIP ENGULFED SJC. RETURN TO PAO WAS ACCOMPLISHED LATER THAT NIGHT UNDER SVFR WITH A CFII. I HAVE SPENT MANY HRS TRYING TO FIND SOMETHING THAT I DID WRONG IN THIS EVENT. IF IT'S MY ERROR; I CAN AVOID IT NEXT TIME. I HAVE BEEN UNABLE TO FIND AN ERROR. NONE OF THE DATA AVAILABLE PREDICTED THE WX OR ITS RAPID CHANGE. I THANK THE CTLRS FOR THEIR PROMPT AND TIMELY ASSISTANCE; AND THIS EVENT HAS ME MORE CONVINCED THAN EVER OF THE NEED FOR MY INST RATING.