C182 SHOOTING LNDGS; ENCOUNTERED IMC WX AND HAD TO GET ATC SVC. HE WAS CLRED TO SJC WHERE HE MADE AN ILS APCH. WX FORECAST DID NOT INDICATE THE ONSET OF LOW CEILINGS. THE RPTR AND THE ACFT WERE INST CERTIFIED.

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SIT OCCURRED WHILE PRACTICING TKOFS/LNDGS (TO FULL STOPS) USING L CLOSED TFC FOR RWY 12 AT PALO ALTO. ALTHOUGH LIGHT RAIN WAS FALLING THE WX APPEARED TO BE ON AN IMPROVING TREND WITH PERIODS OF SUNSHINE. ON MY 5TH CIRCUIT WHILE ON THE DOWNWIND LEG I NOTICED WHAT APPEARED TO BE SOME ISOLATED LOW 'SCUD' FLOATING ACROSS THE APCH END OF RWY 12. I BEGAN A SLIGHT CLB TO REMAIN CLR AND RPTED TO THE TWR CTLR THAT I WOULD NEED TO EXTEND MY DOWNWIND TO MANEUVER PAST THE CLOUD. WITHIN A FEW SECONDS I FOUND MYSELF IN SOLID IMC AT 1000 FT AGL. I IMMEDIATELY RPTED MY SIT TO TWR AND BEGAN A CLB AND HELD A 350 DEG HDG. TWR IMMEDIATELY INSTRUCTED ME TO CONTACT BAY APCH WHO SUBSEQUENTLY INSTRUCTED ME TO CLB AND MAINTAIN 2000 FT AND BEGAN VECTORING ME FOR THE SJC ILS RWY 12 APCH AFTER DETERMINING THAT I WAS INST CAPABLE AND NOT IN A FUEL CRITICAL STATE. I WAS CLRED FOR THE APCH AND LANDED AT SJC WITHOUT INCIDENT. IT TURNED OUT THAT THE ISOLATED 'SCUD' WAS THE LEADING EDGE OF AN UNFORECAST 500 FT CEILING WHICH MOVED IN RAPIDLY FROM THE W. A BETTER PERCEPTION OF THE EXTENT AND NATURE OF RAPIDLY CHANGING METEOROLOGICAL CONDITIONS MAY HAVE ALLOWED FOR MORE TIME AND ROOM TO MANEUVER CLR. OTHER ACFT WERE IN THE TFC PATTERN AND I THEREFORE ELECTED NOT TO ATTEMPT A RAPID 180 DEG TURN.