NMAC IN THE PAO TFC PATTERN ON A HAZY DAY WHEN TFC SIGHTED IS NOT THE TFC CALLED OUT.

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NEAR MISS ON BASE/FINAL RWY 31 PAO ARPT. VISIBILITY WAS BETTER THAN 6 MI; BUT THERE WAS LOW BROWN HAZE THAT WAS AFFECTING VISIBILITY. PATTERN WAS OBVIOUSLY BUSY; SO I PROCEEDED INBOUND SLOWLY AFTER RECEIVING INSTRUCTIONS 'R TFC RWY 31; FOLLOW CESSNA.' I RPTED NO TFC IN SIGHT; REPLY: 'OVER THE BIRD HOUSE' (DOWNWIND 500 FT PAST RWY 31 NUMBERS). STILL NOT IN SIGHT; SLOWED TO MINIMUM SPD TO AVOID OVERTAKING. CALLED AGAIN; REPLY: 'WIDE DOWNWIND OVER YACHT HARBOR.' I DID NOT SPOT MY TFC OVER THE ABANDONED HARBOR; BUT DID SPOT A TAN PLANE THAT APPEARED TO BE TURNING BASE OVER THE CURRENTLY OPERATING MARINA LESS THAN 1 MI FURTHER DOWNWIND. 'IS MY TFC TURNING BASE NOW?' 'AFFIRMATIVE; IN SIGHT.' SOMEWHERE AFTER THIS CALL; THERE WAS A SHIFT CHANGE AT THE TWR. I BEGAN MY BASE TURN AS THE TAN ACFT PASSED ABEAM ON FINAL. I NOTED THAT IT WAS A LOW WING TWIN; BUT DID NOT CALL TWR TO ASK IF THEY MEANT 'TWIN CESSNA.' AS I TURNED ONTO FINAL; THERE WAS A RED AND WHITE HIGH WING C172 MANEUVERING DIRECTLY UNDER ME; PERHAPS LESS THAN 100 FT. I EXECUTED AN EMER CLB TO AVOID COLLISION. THE C172 HAD BEEN ON A WIDE DOWNWIND AND I NEVER SPOTTED HIM. HE MUST HAVE TURNED BACK AND FLOWN UNDER ME AS I WAS ON DOWNWIND. A) I AM A LCL PLT; HOWEVER; EVEN I GOT CONFUSED WHEN I SAW NO TFC OVER TE NO-LONGER-PRESENT PALO ALTO BOAT HARBOR; BUT DID SEE TFC NEAR THE SHORELINE MARINA FURTHER DOWNWIND. THIS TERMINOLOGY MIGHT BE AMENDED TO 'ABANDONED HARBOR' OR 'OLD HARBOR' OR REPLACED WITH A REF TO SOMETHING ELSE (SAME THING; TO A LESSER EXTENT WITH 'BIRD HOUSE'). B) MY CONCENTRATION ON SLOW FLT IN THE SR22 WAS ABSOLUTELY A CONTRIBUTING DISTR. 3) THIS COULD HAVE BEEN AVOIDED IF I HAD ASKED 'IS IT THE TAN ACFT TURNING R BASE OVER SHORELINE PARK HARBOR?' INSTEAD OF 'IS MY TFC TURNING BASE NOW?' D) IN AN INFORMAL DISCUSSION WITH THE CTLR; HE SAID 'WHEN YOU SAY YOU HAVE THE TFC IN SIGHT; WE ASSUME YOU'VE GOT IT IN SIGHT.' TRUE; BUT WHAT TFC? IF HE HAD SAID RED CESSNA IT WOULD HAVE BEEN CLR. E) THE SHIFT CHANGE OR GENERAL TWR WORKLOAD MAY HAVE BEEN A CONTRIBUTING FACTOR; HOWEVER I UNDERSTAND THE RESPONSIBILITY FOR SEPARATION RELIES WITH ME.