RPTR EXPERIENCES TCASII RA FROM INTRUDER CONDUCTING VISUAL THROUGH 300 FT THICK IMC CLOUD LAYER.

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ON ILS APCH TO RWY 25L AT LAX; WE EXPERIENCED AN ALARMING TCASII RA. PRIOR TO THE FINAL APCH FIX; APCH CTL ADVISED US OF COMMUTER TFC FOR RWY 25R WHO WAS INTERCEPTING RWY 25R FINAL FROM THE S. WE WERE TOLD HE HAD US IN SIGHT. AT ABOUT 1500 FT AGL OUR TCASII WHICH WAS SET AT 5 NM RANGE AND 'NORMAL' GAVE US A TA WHICH WAS CLOSE (LESS THAN A MI) AT OUR 5 O'CLOCK POS; PLUS 300 FT. WE ENTERED THE CLOUDS AT ABOUT 1300 FT AGL AND WE RECEIVED ANOTHER TA. HE WAS CLOSING. DSNDING BELOW THE CLOUD LAYER; AT ABOUT 1200 FT AGL THE TCASII TA SHIFTED TO A RA WHICH HAD MERGED WITH THE CTR OF OUR TCASII SCREEN; THE ALT INDICATOR WAS SHIFTING FROM PLUS TO MINUS WHILE THE AURAL ALERT CALLED FOR AN IMMEDIATE CLB IN EXCESS OF 1500 FPM. WE ADVISED TWR WE WERE EXECUTING A TCASII RA AND LAX TWR INFORMED US THAT OUR TFC WHICH WE NEVER SAW HAD US IN SIGHT. WE CONTINUED OUR APCH TO AN UNEVENTFUL LNDG. THIS IS FORMATION FLYING (INCLUDING RUNNING RENDEZVOUS) IN MARGINAL WX. 1 ACFT IFR; THE OTHER VFR. IT SCARES THE PAX AND IT SCARES ME; TOO. IT'S UNACCEPTABLE.