A B737-700 ON AN SNA IMC APCH DEVIATED FROM THE ASSIGNED ALT IN RESPONSE TO A TCAS TARGET THAT GENERATED A TA.

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FLT OPERATED TO SNA. WX AT SNA WAS ABOUT 800 FT BROKEN WITH VISIBILITY 1 1/4 MI; 21 KT WINDS FROM 160-170 DEGS AND GUSTING TO ABOUT 32 KTS. WE HAD ALREADY EXECUTED A GAR FOR A PWS WINDSHEAR WARNING. ACTUAL SHEAR ENCOUNTERED WAS APPROX +/-10 KTS. WE WERE BEING VECTORED ON AN E (L) DOWNWIND ON A NORTHERLY HDG FOR ANOTHER ILS APCH TO RWY 19R AND WERE STAYING E OF A LINE OF CELLS. OUR ALT WAS 5000 FT MSL. THE FO WAS THE PF. WE WERE IMC AND I NOTICED A TCAS ACFT DISPLAY AT ABOUT 12 O'CLOCK AND 5 MI APPROX 100 FT ABOVE OUR ALT. IT APPEARED TO BE DSNDING VERY SLOWLY; AND WHEN IT WAS AT ABOUT 12 O'CLOCK; 2.5 MI AT OUR ALT; AND APPARENTLY CONVERGING; I DIRECTED THE FO TO CLB AND I TOLD SOCAL THAT WE WERE CLBING FOR TFC. WE DID GET A TA; BUT DID NOT GET AN RA. ATC IMMEDIATELY CLRED US TO 6000 FT WITH A DSCNT BACK GO 5000 FT WHEN ABLE. WE HAD CLBED TO ABOUT 5500 FT WHEN THE TA TERMINATED. AT THAT POINT THE TFC WAS ABEAM US APPROX 2 MI TO OUR PORT SIDE AND 1000 FT BELOW US. WE THEN DSNDED BACK TO 5000 FT AND RPTED OUR ALT TO SOCAL. NO OTHER POTENTIAL CONFLICT WAS EXPERIENCED AND THE FLT LANDED WITHOUT FURTHER INCIDENT. OPINION/COMMENTARY: I THINK OUR ACTION PRECLUDED THE RA; INCREASED OUR TFC SEPARATION; AND WAS THE SAFEST COURSE OF ACTION TO PURSUE. THE SOCAL CTLR DID NOT SEEM UPSET AT ALL AND WAS VERY ACCOMMODATING IN INSTANTLY GIVING US THE CLRNC TO 6000 FT. I AM GLAD WE SAW THE INTRUDER WHEN WE DID.