AN EMB145 APCHING OKC RWY 17L ON A VISUAL APCH RESPONDED TO A TCAS RA WITH TFC KNOWN TO ATC.

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APCH HAD DONE A GOOD JOB OF SEPARATING US FROM TFC MOVING FROM N TO S BY KEEPING US AT 4000 FT WHILE ON A L DOWNWIND FOR RWY 17L. AFTER THE TFC PASSED; HE CLRED US FOR THE VISUAL APCH. SINCE WE HAD BEEN HELD UP FOR THE TFC; WE WERE SET UP FOR A VERY DEEP BASE (N OF DOWNTOWN). HE CALLED OUT TFC TO THE W WHICH WAS MOVING FROM W TO E; BUT IT AT LEAST 10 MI AWAY AND WE COULD NOT SEE IT VISUALLY OR ON TCAS. WE TURNED BASE AND BEGAN A DSCNT OUT OF 4000 FT. THIS PUT US HEAD-TO-HEAD WITH THE APCHING TFC. AS WE REACHED 3000 FT IN THE DSCNT IT BECAME APPARENT THAT THERE WOULD BE A CONFLICT. THE FO SAW THE ACFT VISUALLY. I COULD ONLY SEE THE TCAS TARGET. I BEGAN A CLB BECAUSE THE TCAS PRESENTATION INDICATED THAT OUR TRACKS AND ALTS WERE CONVERGING. SHORTLY AFTER I BEGAN THE CLB; THE TCAS ISSUED A CLB RA. THE FO RPTED THAT THE ACFT PASSED UNDER THE NOSE AND HE ESTIMATED 500 FT OF VERT SEPARATION. THE EVENT OCCURRED FOR 2 REASONS: 1) WE WERE ISSUED CLRNC FOR THE VISUAL APCH IN A SITUATION WHERE EXECUTING THE APCH WOULD CREATE A CONVERGING GEOMETRY WITH TFC THAT WE COULD NOT SEE. 2) THE CTLR DID NOT CONTINUE TO GIVE US TA'S TO HELP US FIND THE TFC OR TO ISSUE INSTRUCTIONS WHICH WOULD DE-CONFLICT THE SITUATION. EVASIVE ACTION INITIATED. APCH WAS CONTINUED WHEN CLR OF CONFLICT. SUGGESTIONS: ATC COULD HAVE CONTINUED TO CTL US RATHER THAT TURN US LOOSE ON A VISUAL WHEN HE WAS AWARE OF A TARGET TRACK THAT WOULD ENTER OUR APCH PATTERN. HAVING CLRED US FOR THE APCH; HE COULD HAVE CONTINUED TO ISSUE TA'S TO HELP US ACQUIRE THE TFC. HE COULD ALSO HAVE INCREASED OUR SITUATIONAL AWARENESS WITH MORE INFO (SUCH AS WHETHER HE WAS IN COM WITH THE OTHER ACFT OR WHAT ITS CTL STATUS WAS).