TURNING BASE RWY 31; NMAC WITH UNIDENTED ACFT.

Narrative

ATIS RPTED RWY 31 VISUAL APCH. WE RECEIVED VECTORS FOR A L DOWNWIND TO RWY 31 VISUAL. PRIOR TO ENTERING DOWNWIND WE WERE OFFERED A VISUAL TO RWY 13. THE CAPT DECLINED RWY 13 BECAUSE HE FELT WE WERE TOO CLOSE TO THE ARPT. I (FO FLYING) VERBALLY; TO CAPT; INDICATED I WOULD ACCEPT RWY 13 VISUAL BUT AGREED WITH THE CAPT'S DECISION. WE WERE THEN CLRED FOR THE L DOWNWIND VISUAL APCH TO RWY 31. I CROSSED ABEAM THE APCH END OF RWY 31 AT 2000 FT MSL AND EXTENDED APPROX 3.5 MI TO MAKE THE L TURN FOR CONFLICTING TFC. THE CAPT TOOK THE AIRPLANE AND WE IMMEDIATELY RECEIVED A TCASII RA FOR THE TARGET ACFT INDICATING 100 FT BELOW US. THE CAPT (FLYING) FOLLOWED TCASII ADVISORY AND CLBED AS ACFT PASSED BELOW US. WE CONTINUED AND LANDED UNEVENTFULLY ON RWY 31. CONTRIBUTING EVENTS: 1) NIGHT. 2) LOC SET UP FOR RWY 13 WHEN WE WERE GIVEN VECTORS FOR RWY 31. 3) TWR NO HELP WITH TFC AFTER TCASII MANEUVER. TWR VERY IRRITATED THAT WE WERE VERIFYING CLRNC TO LAND RWY 31 AFTER THE NEAR MISS. 4) CONFLICTING ACFT UNAUTH IN ATA.