POTENTIAL CONFLICT BTWN ARR DEP TFC.

Narrative

WE WERE CLRED TO CROSS ART VOR AT 3000 FT FOR AN APCH TO WATERTOWN ARPT. AT THIS TIME WE WERE STILL TALKING TO CTR. AT 5 MI S OF THE VOR; CTR ADVISED US TO SWITCH TO ADVISORY FREQ; WHICH I DID. I ANNOUNCED OUR POS AND INTENTIONS. AT THE SAME TIME I WAS MONITORING CTR ON THE OTHER RADIO. AT ABOUT THIS TIME; I NOTICED TFC ON THE TCASII AT OUR 12 O'CLOCK AND 5 MI; 600 FT BELOW US. AT THIS TIME; A LIGHT PLANE CAME UP ON CTR FREQ REQUESTING TO PICK UP AN IFR FROM WATERTOWN. CTR ADVISED PLT HE HAD JUST CLRED ACFT INTO ART AND ASKED PLT'S POS. WHEN ADVISED OF POS; CTR SEEMED WORRIED; FOR GOOD REASON. WE WERE AT 3000 FT; IMC; AND THE TFC ON TCASII WAS NOW 12 O'CLOCK AND 2 MI; 200 FT BELOW. I TOLD FO TO TURN WBOUND AS I KNEW THAT THERE ARE NO OBSTRUCTIONS IN THAT DIRECTION. WHILE IN THE TURN; TCASII ISSUED A CLB RA; WHICH WE FOLLOWED. AFTER THIS WE COMPLETED THE APCH WITHOUT INCIDENT. CLOUD BASES WERE AT 2300 FT. PRIMARY CAUSE OF CONFLICT WAS NOT MAINTAINING CLOUD CLRNC (500 FT BELOW) BY OTHER PLT; AND HIS FAILURE TO MONITOR ADVISORY FREQ.