S46 CTLR EXPERIENCED OPERROR AT 4000 FT WHEN FAILING TO NOTICE CONFLICTING TFC ENTERING AIRSPACE.

Narrative

I WAS WORKING THE DEP W SECTOR IN THE N FLOW CONFIGN WHEN THE DH8 DEPARTED SEA RWY 34L ON RWY HDG. AFTER RADAR IDENTING THE ACFT I INSTRUCTED IT TO TURN L TO HDG 210 DEGS; THEN AT THE SHORELINE TO TURN L ON COURSE DIRECT ALDER INTXN. MY INTENTION WAS TO BEAT AN ARRIVING MD80 ACFT COMING FROM THE NW; WELL DISTANT (20+ NM). I WAS ALSO WORKING A SMALLER JET ACFT THAT HAD DEPARTED BOEING FIELD (BFI) 'OUT THE BAY' THEN EVENTUALLY ON COURSE EBOUND. I HAD POINTED THIS ACFT OUT TO THE DEP E CTLR AND OBTAINED APPROVAL TO PLACE THAT ACFT ON COURSE AT LOA ALT AND SWITCH HIM TO THE CTR. THE E CTLR HAD 2 NBOUND DEPS THAT WERE CLBING TO AN AGREED UPON ALT FOR ME TO TURN MY ACFT EBOUND ON COURSE ABOVE THEM. THERE WERE OTHER VFR ACFT IN HDOF STATUS COMING FROM THE NW AS WELL. THE SECTOR WAS MODERATELY BUSY WITH ABOVE-AVERAGE COMPLEXITY. THE ASSOCIATED HDOF POS WAS IN THE PROCESS OF BEING OPENED AT THE TIME OF THE INCIDENT. HE HAD JUST SAT DOWN AND WAS REVIEWING THE TFC AND WX SITUATION IN PREPARATION TO RECEIVE A BRIEFING FROM ME. I HAD NOT YET GIVEN THE BRIEFING. I DID NOT NOTICE THE CHALLENGER (CL60) COMING FROM THE SW AT A LOWER THAN NORMAL ALT NEBOUND; ON VECTORS TO LAND AT BFI. THIS IS PROBABLY BECAUSE MY ATTN WAS FOCUSED NW AND N OF SEA WITH THE ABOVE MENTIONED OPS. THE CL60 WAS ON A CONFLICTING COURSE WITH THE DH8 AT RELATIVELY HIGH GND SPD (ESTIMATE 260-270 KTS). IT WAS LEVEL AT 5000 FT MSL WITH A JUST-ISSUED CLRNC FROM THE ARR CTLR WORKING THE ACFT TO DSND TO 4000 FT. I HAD INITIALLY ISSUED A CLB TO THE DH8 TO 15000 FT; PLACING IT IN CONFLICT WITH THE CL60 APCHING FROM HIS 2 O'CLOCK POS WITHOUT REALIZING THE DANGER. THE ARR CTLR HAD MADE AN AGREEMENT WITH THE OTHER ARR SECTOR WORKING THE BFI FINAL TO RETAIN THE ACFT AND WORK HIM IN ON A VISUAL APCH. THE ARR W CTLR RAN ACROSS THE ROOM AND PHYSICALLY POINTED OUT THE CL60 TO ME WHEN HE REALIZED THAT I PROBABLY DIDN'T SEE IT. THEY WERE ABOUT 6 MI APART BY THEN; ON CONVERGING COURSES. I IMMEDIATELY TOLD THE DH8 TO LEVEL OFF AND ISSUED CONFLICTING TFC. THE DH8 ADVISED TFC NOT IN SIGHT AND THAT HE WAS COMMENCING TCAS DSCNT. HE REACHED 4400 FT MSL AT THE TOP OF HIS CLB. THE CL60 HAD REACHED 4800 FT WHEN IT INITIATED TCAS CLB. THIS WAS PROBABLY A COORDINATED ENCOUNTER WITH TCASII. I ISSUED AN ESCAPE VECTOR TO THE R AND A DSCNT CLRNC TO 3000 FT TO THE DH8. SEPARATION HAD ALREADY BEEN LOST BY THE TIME VISUAL SEPARATION HAD BEEN EFFECTED. I HAD ASKED FOR A HDOF APPROX 1/2 HR EARLIER; BUT LATER DECLINED IT; GIVING INTO SELF-IMPOSED PRESSURE TO KEEP THE RELIEF ROTATION GOING RATHER THAN ADDING ANOTHER PERSON TO THE FLOOR.