AN EMB120 IN THE HOLDING PATTERN CLBS TO AVOID A CLBING C414 DEPARTING VFR IN IMC CONDITIONS FROM JACKSON HOLE; WY.

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THE CAPT AND I WERE HOLDING; AWAITING CLRNC FOR THE VOR DME APCH INTO JACKSON HOLE; WY. WE WERE ASSIGNED 16000 FT MSL WHILE AN ACFT WAS ON THE ILS APCH; 16000 FT BEING NECESSARY FOR US FOR THE ILS MISSED APCH PROC FOR THE INBOUND ACFT. WE WERE MONITORING THE JACKSON UNICOM/CTAF AS WELL TO LISTEN FOR THE PROGRESS OF THE INBOUND AIRPLANE. ANOTHER AIRPLANE; A C414 ON THE GND AT JACKSON; CALLED ON CTR FREQ LOOKING FOR IFR DEP CLRNC AND WAS INFORMED OF THE DELAY DUE TO INBOUND TFC. THE C414 RPTED THAT HE WOULD DEPART VFR AND PICK UP HIS CLRNC AFTER DEP. WE MONITORED HIS CALL ON UNICOM/CTAF THAT HE WAS DEPARTING RWY 18; WHICH WOULD PUT HIM HDG IN OUR DIRECTION. SHORTLY AFTERWARD; CTR ADVISED US OF VFR TFC. I RPTED NEGATIVE CONTACT AND THAT WE WERE IN IMC. THE TARGET APPEARED ON OUR TCASI; ESCALATING IN THREAT FROM THE OPEN WHITE CIRCLE; TO CLOSED WHITE; TO CLOSED YELLOW AND WE RECEIVED THE AURAL 'TFC' ADVISORY. THE CAPT; PF; DISCONNECTED THE AUTOPLT TO INITIATE A CLB AND I RPTED TO ATC THAT WE WERE CLBING. AT APPROX 16300 FT THE CAPT GOT VISUAL CONTACT WITH THE C414 AND OBSERVED IT AT AN ESTIMATED 700 FT AWAY AND AN ESTIMATED 35 DEG BANKED TURN FOR AVOIDANCE. I NEVER SAW THE ACFT; BUT I DID SEE THE CAPT GET PRETTY WIDE-EYED. THE C414 HAD CLRLY BEEN FLYING IN IMC WITHOUT A CLRNC. THE ARPT MAY HAVE BEEN VFR; AND INDEED WAS UPON OUR ARR; BUT HOLDING AT 16000 FT WE WERE AT FAR LESS THAN VFR VISIBILITY. JACKSON HOLE ARPT IS VERY BUSY AND VERY DELAYED WHEN THE WX IS DOWN. THE CTR CTLRS WORK VERY HARD AND PROVIDE GOOD SVC. ALL SORTS OF AIRPLANES; INCLUDING CPR JETS TO B757S; FLY IN AND OUT OF THIS UNCTLED ARPT. PERHAPS MORE COULD BE DONE TO IMPROVE IFR TFC FLOW. AND PERHAPS PLTS OUGHT NOT FLY WITHOUT A REQUIRED CLRNC.