MD80 EXPERIENCES CLOSE ENCOUNTER WITH TRAFFIC DEVIATING WX WHO WAS AT THE SAME FLIGHT LEVEL TO WHICH THEY WERE DESCENDING. ATC ADVISED HE WAS NOT TALKING TO THE TRAFFIC.

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ATC GAVE US INSTRUCTIONS TO DSND FROM FL360 TO FL350. AS WE STARTED DSCNT; WE NOTICED OPPOSITE DIRECTION TCAS TFC HDG DIRECTLY TOWARDS US AT FL350. I THINK THE TFC DISTANCE WAS THEN ABOUT 10-14 MI ON THE TCAS SCALE. MY INITIAL THINKING WAS THAT THIS TFC SHOULD ALSO BE DSNDING. AS WE PASSED ABOUT FL354; WE NOTICED THAT THE TFC WAS NOW HIGHLIGHTED ON THE TCAS DISPLAY; AND WE WERE ON A PERFECT HEAD-ON COLLISION COURSE. DURING THIS TIME WE WERE UNABLE TO QUESTION THE CTLR DUE TO CONSTANT RADIO CHATTER. THE TCAS TFC WARNING ALERT THEN SOUNDED; THEN AFTER ABOUT 2-3 SECONDS; TCAS TFC CLB RA WARNING SOUNDED. AT THE TIME OF THE RA; WE WERE IN AN APPROX 600-800 FPM DSCNT. I THINK THE RANGE TO TFC WAS NOW ABOUT 5 MI AND CLOSING FAST AND TCAS SHOWED THREAT ACFT ABOUT 300 FT BELOW US. I IMMEDIATELY DISCONNECTED THE AUTOPLT; AND STARTED THE REQUIRED CLB. AS WE RE-LEVELED AT FL360; THE CTLR THEN REALIZED THE CONFLICT AND WITH URGENCY IN HIS VOICE; HE INSTRUCTED US TO MAINTAIN FL360. I BELIEVE HE ALSO INITIALLY ASKED US TO TURN ABOUT 80 DEGS L; TOWARDS THE W. AS THE TFC ZOOMED UNDERNEATH AND PAST US; I EXPLAINED TO THE CTLR THAT HIS EARLIER INSTRUCTION TO HAVE US DSND TO FL350 PLACED US DIRECTLY INTO CONFLICT WITH HEAD-ON TFC. THE CTLR RESPONDED 'THE OTHER ACFT WAS DEVIATING AND THAT HE WAS NOT TALKING WITH HIM.' THE CTLR THEN APOLOGIZED AND SAID IT WAS HIS MISTAKE. I TOLD CTLR THAT 'THE TCAS SAVED US TODAY.' AS I REFLECT MORE ON THE INCIDENT; I FIRMLY BELIEVE THAT TCAS DID LIKELY PREVENT POSSIBLE TRAGEDY SINCE THE WX WAS IMC AND THE CTLR DID NOT AMEND OUR INSTRUCTIONS UNTIL THE THREAT ACFT WAS VIRTUALLY PASSING UNDER US -- WITH NO TIME TO REACT. SIDE NOTE: DURING THE RAPID TRANSITION FROM DSCNT TO CLB; WE DID ENCOUNTER A BRIEF 3-4 SECOND G LOAD INDUCED (MODERATE) AIRFRAME BUFFET; WHICH FELT MUCH LIKE THE HIGH ALT STALL BUFFET TRAINING WE GET IN THE SIMULATOR. I DO NOT BELIEVE ANY LIMITATIONS WERE EXCEEDED; HOWEVER; A LOGBOOK ENTRY WAS MADE AS PRECAUTION.