CREW OF B772 REACTED TO DETERIORATION OF ENRTE ETOPS ALTERNATE WX AT CDB BY COORDINATING AMENDED ETOPS RELEASE CRITERIA WITH DISPATCH.

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AT THE TIME OF DISPATCH FROM RJAA; CDB WAS THE SECOND ENRTE ALTERNATE. WE WERE DISPATCHED 180 MINS ETOPS. ETOPS WX SENT BY DISPATCH WHILE ENRTE AND PRIOR TO ENTERING THE ETOPS AREA; SHOWED A FORECAST TEMPORARY CONDITION OF 500 FT OVCST AND 1/2 SM FOG. WE ASKED DISPATCH IF HE HAD ANYTHING BETTER AND HE REPLIED THAT HE DID NOT. LNDG MINIMUMS FOR ILS RWY 14 AT CDB ARE 200 FT AND 1/2 MI VISIBILITY; SO CDB REMAINED A SUITABLE ENRTE ALTERNATE SINCE WE WERE NOW IN THE ENRTE PHASE. AS WE APCHED PET1; WE AGAIN CHKED THE CDB WX AND FOUND THAT IT HAD DROPPED TO 1/4 SM FOG. CDB WAS NO LONGER A LEGAL ALTERNATE. WE IMMEDIATELY CONTACTED DISPATCH AND COORDINATED A CHANGE TO 207 MIN ETOPS USING HKD AND ANC. WX AT BOTH HKD AND ANC WAS SUITABLE. WX AT PKC AND DYR WAS OK; HOWEVER; BOTH WERE NOTAMED CLOSED. SYA AND ADK WX WAS BELOW MINIMUMS. WE DISCOVERED THAT CDB WAS BELOW MINIMUMS AT XA00Z JUST PRIOR TO PET1 (XA06Z). COORD OF THE NEW ALTERNATES TOOK SEVERAL MINS; SO WE MAY HAVE TEMPORARILY NOT HAD A LEGAL ALTERNATE ALTHOUGH WE WERE ALWAYS WITHIN 207 MINS OF HKD AND ANC. IN RETROSPECT; I FEEL THAT IT WOULD HAVE BEEN PRUDENT FOR ME TO HAVE INSISTED THAT 207 MINS ETOPS BE CONSIDERED RATHER THAN ACCEPTING A MARGINAL ENRTE ALTERNATE.