B747-400 CREW WAS REQUIRED TO DECLARE AN EMER TO AVOID FLT INTO A TSTM CELL IN PACIFIC INTL AIRSPACE.

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FLT WAS IN THE VICINITY OF CONVECTIVE ACTIVITY ALONG TRACK #2 FOR THE DATE INDICATED. WE HAD REQUESTED DEV PREVIOUS TO THIS INCIDENT AND HAD BEEN TOLD 'UNABLE' VIA CPDLC. WE DEVIATED ACCORDING TO PACIFIC HI 1 CHART PROCS AT THAT TIME AND REMAINED WITHIN 7.5 NM OF TRACK. AT XA57Z WE ADVISED TOKYO (RJTG) THAT WE WOULD REQUIRE DEV TO THE R OFF COURSE FOR WX AND WOULD DECLARE AN EMER IF NECESSARY TO DEVIATE. WE GOT NO RESPONSE AND AT XB00Z WE SENT THE CPDLC EMER PAGE SAYING WE WERE DEVIATING 20 NM R OFF COURSE; AND WE BEGAN THE DEV. AT XB02Z WE GOT A RESPONSE BACK SAYING 'DEV L 20' WOULD BE APPROVED; STATE INTENTIONS. WE ADVISED THAT WE HAD DECLARED EMER AND WERE DEVIATING R. UPON REACHING 17.5 NM R OFF COURSE WE ADVISED THAT WE WERE THAT FAR R OFF COURSE XB04Z. WE BEGAN RETURNING TO COURSE AFTER PASSING THE CELL. AT XB16Z WE RPTED BACK ON COURSE AND ADVISED THAT THE 'EMER WAS OVER' VIA CPDLC. OUR FLT AVOIDED THE WX WITHOUT INCIDENT. WE ALSO HAD RADIO CONTACT WITH 2 OTHER COMPANY FLTS IN OUR VICINITY (BOTH VISIBLE ON TCASII) AND FOUND THAT THEY TOO WERE HAVING TROUBLE GETTING DEV CLRNCS FROM RJTG. WE KEPT DISPATCH ADVISED OF WHAT WAS HAPPENING DURING THIS ENTIRE EVENT VIA ACARS.