B757-200 FLT CREW WAS UNABLE TO OBTAIN CLRNC TO DEVIATE AROUND TSTM IN OCEANIC AIRSPACE. FLT CREW DECLARED AN EMER AND INITIATED CLIMB TO AVOID THE WX.

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APCHING ENRTE FIX NBOUND; WE PAINTED A LINE OF WX OVER 200 NM LONG FOR WHICH A SIGMET HAD ALREADY BEEN ISSUED. IT EXTENDED OVER 100 NM EITHER SIDE OF COURSE. THE TOPS OF THE TSTMS WERE GROWING. PREVIOUS ACFT RPTED TOPS NEAR FL340 ABOUT 30 MINS PRIOR. JUST TO ENSURE THAT WE HAD AN ALTERNATIVE PLAN OTHER THAN TO PLOW THROUGH IT; WE REQUESTED; VIA HF FROM ATC; EITHER DEV L OR R 100 NM OR A CLB TO A HIGHER ALT. THE ONLY CLRNC THAT ATC WOULD GIVE US WAS TO DEVIATE 20 NM R OF COURSE. THAT ENDED UP NOT BEING A VALID ALTERNATIVE. AS WE APCHED THE STORM SYS; IT APPEARED WE WOULD BE UNABLE TO TOP THE STORMS AT THE ALT WE WERE AT; WHICH WAS FL360. WE BROADCASTED IN THE BLIND ON GUARD AND CLBED TO FL375. WE WERE ABLE TO TOP THE WX BY NO MORE THAN 1000 FT. LIGHTNING HAD APPEARED TO EXIT THE CLOUDS IN THIS SYS UPWARDS VERTICALLY. THE SYS WAS ONLY ABOUT 10 MI ACROSS. WE DSNDED BACK TO OUR ORIGINAL CLRNC OF FL360 AFTER ONLY BEING AT FL375 FOR APPROX 2 MINS. THIS IS THE SECOND TIME IN ABOUT 2 YRS THAT WX HAS BUILT IN THIS SAME AREA AND ATC HAS BEEN UNABLE TO ASSIST AS FAR AS PROVIDING DEV CLRNC. BOTH TIMES WE HAVE HAD TO DEVIATE FROM A CLRNC EXERCISING CAPT'S EMER AUTH.