B777 FLT CREW ENCOUNTERS MODERATE TO SEVERE TURB WHILE IN THE VICINITY OF GREENLAND ON THE ATLANTIC NAT TRACK SYS.

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APPROX 1 HR PRIOR TO ENCOUNTER; REYKJAVIK ATC ADVISED US OF THE POSSIBILITY OF MODERATE TO SEVERE TURB ALONG THE E COAST OF GREENLAND. ACFT TRANSITING THAT AREA HAD SO FAR NOT RPTED ANYTHING MORE THAN MODERATE AND THE DURATION WAS SHORT. WHILE I WAS SENDING A MESSAGE TO DISPATCH; THE FO ADVISED THE PURSER TO HAVE THE CABIN AND FLT ATTENDANTS READY FOR A POSSIBLE TURB EVENT IN 1 HR'S TIME. WHILE WAITING FOR A REPLY FROM DISPATCH; I PUT A TIME FIX ON THE MAP TO REMIND MYSELF TO MAKE A PA TO THE PAX GIVING THEM PLENTY OF TIME TO MAKE ANY EXCURSIONS AWAY FROM THEIR SEATS BEFORE I WANTED THEM STRAPPED IN FOR THE POSSIBILITY OF A BUMPY RIDE. DISPATCH CAME BACK WITH A NEW SIGMET FOR POSSIBLE SEVERE TURB AND WHEN WE PLOTTED THE COORDINATES ON THE MAP; WE WERE GOING TO PASS THROUGH THE CTR OF THE AREA. DISPATCH ALSO ADVISED THAT SOME COMPANY ACFT HAD PASSED THROUGH THE AREA WITH NOTHING MORE THAN LIGHT CHOP. WE CONSIDERED TRYING TO GET A RTE CHANGE N OF THE AREA BUT WITH THE PIREPS BEING MUCH LESS THAN THE SIGMET CALLED FOR; AND THE TIME IT WAS GOING TO TAKE TO COORDINATE A NEW RTE; WAS PROBABLY GOING TO PUT US THROUGH A CORNER OF THE BOX ANYWAY. WE STAYED WITH THE ORIGINAL RTE. 20 MINS PRIOR TO THE AREA; I MADE A PA ANNOUNCEMENT GIVING THE PAX A 10 MIN WARNING FOR THE SEATBELT SIGN. I SHOULD ADD THAT UP TO THIS POINT; THE FLT HAD BEEN SMOOTH WITH JUST SOME VERY LIGHT TURB OF SHORT DURATION. THE SEATBELT SIGN CAME ON 10 MINS LATER AND AS THE PLTS WERE DUE FOR A CREW CHANGE WITHIN THE NEXT FEW MINS; I ASKED THE PURSER TO ADVISE THE PLT IN THE BUNK TO STAY THERE UNTIL WE WERE THROUGH THE AREA AND ADVISED HER I WOULD BE SEATING THE FLT ATTENDANTS IN A MIN OR TWO. AT THAT POINT; I PLACED THE ENG IGNITION ON AND SLOWED TO TURB PENETRATION SPD. I MADE THE 'FLT ATTENDANTS BE SEATED FOR TURB' ANNOUNCEMENT AND SHORTLY AFTERWARDS; WE BEGAN ENCOUNTERING CONTINUOUS LIGHT CHOP. A FEW MINS LATER; THE INTENSITY INCREASED TO OCCASIONAL MODERATE; THEN TO CONTINUOUS MODERATE. THE AUTOTHROTTLES WERE BEGINNING TO LAG BEHIND THE AIRSPD FLUCTUATIONS; SO I HAD TO PROVIDE FASTER REACTIONS KEEPING THE CHANGES SMOOTH SO AS TO NOT INDUCE A COMPRESSOR STALL. I DID NOT TURN OFF THE AUTOTHROTTLES. FOR THE NEXT 10 MINS; THE BASE RIDE WAS CONTINUOUS MODERATE WITH MULTIPLE 10-15 SECOND ENCOUNTERS WITH WHAT CAN ONLY BE DESCRIBED AS SEVERE. BINDERS WERE TOSSED AROUND THE COCKPIT. WE EXPERIENCED +/-30 KT GAINS AND LOSSES IN IAS; HAD AT LEAST 3 MOMENTARY AIRSPD EXCURSIONS 15 KTS BEYOND VMO; 150 FT ALT VARIATIONS; STRONG SIDE GUSTS; ETC. AFTER 10 MINS; THE RIDE QUICKLY IMPROVED BACK TO LIGHT CHOP TO TAPERING BACK TO SMOOTH OVER THE NEXT 20 MINS. I CALLED THE PURSER WHEN I FELT IT WAS SAFE TO GET UP AND SHE ADVISED THAT SHE WOULD CHK WITH THE OTHER FLT ATTENDANTS AND PAX AND RPT ANY PROBS. NONE WERE FOUND AND WHEN I LEFT THE COCKPIT A SHORT TIME LATER TO GO ON BREAK; MOST OF THE FIRST CLASS CABIN WAS ASLEEP. NO OTHER FEEDBACK FROM PAX OR CREW.