ACR WDB ALT DEVIATION EXCURSION FROM CLRNC ALT IN TSTM ACTIVITY.

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HOLDING FOR AN INSTRUMENT APCH AFTER A TRANSATLANTIC FLT; WE WERE ASSIGNED 13000' AT ERICK INTXN. WX WAS POOR WITH A COLD FRONT PASSING THROUGH THE AREA; RAIN SHOWERS; THUNDER SHOWERS; LLWS FORECAST ALONG WITH TURB. WHILE HOLDING; WE FLEW THROUGH A STRONG UPDRAFT (IN A TURN) WHICH CAUSED AN ALT DEVIATION OF APPROX 300'; WHICH WE CORRECTED IMMEDIATELY. OTHER ALT EXCURSIONS WERE NOT AS SEVERE AS THIS ONE. WE NOTIFIED ATC OF THIS AND THE REASON FOR IT. ANOTHER FLT IN THE AREA WITH DIGITAL COLOR RADAR (APPARENTLY) RPTED A CELL IN THE IMMEDIATE AREA AS 'PRETTY RED;' IE; MOST SEVERE. WE REQUESTED HOLDING IN AN AREA AWAY FROM THIS; AND ATC COMPLIED; BUT TURB AND UP; DOWN DRAFTS CONTINUED. ATC INFORMED US THAT WE 'HAD A POSSIBLE PLTDEV' AND ASKED US TO CALL ATC ON THE GND AT ABOUT THE TIME WE HIT THE UPDRAFT. OUR RADAR WAS FUNCTIONING BUT WASN'T SHOWING THE SEVERE CEIL THAT THE OTHER FLT REFERRED TO (OURS IS NON COLOR). WE DID SEE THE RAIN SHOWER HOWEVER. THERE WAS ALSO A DIFFERENCE BTWN THE CAPT'S AND F/O'S ALTIMETERS WHICH CONTRIBUTED TO THIS TO A CERTAIN EXTENT. IN SUMMARY; I THINK THIS IS TYPICAL OF A CONVECTIVE SITUATION AS WOULD BE ASSOCIATED WITH THE AFOREMENTIONED WX; AND ATC SHOULD AVOID ASSIGNING FLTS WITH ONLY 1000' OF ALT SEP DURING HOLDING; WHERE THEY ARE REQUIRED TO MANEUVER THE AIRPLANE UNDER THESE TYPES OF CONDITIONS.