AN A300 FLT CREW TURN OFF THE AUTOPLT TO AVOID A TSTM CELL RESULTING IN A 600 FT DEV FROM ASSIGNED ALT.

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WE DEPARTED ZZZ ENRTE TO KMEM. A LONG LINE OF TSTMS A COUPLE OF HUNDRED MI LONG RAN SW/NE BTWN OUR POS AND MEM. OUR RADAR DEPICTED A LOT OF ATTENUATION SO I TOLD THE FO (PF) TO CONTINUE ON OUR PRESENT HDG TO PARALLEL THE WX. I INFORMED ATC THAT WE NEEDED TO CONTINUE ON OUR HDG FOR ANOTHER 100-200 MI BEFORE WE COULD TURN S DIRECT GQE AS CLRED. WE WERE LEVEL AT FL260. OUR RADAR SHOWED A BREAK IN THE LINE OF CELLS/WX; SO DIRECT GQE WAS NOW POSSIBLE. ATC DSNDED US TO FL240 AND TURNED US TO A 190 DEG HDG DIRECT GQE WITH A CLRNC TO DEVIATE AS NECESSARY FOR WX. WE WERE IMC WHEN WE MOMENTARILY BROKE OUT ONLY TO SEE A BUILD-UP DEAD AHEAD AND .25 MILE. I LOOKED DOWN TO ADJUST THE RADAR TILT WHEN I HEARD THE AUTOPLT CHIME SOUND AS THE FO HAD DISCONNECTED THE AUTOPLT AND WENT INTO A LEFT BANK OF 45 DEGREES TO FLY CLR OF THE CELL. WE PENETRATED THE CELL AND EXPERIENCED A STRONG UPDRAFT AND STARTED A CLB. THE ALT ALERT HORN AND THE AURAL COMMAND OF 'BANK ANGLE; BANK ANGLE' WAS VERY DISTRACTING; COUPLED WITH MOD CHOP FOR THE NEXT 15 TO 30 SEC; UNTIL THE FO LEVELED THE WINGS AND DESCENDED FOR 24600 FT TO OUR CORRECT ALT IF FL240. IN THE MIDDLE OF THE CONFUSION; WE WERE GIVEN A CROSSING RESTR TO CROSS. 10NM W OF GQE AT 10000 FT; WHICH WE COMPLIED WITH. REST OF FLT UNEVENTFUL.