FLC OF A B757 OVERSHOT CLB ALT DUE TO THE CAPT MISTAKENLY SETTING THE FMC ALT SELECTOR TO THE WRONG ALT. ATC INTERVENED AND IMMEDIATELY ASSIGNED A HIGHER ALT.

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AS WE WERE CLBING THROUGH ABOUT 11000 FT ON A HDG OF 060 DEGS; THE CTLR SAID THESE APPROX WORDS; 'ACR X IN ABOUT 5 MI I WILL GIVE YOU A TURN TO 110 DEGS; EVERYONE ELSE HAS GONE THROUGH THAT WAY WITH NO PROB.' RADAR SHOWED A LARGE; YELLOW AND RED CTRED CELL TO OUR R FRONT AND INSTEAD OF INITIALLY TURNING THE HDG KNOB; I TURNED THE ALT KNOB; IMMEDIATELY RETURNING IT TO 14000 FT; WHICH IS WHAT I BELIEVED TO BE OUR ALT ASSIGNMENT. I THEN MOVED THE HDG KNOB TO 110 DEGS WHICH WOULD HAVE TAKEN US THROUGH THE YELLOW AREA OF THE STORM; SO I ASKED FOR AND HAD APPROVED A 100 DEG HDG; WHICH WAS ACCOMPANIED BY A FREQ CHANGE AND INSTRUCTION TO TELL THE NEW CTLR OUR NEW HDG. I CHKED ON WITH THE NEW CTLR SAYING; '...HDG 100 DEGS LEVELING AT 14000 FT.' THE CTLR ASKED IF WE HAD BEEN GIVEN 14000 FT BY ZAU AND INSTRUCTED US TO CLB TO FL230. HE ALSO ASKED OUR AIRSPD AND; I BELIEVE; OUR HDG. THE EXCESS VERBIAGE MAKES ME THINK THAT WHEN I INITIALLY ERRONEOUSLY MOVED THE ALT KNOB I MAY HAVE RETURNED IT TO AN ALT TO WHICH WE WERE NOT ASSIGNED -- 13000 FT SOUNDS PROBABLE. OTHER CONTRIBUTING FACTORS ARE THE HIGH VOLUME OF RADIO TFC; LIGHT TO OCCASIONALLY MODERATE TURB; LIGHTNING; AND OUR EXTREMELY HIGH RATE OF CLB CAUSED BY COLD GROSS WT AND RELATIVELY COLD AIR; PLUS A RELATIVELY LOW EXPERIENCE LEVEL WITH THE B757; AND A VERY HIGH WORKLOAD SIT. IN ORDER TO PREVENT THIS SIT FROM HAPPENING; AS SOON AS I REALIZED THAT I HAD CHANGED THE ALT WINDOW ERRONEOUSLY; I SHOULD HAVE HAD THE FO AGGRESSIVELY REDUCE THE RATE OF CLB IN ORDER TO OBTAIN A POSITIVE ALT CHK FROM THE CTR CTLR.