ACR FLC IN B727 ATTEMPTED TO CLB ABOVE ADVERSE WX BUT WERE UNABLE TO MAINTAIN THEIR CLB TO THE ASSIGNED ALT BECAUSE OF THEIR WT AND USE OF ENG ANTI-ICE.

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WE WERE ON A FLT FROM MEM DEPARTING TO THE W. THERE WAS A LINE OF TSTMS ABOUT 20 MI WIDE; 70 MI TO THE NW OF MEM. IN OUR CLBOUT TO OUR ASSIGNED ALT OF FL350; WE NOTICED SOME VERY INTENSE ECHOES BEING DISPLAYED ON OUR RADAR SCREEN AT ABOUT 20-30 MI AHEAD OF OUR POS. IN THE MOONLIGHT WE COULD SEE THAT THESE TSTM CLOUD TOPS WERE BUILT UP TO A HIGHER ALT THAN OURS. BASED ON OUR WX RADAR PRESENTATION WE DECIDED TO ASK ATC FOR A DEV TO THE R (N) OF OUR COURSE. CTR APPROVED THE DEV. WE FELT THIS WOULD BE THE BEST WAY TO GO SINCE THE AREA TO THE L (S) OF OUR COURSE SEEMED MORE CONCENTRATED WITH TSTMS. HOWEVER; AS WE TURNED; WE NOTICED WE WOULD HAVE TO FLY BTWN 2 TSTM CELLS WHOSE ECHOES APPEARED TO BE ABOUT 20 MI APART. AS WE CONTINUED; THE CLOUD TOPS CONTINUED TO RISE AND WE ASKED ATC IF WE COULD CLB TO A HIGHER ALT OF FL390. CTR APPROVED THE ALT CHANGE AND ISSUED US A CLRNC TO CLB AND MAINTAIN FL390. DURING THE CLB WE NOTICED THE OUTSIDE AIR TEMP GAUGE HAD DECREASED TO A POINT IN WHICH WE NEEDED TO TURN ON ENG ANTI-ICE. TO PREVENT ICE BUILD-UP ON THE ENGS (AT THIS TEMP IN THE CLOUDS) WE HAD TO ACTIVATE THE ENG ANTI-ICE SYS PRIOR TO ENTERING THE CLOUDS. ALSO DURING THE CLB; WE EXPERIENCED LIGHT TO MODERATE TURB. IN TRYING TO CLB OVER THESE STORMS; WE INITIALLY USED A HIGHER RATE OF CLB; HOWEVER; OUR AIRSPD BEGAN DECREASING. SO BTWN FL380 TO FL390 IN ORDER TO MAINTAIN OUR AIRSPD ABOVE THE MINIMUM BUFFET SPD; OUR RATE OF CLB STOPPED. WE HAD TO LEVEL OFF AND START A SLIGHT DSCNT TO ALLOW THE AIRSPD TO GRADUALLY BUILD BACK UP SO WE COULD CONTINUE OUR CLB TO FL390. (AFTER DSNDING TO FL383 IT TOOK A COUPLE OF MINS FOR THE AIRSPD TO BUILD UP SO WE COULD CONTINUE THE SLOW CLB TO FL390.) AFTER REACHING FL390; WE WERE ON TOP OF ALL THE WX AND CONTINUED WITH THE EST OF THE FLT WHICH WAS UNEVENTFUL. WHAT I BELIEVE CAUSED THIS PROB WAS NOT USING OUR ONBOARD WX RADAR MORE THOROUGHLY TO ALLOW US TO CIRCUMNAV AROUND THESE STORMS AT A MUCH FURTHER DISTANCE. (WE COULD HAVE CHANGED RANGE AND TILT SETTINGS MORE FREQUENTLY.) ALSO REALIZING THAT ONCE ENG ANTI-ICE IS TURNED ON; THERE IS LESS AVAILABLE THRUST BEING PRODUCED BY THE ENGS (ENG BLEED AIR IS USED TO OPERATE THE SYS). WITH LESS THRUST AVAILABLE; A LOWER RATE OF CLB IS NECESSARY IN ORDER TO MAINTAIN A CERTAIN MINIMUM AIRSPD. AND ASKING ATC FOR A 'BLOCK ALT' IF YOU'RE UNABLE TO REACH YOUR ASSIGNED ALT. TO PREVENT A RECURRENCE; I BELIEVE CREWS SHOULD FULLY USE THEIR ONBOARD WX RADAR SYS TO DETERMINE THE BEST COURSE OF ACTION TO CIRCUMNAV TSTMS. ALSO; WHEN USING ENG ANTI-ICE; MAINTAIN A LOWER RATE OF CLB TO ENSURE THE AIRSPD WILL NOT BLEED OFF TO THE POINT OF HAVING TO LEVEL OFF BEFORE YOU REACH YOUR ASSIGNED ALT; AND TO ASK ATC FOR A BLOCK ALT CLRNC IF YOU SEE THAT YOU ARE NOT GOING TO REACH YOUR ASSIGNED ALT.