B727 CREW ENCOUNTERED A TSTM AFTER TKOF.

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WHILE TAKING OFF FROM IAH A STRONG TSTM WAS APCHING FROM THE NW. WE WERE USING RWY 15 AND GIVEN AN IMMEDIATE R TURN TO 360 DEGS AND CLB TO 4000 FT. THE CAPT WAS FLYING. I WAS WORKING THE RADIOS; RADAR AND MONITORING THE INSTS. UPON LEVELING AT 4000 FT MSL; THE CTLR SOUNDED VERY URGENT AND SAID TO TURN BACK L TO 100 DEGS AND CLB TO 5000 FT. WE BEGAN TO EXPERIENCE STRONG TURB. I LOOKED OUT THE WINDOW AND SAW A VERY STRONG LOOKING CELL AND WE PENETRATED IT. I ANNOUNCED WE WERE OUT OF 4000 FT FOR 5000 FT AND NOTICED WE HAD APPROX WENT FROM A 500 FPM CLB RATE TO ONE APCHING OVER 2500 FPM. I VERBALLY SAID LEVEL AT 5000 FT. AT 5100 FT MSL I SAID WE NEED TO LEVEL AT 5000 FT. THEN IT SEEMED AN UPDRAFT CAUGHT US AND I PUSHED FORWARD ON THE YOKE AND ANNOUNCED AGAIN PUSH IT OVER. I COULD NOT TELL IF THE CAPT WAS PULLING BACK AGAINST ME BUT IT FELT LIKE IT. AT APPROX 5500 FT MSL WE IMMEDIATELY BEGAN TO DSND AND LEVEL AT 5000 FT. THE CTLR CAME ON AND SAID WE NEED TO BE AT 5000 FT. CONTRIBUTING: THE VERY STRONG CELL MOVING INTO THE ARPT AREA. THE INSTRUCTIONS FROM THE CTLR TO TURN FROM 150 DEGS TO 360 DEGS AND THEN IMMEDIATELY BACK TO 100 DEGS. THE CTLR'S CONCERN ABOUT WHAT WE WERE ABOUT TO FLY INTO HEIGHTENED OUR CONCERN. I BEGAN TO FOCUS ON THE CELL AND TRY TO GET THE RADAR TO 'PAINT IT' AND TOOK MY ATTN FROM WATCHING THE INSTS TO BACK UP THE CAPT. ALSO; THIS IS A NEW CAPT AND I AM A NEW FO WITH THE COMPANY. I JUST FINISHED IOE AND HAVE NOT BEEN FLYING FOR APPROX 9 MONTHS. THE CTLR'S WARNINGS OF WINDSHEAR IN THE NW QUADRANT HAD ME FOCUSING ON THE WX OUTSIDE MORE THAN THE INSTS AND THE CAPT'S PERFORMANCE INSIDE THE ACFT.