AN ACR MLG CREW WAS UNABLE TO LINE UP WITH THE RWY CTRLINE DUE TO POOR BRAKING AND FREEZING RAIN.

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WE DEPARTED THE GATE AT APPROX AM15 AFTER BEING DEICED. LIGHT FREEZING RAIN HAD BEEN FALLING FOR ABOUT 45 MINS AND WE WERE DEICED AGAIN AT THE HOLD SHORT POS FOR 13L. WE WERE CLRED FOR TKOF AS SOON AS THE DEICING CREW FINISHED. WE HAD BRIEFED; AND THE CAPT BRIEFED THE PAX THAT WE WOULD BE TAXIING INTO POS AND RUNNING THE ENGS UP PRIOR TO RELEASING THE BRAKES FOR TKOF. AT THIS POINT; THE ONLY BRAKING RPT WE HAD RECEIVED WAS 'FAIR' RPTED BY LNDG ACFT. OUR TAXI FROM THE RAMP TO THE END OF THE RWY WAS NORMAL. WHILE WE WERE TAXIING INTO POS AND AS WE STARTED OUR TURN TO LINE UP ON RWY 13L CTRLINE; THE ACFT STARTED SLIDING TO THE R. THE NOSEWHEEL ENTERED THE SNOW ON THE R EDGE OF THE RWY AND THE ACFT CAME TO A GENTLE STOP HDG 145 DEGS. THE NOSEWHEEL WAS PARTIALLY OFF THE PAVEMENT AND IN SNOW APPROX 15 INCHES DEEP. THE MAIN WHEELS WERE ABOUT HALF WAY BTWN 13L CTRLINE AND THE R EDGE OF THE RWY. OUR TAXI SPD NEVER EXCEEDED MORE THAN A COUPLE MPH AS WE TOOK THE RWY. I DON'T THINK WE COULD HAVE DONE ANYTHING TO PREVENT WHAT HAPPENED SHORT OF STAYING AT THE GATE; WHICH WE WOULD HAVE; HAD BRAKING RPTS BEEN 'NIL.' OUR PAX WERE DEPLANED AND RETURNED TO THE TERMINAL BY BUS AFTER THE TXWYS WERE DEICED AND SANDED. THE ACFT WAS THEN TOWED BACK TO THE GATE. FACTORS: BRAKING ACTION: 'NIL;' LAST RPTED AS 'FAIR.' XWIND: 080 DEGS/08. AMOUNT OF ICE AT THE TKOF END OF RWY 13L.