A MD80 CAPT REPORTS MODERATE TURB AT FL330 DOWN TO 17000 FT WHERE SEVERE TURB BEGAN DOWN TO 15000 FT WHERE SMOOTH AIR PREVAILED. PAX AND CREW WERE PREVIOUSLY SEATED.

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WE HAD BEEN RECEIVING RPTS ABOUT LIGHT TO MODERATE TURB IN THE DSCNT TO ZZZ. I HAD THE FLT ATTENDANTS PREPARE THE CABIN FOR LNDG AND TAKE THEIR JUMPSEATS ABOUT 50 MIS PRIOR TO LNDG; BEFORE WE STARTED THE DSCNT FROM FL330. WE STARTED PICKING UP MODERATE TURB AT FL330 AND REQUESTED LOWER. WE DSNDED AND LEVELED AT FL270 BUT WERE IN THE SAME CONDITIONS; CONSTANT MODERATE TURB. WE ASKED FOR LOWER AGAIN. CTR SAID WE'D HAVE TO GO DOWN TO 17000 FT TO FIND ANYTHING REASONABLY SMOOTH. WE TOOK IT. 17000 FT WAS PRETTY GOOD INITIALLY. THE INTERESTING THING ABOUT THIS IS THE WX WAS BEAUTIFUL. IT WAS A BEAUTIFULLY CLR WINTER SKY. THE WINDS WERE SURPRISINGLY LIGHT; APPROX 25 KTS AT ALL ALTS INCLUDING THE HIGHER FLT LEVELS. ABOUT 30 MI W OF ABC; WE WERE SUDDENLY HIT BY SEVERE TURB. THE AUTOPLT AND AUTOTHROTTLES KICKED OFF; THE ACFT WAS MOVING UP AND DOWN THROUGH ABOUT 200 FT OF ALT. IT WAS BARELY CTLABLE. THE INSTS WERE UNREADABLE. THE FO WAS FLYING AND HAD HIS HANDS FULL JUST TRYING TO KEEP THE WINGS LEVEL. I TOLD ATC WE WERE IN SEVERE TURB AND WERE MAKING AN IMMEDIATE DSCNT. WE WERE CLRED TO 14000 FT. AT APPROX 16000 FT WE ENCOUNTERED MORE SEVERE TURB. WE FLEW INTO SMOOTH AIR AT ABOUT 15000 FT. I WAS VERY THANKFUL THAT WE'D RECEIVED WORD OF THE ROUGH RIDE IN THE DSCNT AND HAD PREPARED THE CABIN FOR LNDG EARLY.