CAPT OF A B767 DECLARED AN EMER DUE TO TURB AND DSNDED 1000 FT AFTER ARTCC CTLR DENIED DSCNT.

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WE WERE DSNDING INTO JFK ON THE LENDY ARR. WE WERE ADVISED BY ZNY THAT THERE HAD BEEN RPTS OF MODERATE TURB BELOW FL240. WE HAD THE FLT ATTENDANTS SEATED AND THE SEATBELT SIGN ON PRIOR TO REACHING FL240. CLRED TO FL180; WE BEGAN TO EXPERIENCE THE TURB AS ADVISED. AS WE LEVELED OFF AT FL180; STILL IN THE MODERATE TURB; CAPT REQUESTED A LOWER ALT DUE TO THE RIDE. ZNY SAID THAT IT MIGHT BE A MIN OR 2 DUE TO WBOUND DEPS. CAPT THEN DECLARED AN EMER AND ADVISED ATC THAT WE WERE DSNDING TO 17000 FT. ZNY ACKNOWLEDGED; THEN CLRED US TO 17000 FT. DURING THE DSCNT TO 17000 FT; WE DID NOT SEE ANY CONFLICTING TFC VISUALLY OR ON TCASII. SHORTLY THEREAFTER; WE WERE HANDED OFF TO NEW YORK APCH. APCH ASKED US IF WE WERE STILL AN EMER ACFT; TO WHICH CAPT RESPONDED NO; THE RIDE HAD IMPROVED SUBSTANTIALLY. THE REST OF THE APCH AND LNDG OCCURRED WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 492421: THE REQUEST WAS DENIED. I THEN ADVISED NEW YORK THAT DUE TO MODERATE TURB WE REQUIRED A LOWER ALT AND IF THEY WERE UNABLE I WOULD DECLARE AN EMER AND DSND TO 17000 FT. I BELIEVE 17000 FT WAS OUT OF THEIR AREA OF CTL. OUR TCASII SHOWED NO CONFLICTING TFC AT 17000 FT OR CLBING TFC THAT COULD BE A PROB; AND I DECLARED AN EMER AND DSNDED TO 17000 FT. LEAVING ABOUT 17700 FT; THE MODERATE TURB DECREASED TO UPPER LIGHT TURB. WE WERE HANDED OFF TO APCH CTL. THE MODERATE TURB DUE TO A LOW LEVEL JET STREAM WAS ONLY BTWN FL200 AND FL180.