A B757-200 EXECUTED A GAR ON FINAL TO JFK DURING A WINDSHEAR ENCOUNTER WITH SEVERE TURB. THE FLT DIVERTED TO AN ALTERNATE ARPT.

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MY FO WAS FLYING THE APCH AND WE WERE CLRED FOR THE APCH TO RWY 4R. HE HAD SOME DIFFICULTY SLOWING THE ACFT TO FLAPS 30 DEG SPD BECAUSE WE ENCOUNTERED A 38 KIAS TAILWIND ON FINAL. WE WERE CLRED TO LAND BY THE TWR INSIDE THE MARKER AND WERE GIVEN THE FIRST RPT OF A LOSS OF 20 KIAS ON FINAL AT 700-800 FT BY THE PRECEDING ACFT. WE STILL COULD NOT SLOW TO FLAPS 30 DEG EXTEND SPD AND THE SPD WAS HANGING AT 160-165 KIAS AT 1000 FT AGL IN THE WX. AT THAT TIME; I DIRECTED HIM TO GO AROUND. THE MISS WAS STARTED AND THE TWR DIRECTED THE MISS ALT AT 1500 FT AND A HDG OF 100 DEGS. WE WERE IN THE CLEAN-UP WITH FLAPS RETRACTING TO 1 DEG WHEN WE LOST APPROX 25 KIAS AND THE ACFT BEGAN DSNDING RAPIDLY WITH A GPWS TERRAIN WARNING OVERRIDING THE WINDSHEAR ENCOUNTER. WE STOPPED THE DSCNT AT APPROX 800 FT AGL AND THE AIRSPD RAPIDLY ACCELERATED TO APPROX 255 KIAS WITH THE FLAPS STILL AT 1 DEG. WE NOTIFIED THE TWR OF THE SHEAR AND THE SEVERE TURB WE ENCOUNTERED DURING THE GAR. ALL OTHER ACFT CEASED THE APCHS AND WE HELD UNTIL OUR DIVERT FUEL WAS REACHED AND WE WENT TO IAD. I RPTED THE OVERSPD VIA ACARS TO MAINT CTL AND WE TURNED THE ACFT AT IAD AND RETURNED TO JFK. THIS EVENT WAS NOT EXPECTED GIVEN THE NATURE OF THE WX IN AND AROUND JFK AT THE TIME OF THE APCH. THE STORM ACTIVITY INCREASED IN INTENSITY VERY QUICKLY AND CAUGHT US AND THE TWR OFF GUARD.