ATC TRAFFIC VECTORS AND AN INTENSE THUNDERSTORM ON THE VECTOR PATH LEAD TO AN EMERGENCY DECLARATION BY B757-200 FLT CREW TO AVOID THE CELL.

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TRACKING INBOUND TO SJU VOR; SJU APCH REQUESTED THAT WE TURN L TO 270 DEGS FOR TFC. THERE WAS A LARGE CELL ON THAT HDG; AND I TOLD ATC THAT WE WOULD MAKE THE TURN; BUT COULD ONLY MAINTAIN THAT HDG FOR A FEW MINS BEFORE NEEDING A TURN BACK TO THE R TO AVOID THE CELL. THE TSTM WAS PAINTING ALMOST SOLID RED ON RADAR; AND WAS QUITE LARGE. I SAW TFC PASS BY ON TCAS AFTER A FEW MOMENTS; AND REQUESTED A R TURN AWAY FROM THE CELL. ATC DENIED THE TURN; AND ADVISED THAT THERE WAS YET ANOTHER ACFT APCHING ON OUR R. HE ALSO ADVISED THAT PREVIOUS ACFT HAD FLOWN THROUGH THE CELL WITH NO PROBS. BY THIS TIME; THE CELL WAS DIRECTLY IN FRONT OF US. I DECLARED AN EMER; ADVISED ATC THAT WE WERE MAKING AN IMMEDIATE R TURN AWAY FROM THE CELL; AND INSTRUCTED MY FO TO TURN N IMMEDIATELY. WE WERE UNABLE TO COMPLETELY AVOID PENETRATING THE CELL; AND ENCOUNTERED MODERATE TURB IN THE TURN. FORTUNATELY; THE FLT ATTENDANTS HAD COMPLETED THEIR LNDG PREPARATIONS BY THIS POINT; AND WERE IN THEIR SEATS. I WILL BE EXTREMELY RELUCTANT TO ACCEPT ANY SUCH CLRNC FROM ATC IN THE FUTURE -- ABSOLUTELY UNACCEPTABLE!