POTENTIAL CONFLICT WITH A JFK DEP ACR FEARED BY N90 APCH CTLR WHEN THE PIC OF AN ARR B737 CANNOT TURN TO ASSIGNED HEADING BECAUSE OF TSTM ACTIVITY NEAR HIS DIRECT COURSE TO LGA; NY.

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PROCEEDING DIRECT TO LGA VOR FROM ABOUT 210 DEG RADIAL AT 30 NM. LINE OF STRONG TSTMS MOVING W TO E WAS JUST TO THE L OF ARR CORRIDOR. REQUESTED DEVS E. APCH GRANTED DEVS UNTIL WITHIN ABOUT 20 NM OF LGA; THEN TOLD US WE NEEDED TO TURN L TO A 010 DEG HDG. THAT HEADING WOULD HAVE PUT US INTO A 'WALL OF RED' ON OUR RADAR SCREEN. APCH TOLD US WE HAD 12 O'CLOCK POS TFC CLBING OUT OF 3000 FT AND EMPHASIZED WE HAD TO MAKE THE TURN TO 010 DEGS. I KNOW JFK WAS A FACTOR; SO WE COULDN'T TURN RIGHT WHERE RADAR SHOWED EVERYTHING WAS CLR OF WX. I TOLD FO TO DECLARE AN EMER IN HOPES THAT APCH COULD VECTOR TFC AWAY FROM US; SO WE COULD TURN AWAY FROM WX. APPARENTLY; APCH COULD NOT CONTACT TFC; BECAUSE HE TOLD US THE ONLY HEADING HE COULD GIVE US WAS 010 DEGS. WE GOT A TA ON TCASII FOR TFC JUST OUTSIDE 5 NM; AROUND 100 FT BELOW OUR ALT. I STARTED TO TURN SLIGHTLY L OF OUR 030 DEGS. APCH ASKED IF WE COULD TAKE 290 DEG HDG; WHICH WAS MUCH BETTER THAN 010 DEGS ON RADAR; SO WE TOO THE TURN AND CLRED TSTM AND TFC. REMAINDER OF FLT UNEVENTFUL. COULD HAVE ASKED FOR A TURN W MUCH EARLIER WHEN I SAW LINE ON RADAR. IT WAS FASTER MOVING THAN I EXPECTED. SUPPLEMENTAL INFO FROM ACN 598744: AN ACFT APPEARED ON TCASII AT OUR ALT. THAT WAS WHY THE CTLR WOULDN'T TURN US. IT WAS A VERY UNCOMFORTABLE SIT; BTWN AN AIRPLANE AND A CUMULO NIMBUS. WE HAD NO CHOICE; BUT TO FLY THROUGH THE BAD WX. THE OTHER ACFT WAS APPARENTLY ON JFK DEP FREQ; AND WE ON LGA ARR FREQ.