AN ACR B767 FLC REFUSED A HDG IN ORDER TO AVOID A TSTM AND THE ARTCC CTLR BECAME UPSET WITH THE FLC.

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HAD BEEN CLRED DIRECT DUBLIN; SINCA ARR; ATL. DEVIATED L (W) OF A LINE OF TSTMS WITH INSTRUCTIONS TO TURN DIRECT DUBLIN WHEN ABLE. ACKNOWLEDGED WITH INFO THAT THIS WOULD BE IN APPROX 80 MI. HDG WAS 335 DEGS PARALLELING LINE. AFTER ABOUT 10-20 MI; ZTL 119.77 SAID TO TURN TO 345 DEGS. I RESPONDED; 'UNABLE DUE TO WX; HDG NOW 335 DEGS.' HE SAID; 'TURN TO 345 DEGS IMMEDIATELY.' I AGAIN RESPONDED; 'UNABLE DUE TO WX.' HE SAID; 'THIS IS NOT A SUBJECT FOR DEBATE; TURN TO 345 DEGS NOW.' I AGAIN RESPONDED; 'UNABLE 345 DEGS FOR TSTMS; HDG NOW 335 DEGS.' HE SAID; 'FINE. YOU ARE EXERCISING YOUR EMER AUTH. THANK YOU.' (THE LAST WAS SARCASTIC AND CONFRONTATIONAL IN TONE.) I STUTTERED A BIT AND SAID; 'I; UM; I; I'M JUST NOT TURNING TO 345 DEGS RIGHT NOW.' HE THEN GAVE A FREQ CHANGE. THE NEXT CTLR TOLD US DIRECT DUBLIN WHEN ABLE; WHICH WE ACKNOWLEDGED. TO TURN TO 345 DEG HDG WHEN INSTRUCTED BY ATC WOULD HAVE CAUSED US TO FLY DIRECTLY INTO A SOLID LINE OF LEVEL 3 AND HIGHER TSTMS. THESE WERE CLRLY VISIBLE AND BOLDLY MARKED ON OUR PROPERLY WORKING WX RADAR. WE WERE SIMPLY NOT GOING TO FOLLOW THIS INSTRUCTION AND INFORMED HIM OF SUCH PLAINLY AND CLRLY 3 TIMES. HIS HOSTILE RESPONSE WAS INAPPROPRIATE AND OUT OF PLACE. HIS FAILURE TO ADJUST TO CHANGING WX AND TFC CONDITIONS IS CAUSE FOR ALARM. AT NO TIME WERE WE IN AN EMER SIT; NOR DID WE DECLARE ONE. WE WERE MERELY DEVIATING FOR WX WITH PERMISSION FROM ATC. TO HAVE ATC 'DECLARE' AN EMER FOR US WAS CONFUSING; DISTRACTING AND INAPPROPRIATE.