B737 CREW LNDG MIA WAS UNABLE TO ACCEPT A TURN TO FINAL BECAUSE OF SEVERE WX.

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WE WERE BTWN WORPP INTXN AND MIA AND N OF THE RWY 9L FINAL APCH COURSE. THE CTLR GAVE US A R TURN FROM OUR CURRENT HDG (110 DEGS) TOWARDS THE S; WHICH HAD US JUST W OF SOME BUILDUPS AND MODERATE TO SEVERE RADAR RETURNS. I ASSUMED HE WAS VECTORING US TO THE S OF THE WX TO INTERCEPT RWY 9R LOC FROM THE S. TO OUR SURPRISE; HE GAVE US A HDG RIGHT INTO THE WX (120 DEGS). I READ BACK THAT; 'I DIDN'T THINK WE COULD TAKE THAT HDG' AND HE DIDN'T RESPOND. WE CONTINUED ON THE SOUTHERLY HDG WHILE SOME RADIO CHATTER OCCURRED. HE THEN ASKED US WHY WE HADN'T TURNED AND TOLD US TO TURN TO 080 DEGS TO INTERCEPT THE FINAL. HE WAS NOT VERY HAPPY; NOT VERY NICE; TELLING US THAT WE NEEDED TO TELL HIM WHEN WE CAN'T TAKE A HDG. I TOLD HIM WHAT WE HAD READ BACK AND THAT WAS THE END OF THE SUBJECT. AS WE TURNED TO INTERCEPT THE FINAL; I LOOKED BACK AT THE WX THAT WE HAD REFUSED TO TURN INTO AND SAW A HVY TO WIDE RAIN SHAFT COMING OUT OF IT (THE CTLR SEEMED TO QUESTION WHETHER THERE WAS ANY WX AT ALL OUT THERE). I AM GLAD WE DIDN'T TURN INTO IT; HOWEVER; I THINK THAT MORE STANDARD PHRASEOLOGY AND AN INCREASE IN THE LEVEL OF MY VOICE MAY HAVE HELPED THE CTLR TO HEAR MY READBACK. I SHOULD HAVE SAID 'UNABLE' WHEN HE GAVE US THE HDG INTO THE WX.