ACR AT FL330 DESCRIBED EVENT WHEN ATC'S REFUSAL TO PERMIT WX DEVIATION REQUIRED EMER DECLARATION.

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CRUISE FLT ABOUT MIDWAY OBSERVED LINE OF TSTMS DEVELOPING AND CLOSING IN APPROX ALONG A N/S LINE. OUR RTE WAS THROUGH THE LINE OF TSTM. APPROX 120 MI PRIOR ADVISED FLT ATTENDANTS THAT THEY SHOULD PLAN TO BE SEATED AND EQUIP STOWED IN 15-20 MINS. APPROX 40-50 MI PRIOR TO TSTMS; REQUESTED A DEV TO THE 'R.' ATC CLRED US TO A 180 DEG HDG. IT APPEARED THIS WOULD NOT BE QUITE ENOUGH. TO MAKE AN END RUN AROUND WHAT APPEARED TO BE THE OPEN AREA AND ALSO THE UPWIND (85 KTS) SIDE WE REQUESTED A HDG OF 190 DEGS. CTLR ADVISED UNABLE; HE WOULD BE DSNDING US AND THEN TURNING US. AS WE GOT CLOSER; CTLR WANTED US TO TURN L (E) THROUGH LINE OF TSTMS. WE REPLIED UNABLE BUT WE NEEDED TO TURN FURTHER R. AT A PARTICULAR POINT IN TIME; IT SEEMED WE COULD MAKE A L TURN AND GET BTWN SOME LARGE CUMULO NIMBUS; SEEING CLR ON THE OTHER SIDE; OR WE WOULD NEED TO TURN IMMEDIATELY TO R TO AVOID TSTM IN OUR 12 O'CLOCK POS. FO ADVISED ATC; 'WE CAN TURN L OR R RIGHT NOW.' ATC RESPONDED 'ROGER.' AT THIS POINT; OUR ABILITY TO TURN L HAD PASSED. I PICKED UP MIKE AND STATED; 'WE ARE TURNING R.' ATC OBJECTED SO I DECLARED AN EMER. ATC ASKED HOW FAR R WE NEEDED TO TURN AND I REPLIED OUR CURRENT HDG WOULD DO FOR ABOUT 5-10 MI THEN WE COULD TURN L AROUND TSTM. CTLR TOLD US TO TURN R AND I REPLIED; 'NEGATIVE.' HE AGAIN ASKED US TO TURN R AND I REPLIED; 'NEGATIVE; NEGATIVE; WE ARE EMER ACFT; ONCE WE GET AROUND TSTM WE WILL CANCEL EMER.' HE IMMEDIATELY SWITCHED FREQS TO ZFW. ZFW ASKED THE NATURE OF EMER AND I REPLIED WE DID NOT GET COOPERATION TO AVOID TSTM FROM PRIOR CTR. VIA ACARS WAS ASKED TO CONTACT CTR MGR UPON LNDG. I DID AND THE CTLR MGR SAID SHE COULD NOT SEE WHY WE DECLARED EMER. I TOLD HER TO AVOID POSSIBLE INJURY/DAMAGE TO ACFT. I ALSO INDICATED I THOUGHT CTLR HAD LOST SITUATIONAL AWARENESS AND TO AVOID FLYING INTO TSTM I EXERCISED MY EMER AUTH. NO TURB OR CHOP WAS EXPERIENCED DURING THIS INCIDENT. MGR CTLR SAID SHE WAS TURNING THIS OVER TO THEIR QUALITY REVIEW DEPT.