ACR X REFUSED CLRNC UNAUTH DSCNT FOR WX AVOIDANCE.

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ACR X IN A CLB TO FL310; ASSIGNED. THE CAPT WAS FLYING AND I WAS HANDLING THE RADIO CALLS. WE WERE SBOUND AND PASSING W OF A CELL THAT WAS ON OUR RADAR. AT FL270; TURB BEGAN INCREASING IN INTENSITY AND BY FL290 IT WAS MODERATE AND STILL GETTING WORE. WE WERE GIVEN A FREQ CHANGE IN THE MIDDLE OF THIS AND THE NEW CTLR WAS EXTREMELY BUSY; GIVING NONSTOP INSTRUCTIONS TO SEVERAL ACFT AND I WAS UNABLE TO CHK IN WITH HIM. THE CAPT ASKED ME TO GET US A DSCNT TO FL270. HE SLOWED HIS RATE OF CLB AT ABOUT FL290. MY FIRST XMISSION WAS BLOCKED BY ANOTHER RADIO. MY SECOND RADIO CALL WAS 'ZLC; CALL SIGN; FL295 (APPROX) IN MODERATE TURB; ASSIGNED FL310; REQUEST A DSCNT TO FL270.' THE CTLR INSTRUCTED US TO GO TO FL310. WE COULD NOW FAINTLY SEE AN OVERHANG ABOVE US FROM THIS TSTM AND WITHOUT EVEN DISCUSSING IT; IT WAS CLR TO BOTH OF US THAT CLBING TO FL310 WOULD BE DANGEROUS. THAT CAPT TOOK OVER THE RADIOS AND TOLD SLC HE WAS DSNDING BECAUSE OF THE TURB; OVERHANG; ETC; AND THE CTLR SAID UNABLE; MAINTAIN FL310. THE EXCHANGE BECAME HEATED AND THE CTLR ASKED IF WE WERE DECLARING AN EMER AND THE CAPT TOLD HIM NO; WE'RE JUST DSNDING TO FL270. I DID NOT OBSERVE ANY TFC VISUALLY OR ON TCASII AND DON'T BELIEVE THERE WAS ANY MIDAIR POTENTIAL BASED ON THE RADIO CALLS I HEARD. FACTORS: INABILITY TO CHK IN WITH NEW SECTOR DUE TO SATURATED RADIO. I'M SURE I COULD HAVE WORKED IT OUT IF I COULD HAVE TALKED SOONER. AN OVERHANG THAT DID NOT SHOW ON RADAR AND IT WAS DARK SO WE COULD NOT SEE IT. THE CAPT SHOULD HAVE DECLARED AN EMER. THAT WOULD HAVE PREVENTED A RADIO SATURATING ARGUMENT AND WE COULD HAVE DSNDED QUICKER. IN MY ESTIMATION THE TURB WAS ON ITS WAY TO SEVERE. THE CTLR EVENTUALLY GAVE US FL270 WHEN WE WERE AT ABOUT FL285 AND DSNDING SLOWLY; BUT HE WAS PRETTY HOT!