WX FACTORS CAUSE MD80 SPD LOSS REQUIRING IMMEDIATE DSCNT AS THEY SIMULTANEOUSLY ADVISE ZTL.

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ACFT WAS IN LEVEL FLT AT FL350; WITH SOME DEVS OFF COURSE DUE TO WX. AT XA35 THE PLT UNEXPECTEDLY ANNOUNCED 'ACR X; WE NEED TO CHANGE ALT RIGHT NOW.' SINCE I DID NOT CTL THE AIRSPACE BELOW FL350; I WAS UNABLE TO PROVIDE A DSCNT CLRNC RIGHT AWAY. THE PLT INITIATED A DSCNT AND I ADVISED THE PLT THAT I WAS DECLARING AN EMER AND TO PLEASE FILL ME IN ON THE SIT WHEN CIRCUMSTANCES PERMITTED. DURING THIS TIME; COORD WITH THE SECTOR BELOW ME WAS BEING PERFORMED AND IT WAS DETERMINED THAT NO ACFT WERE IN CONFLICTION. SUBSEQUENT DISCUSSION WITH THE PLT INDICATED THAT; DUE TO TURB; THE ACFT WAS UNEXPECTEDLY UNABLE TO MAINTAIN ALT. AFTER DSNDING TO ABOUT FL330; THE PLT INDICATED THAT HE COULD THEN CLB BACK TO FL350. I OFFERED THE PLT A CLB BACK TO FL350 OR DSCNT DOWN TO FL310. THE PLT ASKED FOR AND RECEIVED CLRNC TO FL310. BECAUSE I WAS SITTING IMMEDIATELY ADJACENT TO THE CTLR WITH AIRSPACE BELOW ME; I WAS ABLE TO COORDINATE THE UN-CLRED DSCNT QUICKLY ENOUGH TO AVOID CASCADING PROBS WITH OTHER ACFT. SUPPLEMENTAL INFO FROM ACN 547375: ACFT WAS IN CRUISE FLT AT FL350 WITH AIRFOIL AND ENG ANTI-ICE ON. FLT WAS IN IMC DEVIATING TO THE E OF TSTMS. ACFT SPD INCREASED INITIALLY FROM .76 TO .78 MACH; THEN DETERIORATED TO .69 MACH. PERFORMANCE DID NOT INCREASE SO WE IMMEDIATELY DSNDED TO LOWER ALT TO REGAIN SPD AND ACFT CTL. THERE WAS NO TIME TO REQUEST AND RECEIVE CLRNC FOR THE ALT CHANGE. I BELIEVE WE MUST HAVE FLOWN INTO RELATIVELY WARM; MOIST AIR BLOWING OFF THE TOP OF A STORM TO OUR L; CAUSING MARKED DETERIORATION IN ACFT PERFORMANCE.