AN EMER IS DECLARED AT ATC'S SUGGESTION WHEN A B757 CREW IS UNABLE TO ACCEPT THE FIFTH HOLDING CLRNC OFFERED BECAUSE OF THE LOCATION'S PROX TO CONVECTIVE WX.

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ENRTE TO ZZZ1 WE WERE ON THE ARR. ON DSCNT WE WERE GIVEN HOLD OVER FIX 1 BUT PRIOR TO ENTERING THE HOLD WE WERE CLRED ON COURSE. THEN GIVEN A HOLD OVER FIX 2 WHERE WE DID 1 TURN IN MODERATE TURB AND ATC OFFERED A HOLD AT FIX 1; AFTER 1 TURN IN THE HOLD AND DSNDING TO FIND SMOOTH AIR ATC DIRECTED US TO HOLD AT FIX 3. WE PROCEEDED ON TO HOLD AT FIX 3 AND AFTER 1 TURN AT THAT LOCATION THERE WAS CONVECTIVE ACTIVITY WITH BUILDUPS AND MODERATE TURB. ATC THEN APPROVED A HOLD OVER FIX 4 AFTER DSNDING TO 11000 FT WE WERE CLRED DIRECT. UPON REACHING FIX 4 THERE WAS A TSTM ABOUT 1 MI TO THE R OF THE VOR; WE WERE CLRED FOR R TURNS WHICH WOULD FLY US RIGHT INTO THE CELL. WE ASKED FOR L TURNS AND THAT WAS NOT APPROVED FOR AIRSPACE LIMITS. WE PROCEEDED PAST THE VOR BECAUSE WE TOLD ATC WE COULD NOT ACCEPT A HOLD AT THAT LOCATION WITH R TURNS. HE TOLD US TO TURN DIRECT FIX 4 OR TO DECLARE AN EMER AND SQUAWK 7700. WE DECLARED AN EMER AND SQUAWKED 7700. AND TOLD THEM WE WERE GOING TO DIVERT TO ZZZ2. WE WERE THEN CLRED TO CONTINUE A L TURN TO HDG 090 DEGS AND CLRED ON TO ZZZ. THEN SHORTLY AFTER CLRED DIRECT FIX 5 AND TO RETURN TO PREVIOUS XPONDER CODE. REMAINDER OF APCH AND LNDG UNEVENTFUL. THERE WAS CONVECTIVE ACTIVITY THROUGHOUT AND A LARGE AMOUNT OF CONGESTION ON THE FREQ AND WE WERE UNABLE TO COMMUNICATE WITH ATC IN A TIMELY MANNER TO INFORM THEM THAT FIX 4 WOULD NOT BE AN ADEQUATE FACILITY TO HOLD AT. ATC WAS VERY BUSY AND QUITE HELPFUL BUT THE AMOUNT OF FREQ CONGESTION; ATC WORKLOAD AND PLT WORKLOAD RESULTED IN COMS IN A LESS THAN TIMELY MANNER. IN ORDER TO MAINTAIN THE SAFETY OF THE FLT AND PAX WE HAD TO DECLARE AN EMER TO AVOID FLYING INTO CONVECTIVE WX.