MD80S ACFT ON APCH HAD FLUCTUATING AIRSPD RELATED TO 2 WX CELLS BTWN ACFT AND ARPT. RPTR CAPT ELECTED TO GO MISSED APCH WHEN FLYING OVER THE CELLS RECEIVED A GPWS 'TERRAIN; TERRAIN' WARNING. IN ACCORDANCE WITH SOP'S FLC ADDED PWR AND COMMENCED THE ESCAPE MANEUVER. RPTR FLC RELATED THE GPWS TO THE WX CELL ACTIVITY.

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ON APCH INTO PHX ON FLT XX AT APPROX XX45Z THE CAPT WAS EXECUTING THE ILS RWY 8R; GS OUT (LOC ONLY) APCH. AT APPROX 2000 FT MSL; INBOUND FROM THE FAF; IN DSCNT; PLT NOTICED A PLUS AND MINUS 15 KT AIRSPD FLUCTUATION ACCOMPANIED BY 2 DISTINCT CELLS BTWN ACFT AND ARPT. LIGHTNING AND THUNDER WERE ALSO NOTED IN IMMEDIATE VICINITY. PLT ELECTED TO GAR. MISSED APCH WAS INITIATED WITH INTENT TO FLY TO THE PUBLISHED MISSED APCH ALT OF 5000 FT MSL. CLBING THROUGH APPROX 3200 FT MSL; TWR INSTRUCTED US TO MAINTAIN 3000 FT MSL. UPON REACHING 3000 FT MSL; GPWS SOUNDED 'TERRAIN; TERRAIN.' ESCAPE MANEUVER WAS INITIATED WITH PLT APPLYING FULL PWR. CLBING THROUGH 5000 FT MSL (WITH TERRAIN; TERRAIN STILL SOUNDING) TWR ADVISED US TO MAINTAIN 4000 FT MSL. WE IMMEDIATELY DECLARED AN EMER AND KEPT CLBING. GPWS WARNING DID NOT EXTINGUISH UNTIL APPROX 6200 FT MSL. ACFT WAS LEVELED AT 7500 FT MSL. WE THEN DIVERTED TO TUCSON AND LANDED UNEVENTFULLY. IN REVIEWING THE ENTIRE EVENT; BOTH PLTS BELIEVE THAT THEY EXPERIENCED A FALSE GPWS WARNING THAT WAS POSSIBLY GENERATED BY CELL ACTIVITY DIRECTLY OVER THE PHX ARPT. THE ENTIRE EVENT OCCURRED ON THE EXTENDED CTRLINE OF RWY 8R WHERE NO TERRAIN EXISTS AT THE ALT WE WERE AT WHEN WE EXPERIENCED THE GPWS WARNING.