AN ACR B737-400 HAD TO DIVERT LAND FOR FUEL. WX FACTORS HIGH ALT AND HIGH TEMP AT DEP ARPT; CUMULO NIMBUS AND ARRS DELAYS AT DEST.

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FLT DEPARTED PHX FULL (150 PAX/CREW); ARPT TEMP 97 DEGS F FOR A NON-STOP FLT TO PHL. FLT PLAN ARR FUEL WAS 6300 LBS. ENRTE FUEL OVER-BURN; WX AND TFC DEVS COUPLED WITH EARLY DSCNT AND ATC SPD REQUIREMENT ALL COMBINED TO A PROJECTED UNACCEPTABLE LOW ARR FUEL AT PHL. OVERHEAD MDT; WHEN THE LAST ALT ASSIGNMENT (15000 FT) WAS PROGRAMMED INTO THE FMC; ARR FUEL WAS INDICATED TO BE 3900 LBS (WHICH ALSO WAS OUR FLT PLAN RESERVE FUEL). I ESTIMATED OUR LNDG FUEL WOULD BE 3000 LBS DUE TO METERING OF TFC; CUMULO NIMBUS CLOUDS IN THE AREA AND FUEL FOR APCH AT PHL. WE REQUESTED AND RECEIVED VECTORS FOR DIVERSION INTO MDT; ARRIVED AT THE GATE WITH 4300 LBS OF FUEL. FLT WAS REFUELED AND RE-DISPATCHED FOR PHL FOR AN UNEVENTFUL FLT; ARRIVING APPROX 1:20 HRS LATE. DECISION TO DIVERT WAS A CREW DECISION. THIS WAS A CLASSIC CASE OF INCREMENTAL INCREASED DEMAND ON OUR FUEL RESERVE WHICH COULD NOT BE FULLY ANTICIPATED NOR COULD WE ANTICIPATE WHAT FURTHER DEMANDS WOULD BE MADE (RWY CLOSURE; EMERS; ETC). A SCHEDULED FUEL STOP WILL BE DISCUSSED WITH DISPATCHER ON ANOTHER SIMILAR FLT.