FLT CREW OF EMB140 DISPATCHED WITH FUEL LOAD BASED ON EXPECTED GOOD WX MUST DECLARE MINIMUM FUEL AND DIVERT TO AN UNPLANNED ALTERNATE WHEN ARRIVAL DELAYED AT DEST BY TSTMS.

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FLT DEPARTED MIA FOR SDF WITH 8400 LBS FOB. PLANNED ARR FUEL WAS TO BE 2881 LBS. 900 LBS DISPATCH ADD AND 281 LBS HOLD FUEL -- 10 MIN -- WERE INCLUDED. SDF FORECAST WAS VFR SO NO LNDG ALTERNATE FUEL WAS REQUIRED NOR PLANNED. ENRTE BURN WAS SLIGHTLY HIGHER THAN PLANNED DUE TO WX DEVIATIONS AND STEEP CLB. ATC ISSUED HOLDING FOR US APPROX 40 DME SE OF IIU VOR WITH EFC AT LEAST 30 MIN DUE TO UNFORECAST TSTM AND WINDSHEAR MICROBURSTS IN EXCESS OF 50 KTS ON FIELD. AS WE MADE A FEW TURNS IN THE HOLD; ATC EXTENDED EFC 10 MORE MINS. WE ENTERED THE HOLD WITH APPROX 2500 LBS FOB AND EVENTUALLY LEFT THE HOLD WITH APPROX 2100 LBS FOB. WE DETERMINED THAT WITH PRESENT FOB WE COULD NOT COMFORTABLY HOLD THAT LONG; SO WE DECIDED TO DIVERT TO BNA TO REFUEL AND INFORMED ATC OF OUR FUEL STATUS. DURING HOLDING; I TRIED UNSUCCESSFULLY TO CONTACT OPS CTL. I CONTINUED TO SUCCESSFULLY; BUT MORE SLOWLY; COM WITH OPS CTL VIA ACARS AND SDF RADIO RELAY. WE HAD TO DEVIATE AROUND WX AT 12000 FT BETWEEN LEAVING HOLD AND BNA. I RAN THE AOM VOL 1 'FUEL LOW LEVEL' CHKLIST EARLY IN ANTICIPATION OF RECEIVING THE EICAS MSG. WHEN FOB REACHED 1600 LBS; I DECLARED MINIMUM FUEL STATUS TO ATC. I DID NOT DECLARED AN EMER. ATC CLRED US DIRECT TO THE OM OF RWY 2R AT BNA. WE MADE AN EXPEDITED; BUT UNEVENTFUL LNDG WITH 1100 LBS FUEL AT TOUCHDOWN. I THEN PHONED OPS CTL WITH A RPT AND ATTEMPTED TO CONTACT THE CHIEF PLT'S OFFICE VIA PHONE. WE REFUELED AND CONTINUED UNEVENTFULLY TO SDF. THE PAX WERE KEPT INFORMED WITH DELAY AND GENERAL DIVERSION INFORMATION; BUT I DID NOT WANT TO ALARM THEM AND DID NOT GIVE PAX ANY CRITICAL OR ALARMING FUEL STATUS INFO. FLT ATTENDANT WAS KEPT TOTALLY ADVISED OF EVERYTHING AND CRM BETWEEN ALL CREWMEMBERS WAS EXCELLENT. FO DID AN OUTSTANDING JOB AS FLYING PLT; WHILE CAPT PERFORMED NON-FLYING PLT DUTIES; PLANNED AND EXECUTED DECISIONS; COMMUNICATED; AND COORDINATED ALL THE NECESSARY ACTIVITIES. OUR DECISION TO DIVERT TO BNA AS AN UNPLANNED ALTERNATE WAS A GOOD ONE BASED ON THE INFO WE AS A CREW HAD AVAILABLE TO US AT THE TIME; DESPITE OUR INITIAL INABILITY TO MAKE TIMELY CONTACT WITH OPS CTL. MY MAIN OBSERVATION ON ALL OF THIS IS THAT I'VE BEEN NOTICING A MARKED DECREASE IN DISPATCH RELEASE FUEL LOADS LATELY. WHILE I COMPLETELY UNDERSTAND; APPRECIATE; AND AGREE WITH THE COMPANY'S NEED FOR A FUEL CONSERVATION POLICY; I MUST RESPECTFULLY POINT OUT THAT REDUCED FUEL LOADS; WHILE EFFICIENT; DO NOT LEAVE MUCH ALLOWANCE FOR MULTIPLE UNFORESEEN CIRCUMSTANCES SUCH AS OCCURRED THIS FLT.