A320 LANDS IAD WITH LOW FUEL STATE.

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WE TOOK OFF FROM SAN DIEGO; CA (SAN) RWY 27 ENRTE TO WASHINGTON DULLES (IAD). WE WERE WT RESTR ON TKOF DUE TO THE RWY. ENRTE WX WAS GOOD AND OUR DISPATCH HAD US LNDG WITH 6000 LBS OF FUEL (ABOUT 1 HR). WX IN DULLES AT TKOF WAS IFR DUE TO CEILING (600 FT); BUT IT WAS FORECAST TO BE VFR AT OUR ARR TIME. STARTING JUST W OF THE ROCKIES; WE ENCOUNTERED LIGHT TURB AT ALL FLT LEVELS. OVER MID KANSAS WE HAD BEEN SCHEDULED TO GO FROM FL330 TO FL370; BUT ATC SAID THAT FL370 WAS A WORSE RIDE. WE DECIDED TO STAY AT FL330. DUE TO DIRECT ROUTING FROM ATC OUR FUEL BURN WAS ON SCHEDULE EVEN THOUGH WE WERE AT A LOWER ALT. SOMEWHERE OVER INDIANA; OUR DISPATCH INFORMED US THAT IAD HAD NOT IMPROVED AS FORECAST; AND WE SHOULD PLAN ON BWI AS AN ALTERNATE IF NEEDED. THE ESTIMATED ALT FUEL BURN WAS 1600 LBS. CROSS NEAR PIT WE NOTED THAT THE FUEL BURN WAS 500 LBS BELOW PLANNED; BUT STILL ENOUGH. AT THAT POINT; IT WAS NOT WORTH REQUESTING FL370 DUE TO THE PROX OF IAD. ATC STARTED US DOWN A LITTLE EARLY AND GAVE US EXTRA VECTORING DUE TO MVFR WX AND HIGH VOLUME TFC. NEITHER OF US NOTED THE FUEL UNTIL VECTORED ONTO FINAL. IT WAS ABOUT 4300 LBS. THE CAPT AND I BOTH AGREED THAT A GAR MIGHT WARRANT PRIORITY HANDLING OR EMER DUE TO LOW FUEL. WE LANDED ON RWY 1L AT IAD UNEVENTFULLY. WHEN WE BLOCKED IN THE FUEL WAS 3900 LBS (FAR MINIMUM 4100 LBS). GIVEN OUR TAXI TIME TO THE GATE WE PROBABLY LANDED AT ABOUT 4100 LBS. DUE TO THE VECTORING AND EARLY LET DOWN WE WOULD HAVE BEEN PRESSED TO GET TO BWI WITH A COMFORTABLE FUEL LEVEL. TO MY KNOWLEDGE THE IAD TERMINAL FORECAST HAD NOT BEEN AMENDED TO REFLECT THE 1200 FT BROKEN CEILING AND 5 SM VISIBILITY AT OUR ACTUAL ARR TIME.