B737-700 FLT CREW ENCOUNTERS WX AND IS UNABLE TO HOLD DUE TO LACK OF FUEL AND NO ALTERNATE IS FILED.

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WE WERE ENRTE TO LAS; SCHEDULED TO ARRIVE WITH 5400 LBS FUEL; AND NO ALTERNATE REQUIRED. DISPATCH SENT ACARS MESSAGE ADVISING US OTHER ACFT HAD BEEN HELD BRIEFLY AT FUZZY; SO WE SLOWED DOWN IN ANTICIPATION. ZLA GAVE US HOLDING INSTRUCTIONS; WITH AN EFC OF XB15; PRESENT TIME WAS XA45. WE ENTERED THE INFO INTO THE CDU HOLDING PAGE; AND IT CALCULATED WE COULD HOLD FOR 4 MINS; AND THEN WOULD LAND WITH 3800 LBS. I ADVISED ATC THAT WE COULD DO ONE TURN; AND THEN WOULD HAVE TO CONTINUE THE ARR. THE CTLR REPEATED THE EFC TIME; AND SUGGESTED WE COULD PROCEED TO OUR ALTERNATE IF NECESSARY. HE SEEMED SURPRISED WHEN I INFORMED HIM THAT WE HAD NO ALTERNATE ON OUR RELEASE; THAT NONE HAD BEEN REQUIRED. I TOLD HIM WE COULD DO 1 TURN; AND WOULD HAVE TO PROCEED; OR ELSE WE WOULD HAVE TO DECLARE AN EMER. 1 MIN LATER WE WERE VECTORED OFF THE ARR; AND AROUND TO THE S; AND FINALLY CLRED DIRECT TO LAS; THEN BLD; WHICH PUT US ON DOWNWIND FOR RWY 25L. WE FLEW A NORMAL LAS 12 MI DOWNWIND; AND LANDED. ON TAXI IN; WE WERE GIVEN PHONE NUMBERS TO CALL -- ONE FOR ZLA; AND THE OTHER FOR LAS APCH. AFTER TURNING THE PLANE OVER TO THE NEXT CREW; I WENT DOWN TO OPS AND CALLED BOTH FACILITIES; AND EXPLAINED THE SITUATION. BOTH MGRS APPRECIATED THE INPUT; AND WERE SATISFIED WITH MY EXPLANATION. (AS AN ASIDE; THE FUEL LOAD WAS CORRECT; AND THE UPLIFT WAS WITHIN 7 GALLONS OF MY CALCULATED VALUE). SUPPLEMENTAL INFO FROM ACN 695277: THE FLT PLAN CALLED FOR 12.0 OF FUEL WHICH WOULD GIVE US AN ESTIMATED 5.4 FUEL UPON ARR IN LAS. ENRTE; THE FMC GAVE US THE MESSAGE THAT WE WERE USING RESERVE FUEL AND WOULD LAND WITH 3.8 FUEL. OUR OWN ESTIMATES WERE ABOUT 5.0 SO WE WEREN'T VERY CONCERNED UNTIL WE WERE TOLD THAT WE WERE TO HOLD AT FUZZY FOR APPROX 20 MINS BECAUSE AN UNFORECAST MIRCOBURST HAD PASSED THROUGH LAS FORCING INBOUND ACFT TO HOLD. WE ADVISED THE CTR THAT WE COULD HOLD FOR ONLY 1 TURN IN THE HOLD AND THEN WE NEEDED TO CONTINUE INBOUND TO THE ARPT. NO EMER WAS ACTUALLY DECLARED AND WE LANDED AT LAS RWY 25R WITHOUT INCIDENT AND WITH MORE THAN 5.0 FUEL.