B737-700 DIVERTS TO AN UNPLANNED ALTERNATE WHEN VISIBILITY AT THE DESTINATION AIRPORT DROPS DRAMATICALLY. CAPTAIN FAILS TO ADVISE ATC OF EMERGENCY FUEL STATUS.

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DISPATCH AND WX PACKAGE SHOWED FOLLOWING: MINIMUM FUEL: 10300 LBS. PLANNED ARR FUEL: 5000 LBS. NO LNDG ALTERNATE REQUIRED. SAN OBS CLR; 7 SM; 16/14 CALM. SAN TAF ALL VIS WERE TAF 3 SM OR GREATER. APPROX MIDPOINT IN FLT RECEIVED ACARS FROM DISPATCH THAT FOG WAS MOVING INTO SAN AREA. IF UNABLE LNDG AT SAN; PROCEED TO ONT; AND I QUOTE; 'PUT THE PEDAL TO THE METAL.' ON R DOWNWIND WE COULD SEE THAT FOG HAD INDEED OBSCURED THE ROLLOUT END OF RWY 27 TO APPROX TXWY B2. WE WERE GIVEN A CLRNC TO FOLLOW COMPANY ACFT; SWITCH TO TWR. CHKING IN WITH TWR; ADVISED THAT TOUCHDOWN RVR WAS GREATER THAN 6000 FT BUT ROLLOUT WAS 1400 FT. WE TURNED FINAL AT 3000 FT AND 8 NM. ACFT WAS CONFIGURED PRIOR TO TURNING FINAL. AS APCH WAS CONTINUED IT WAS BECOMING INCREASINGLY DIFFICULT TO SEE THE RWY 27 TOUCHDOWN ZONE; ALTHOUGH THE DISPLACED THRESHOLD WAS STILL IN THE CLR. AT A 3 NM FINAL TWR CALLED TOUCHDOWN 1400 FT RVR ROLLOUT 1400 FT RVR. AT THE 1000 FT CALL I COULD NO LONGER MAKE OUT ANY OF THE RWY ENVIRONMENT EXCEPT THE FIRST 500 FT OF THE DISPLACED THRESHOLD. THE VISIBILITY HAD GONE FROM 6000 + RVR TO 1400 FT RVR IN LESS THAN 2 MINS. I ELECTED TO ABANDON THE APCH AND ACCOMPLISHED A MISSED APCH. TWR GAVE US A HDG AND ALT WHICH WAS ACCOMPLISHED. AT THE BEGINNING OF THE MISS; OUR FUEL STATUS WAS 4800 LBS. WE WERE FINALLY HANDED OFF TO SOCAL DEP. BY THIS TIME WE HAD BEEN AT 2500 FT HDG OF 265 DEGS FOR ABOUT 6 NM. BECAUSE OF THE PRECARIOUS NATURE OF THE FOG AND THE FUEL STATUS; I MADE THE DECISION TO NOT ATTEMPT ANOTHER APCH IN SAN. PNF REQUESTED AN IMMEDIATE CLRNC TO ONT. WE CLBED TO 11000 FT. I EXPLAINED SITUATION TO FLT ATTENDANTS AND THEN MADE A PA TO PAX. PNF THEN COORDINATED WITH DISPATCH VIA ACARS AND THEN VIA COMPANY FREQ. DURING THIS TIME OF ATC; COMPANY; FLT ATTENDANT; PAX COMS; CHKLISTS; OPC; ETC; TIME WAS VERY CONSTRAINED AND I DID NOT DECLARE MINIMUM FUEL TO ATC AS PRESCRIBED IN FOM WHEN OUR FUEL STATUS WENT BELOW 4000 LBS. BY THE TIME WE WERE ON SHORT FINAL TO ONT; OUR FUEL STATE WAS SUCH I SHOULD HAVE ALSO DECLARED EMER FUEL AS IT APPEARED WE WOULD LAND WITH LESS THAN 3000 LBS OF FUEL. 1) MY EXPERIENCE LEVEL IN SOCAL CORRIDOR SHOULD HAVE RAISED A WARNING FLAG ON FOG POTENTIAL IN SAN. MORE FUEL (5000 LBS) WOULD HAVE GIVEN US THE OPPORTUNITY TO LOITER AND POSSIBLY GET INTO SAN. 2) WHEN DISPATCH GAVE THE INFLT ACARS; I SHOULD HAVE GOTTEN ONT AS THE SPECIFIC ALT WITH FUEL BURNS FROM THE MAP IN SAN; CLBOUT; AND CRUISE; APCH INTO ONT. I DID A QUICK HEAD CALCULATION THAT IT WOULD TAKE 1200 LBS TO GET OVER TO ONT. HOWEVER; WHEN WE GOT VOICE COM WITH DISPATCH; THEIR BURN ESTIMATE WAS 1600 LBS. 3) I WAS TOLD THAT 7 FLTS HAD DIVERTED TO ONT THE DAY BEFORE BECAUSE OF FOG WITH THE SAME WX CONDITIONS. AND WE WERE STILL DISPATCHED WITH MINIMUM LNDG FUEL?