AN E170 PLT RPTS DEP WITH NO ALTERNATE REQUIRED. WITH MIN FUEL THE FLT WAS DIVERTING TO AN ALTERNATE WHEN THE ARPT VISIBILITY INCREASED ABOVE MIN FOR LNDG.

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BEFORE LEAVING I WAS GOING OVER THE WX AND NOTICED THERE WAS NOTHING OF CONCERN. THE CURRENT WX AND FORECAST WAS VMC ALL DAY WITH NO TEMPOS. WE DEPARTED WITH NO ALTERNATE OR EXTRA FUEL AND ANTICIPATION OF GETTING INTO ZZZ SMOOTHLY. ABOUT A 100 MILES OUT WE GOT ATIS; NOTICED THAT BOTH RWYS IN USE WERE RPTING GREATER THAN 5000 RVR. THIS MADE US A LITTLE CURIOUS SEEING HOW WE WERE NOT EXPECTING ANY REDUCED VISIBILITY AT ALL. HOWEVER WE CONTINUED WITHOUT WORRY BECAUSE 5000 RVR IS MORE THAN ENOUGH TO COMFORTABLY SHOOT AN APCH AND WITH NO FORECAST OF THIS AT ALL WE DID NOT IMAGINE IT COULD GET MUCH WORSE. AS WE STARTED OUR DOWNWIND THE CTLR STARTED UPDATING THE RVRS. AS WE CONTINUED OUR VECTORS AND APCH THE CTLR KEPT RPTING WORSE CONDITIONS. WHEN WE WERE ABOUT 2-3 MILES FROM THE FAF WE GOT A RPT OF 1200 RVR WHICH DOES NOT ALLOW US TO CONTINUE THE APCH. TOWER ASKED US OF OUR INTENTIONS AND WE SAID WE WOULD LIKE TO TRY ANOTHER RWY. UNFORTUNATELY ALL RWYS WERE BELOW LEGAL MINIMUMS TO TRY THE APCH. THEY THEN ASKED US OF OUR INTENTIONS AGAIN AND WE TOLD THEM TO GIVE US VECTORS TO FIGURE THIS OUT. THIS SITUATION CAUGHT US ALL BY COMPLETE SURPRISE AND UNFORTUNATELY THERE WAS NO FUEL OR ALTERNATE GIVEN TO US FOR THIS. WE IMMEDIATELY ACARS DISPATCH AND ASKED FOR AN ALTERNATE. THEY GAVE US ZZZ1 AND UPON ENTERING IT INTO THE FMS WE WOULD LAND WITH 1500 LBS OF FUEL IF WE WENT THERE. I ASKED FOR CLOSEST AVAILABLE BECAUSE WE DIDN'T HAVE A SECOND TO SPARE. WE STARTED TO HEAD TOWARDS ZZZ1 AND WERE STILL IN A CRITICAL FUEL SITUATION. THANKFULLY AS WE WERE STARTING OUR DIVERSION WE MONITORED TOWER AND NOTICED THE RVR WAS STARTING TO IMPROVE. WE GOT APPROVAL TO TURN AROUND AND SHOOT THE APCH AGAIN WHICH WORKED OUT WITHOUT INCIDENCE. THERE WAS NO WRONG DOING ON EITHER END; IT WAS A RARE OCCURRENCE; BUT I DID WANT TO FILE THIS TO SHOW WHAT WE NEED TO CARRY MORE FUEL FOR CIRCUMSTANCES SUCH AS THIS; OR AT LEAST CARRY EXTRA FUEL ON MARGINAL WX DAYS OR THE HEAVY CONGESTION ARPTS. EVERY PLANE THAT CAME IN AROUND THAT 30 MIN WINDOW HAD TO DIVERT. THE OTHER PROB WAS THERE WAS NO ACARS RECEIVED FROM DISPATCH TO GIVE US A HEADS UP FOR THE DECLINING WX. IF WE COULD HAVE HAD A LEAST AN ALTERNATE LISTED BEFORE THE APCH IT MIGHT HAVE ALLEVIATED SOME OF THE PROBLEM.