SF340B ACFT. ENRTE PVD TO JFK THE WX WENT BELOW MINIMUMS; FLC HELD FOR ALMOST AN HR AND A HALF BEFORE THE RVR WAS HIGH ENOUGH TO COMMENCE AN APCH TO JFK. THEN THE VISIBILITY AGAIN WENT BELOW MINIMUMS SO FLC DIVERTED BACK TO PVD. THE FUEL WAS GETTING LOW AND FLC DECLARED LOW FUEL; BUT FAILED TO DECLARE AN EMER. THOUGH THE VISIBILITY AGAIN WENT BELOW MINIMUMS AT PVD; FLC MADE APCH AND LANDED. RPTR FO SAYS THAT PERHAPS THEY SHOULD HAVE DECLARED AN EMER.

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ON JAN/XX/1997; WE DEPARTED PVD FOR JFK AT XX50. WX IN JFK WAS RPTED TO BE VV1; 1800 FT RVR FOR RWY 4R. ENRTE THE WX AT JFK DETERIORATED TO LESS THAN 1/4 MI OR 1000 FT RVR. WE HELD AT CCC FOR NEARLY AN HR AND A HALF BEFORE DECIDING TO RETURN TO PVD. AFTER RECEIVING A CLRNC AND AN INITIAL VECTOR FOR PVD; WE WERE TOLD JFK VISIBILITY WAS NOW MORE THAN 1800 FT RVR. WE THEN ELECTED TO FLY THE APCH INTO JFK RWY 13R. AS WE WERE RECEIVING VECTORS THE VISIBILITY WENT BACK DOWN BELOW MINIMUMS BUT WE ELECTED TO CONTINUE THE APCH TO THE MARKER BEFORE ELECTING TO MISS JUST IN CASE THE VISIBILITY CAME UP. UPON REACHING THE MARKER; WE QUERIED THE TWR AS TO THE VISIBILITY. THE TWR WOULD NOT RESPOND. SO UPON REACHING THE MARKER WE HAD NO CHOICE BUT TO MISS AND PROCEED TO OUR ALTERNATE. HAVING HELD FOR AS LONG AS WE DID AND THEN RETURNING TO JFK PUT US AT ABOUT 1250 LBS OF FUEL WHICH EQUALED 1:30 MINS OF TOTAL FUEL REMAINING -- ENOUGH TO GET TO OUR ALTERNATE OF PVD WITH 45+ MINS REMAINING. AS WE APCHED PVD THEIR WX WENT BELOW MINIMUMS (1200 FT RVR). WE HAD ALREADY DECLARED MINIMUM FUEL AND DIDN'T FEEL WE REALLY NEEDED TO DECLARE AN EMER IN ORDER TO BE LEGAL FOR THE APCH. BUT IN HINDSIGHT WE AGREED MAYBE WE SHOULD HAVE JUST TO COVER OUR BASES.