A CRJ700 WITH NO FLT PLANNED ALTERNATE DIVERTED TO A NEARBY VMC FIELD WITH A LOW FUEL STATE. BECAUSE OF FORECAST WX; DISPATCH SAID DIVERSION ARPT WAS NOT LEGAL.

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THE SIT EVOLVED BECAUSE OF THE FOLLOWING: ORIGINALLY; THE FLT FROM ORD TO HPN DID NOT REQUIRE AN ALTERNATE. THEREFORE; MINIMUM FUEL FOR THE FLT WAS ISSUED; IE; ORD TO HPN PLUS RESERVE FUEL PLUS A LITTLE ADD FUEL AND HOLD FUEL; AND NO ALTERNATE FUEL. WHEN WE ARRIVED NEAR HPN; SOME TSTMS OCCURRED; SHUTTING DOWN HPN FOR LNDG. WE WERE HOLDING OVER KINGSTON VOR -- JUST A BIT N OF HPN. WE DID NOT HAVE ENOUGH FUEL TO HOLD TO THE EFC TIME GIVEN BY ATC. WE IMMEDIATELY CONTACTED DISPATCH WITH OUR SIT AND REQUESTED THEIR INPUT ON WHERE THEY WOULD LIKE US TO DIVERT. THE WX WAS QUICKLY CLOSING OUT THE OPTIONS OF THE NEW YORK METRO AREA -- IE; LGA; JFK; AND EWR -- DUE TO THE VICINITY OF THE STORMS CROPPING UP. SWF WAS JUST TO THE IMMEDIATE W OF US AND IN THE CLR. ONE OF OUR OTHER ACFT WAITING FOR A RELEASE TO ORD WAS ON THE GND AT SWF. THEY INFORMED US THAT THE WX WAS NICE VFR CONDITIONS. WE COULD CONFIRM THAT ON OUR RADAR AND BY LOOKING OUT TO THE W OF OUR POS THROUGH THE WINDOW. WE REQUESTED DISPATCH TO LET SWF KNOW THAT WE WERE GOING TO BE THERE VERY SOON. AGAIN; OUR FUEL SIT WAS CRITICAL. SWF WAS IN THE CLR; WE WERE NEAR; AND WOULD BE ON THE GND IN 10 MINS. DISPATCH FINALLY ACKNOWLEDGED US. THEY WERE HAVING A HARD TIME MAKING A DECISION. TIME WAS OUT; AND I MADE IT DUE TO THE FUEL. AS WE WERE ON FINAL TO SWF; DISPATCH SENT US AN ACARS SAYING THAT SWF WAS NOW NOT A LEGAL ALTERNATE. IT WAS RECEIVED ON SHORT FINAL; FIELD CONDITIONS BETTER THAN 10000 FT AND 6 MI VISIBILITY AT THE TIME -- WE LANDED. I SPOKE WITH DISPATCH AFTER LNDG AS WE WERE GETTING FUELED. THE DISPATCHER APOLOGIZED FOR GETTING THE INFO TO US SO LATE -- COMPUTER SLOWNESS WAS THE REASON. THE UPDATED FORECAST FOR SWF HAD MADE IT AN UNSUITABLE ALTERNATE; I GUESS DUE TO THE TSTMS JUST CROPPING UP. AGAIN; I EXERCISED MY AUTH OF PIC UNDER THE SIT OF LOW FUEL; NEW YORK METRO BLOCKED BY WX; AND SWF WITHIN 5-10 MINS OF LNDG FROM OUR HOLDING POS AND IN GOOD VFR CONDITIONS; AND NO IMMEDIATE RESPONSE FROM DISPATCH. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT STATED THAT HE DID NOT DECLARE AN EMER. HE SAW THE ARPT NEARBY WAS CLR AND AVAILABLE; SO HE LANDED TO TAKE ON ADDITIONAL FUEL. THERE HAS BEEN NO CONSEQUENCE TO HIS ACTION OTHER THAN HIS DISPATCHER'S CONCERN THAT ON PAPER THAT ARPT'S WX MADE IT AN ILLEGAL ALTERNATE.