LGT DIVERTING TSTMS HAS FUEL CRITICAL CONDITION. REQUESTS TO DIVERT TO ALTERNATE. REQUEST DELAYED DUE TO WX AND DEPS. DECLARED EMER DIVERT TO ILLEGAL ALTERNATE FOR ACFT.

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WE WERE APCHING ORD FROM THE W OVER BDF IN VFR WX. WE HAD BEEN RERTED FROM THE JVL2 ARR CORRIDOR TO THE BDF ARR CORRIDOR DUE TO UNEXPECTED (NOT FORECAST) WX JUST N OF ORD. WE HAD BEEN PREVIOUSLY SLOWED; VECTORED; AND GIVEN HOLDING FOR SEQUENCE SPACING AND WERE THEN RTED OVER BDF TO ORD AS ALL ACFT ARRIVING FROM THE W WERE NOW BEING VECTORED OVER BDF. AS WE CAME CLOSER TO ORD WE WERE TOLD THAT ONCE 'IN TRAIL' SPACING WAS ESTABLISHED; THERE SHOULD NOT BE FURTHER DELAYS TO ORD. APCHING BENKY INTXN AT 11000 FT AND 270 KTS; WE WERE TOLD TO ENTER AN IMMEDIATE HOLD AT BENKY AS ORD HAD JUST CLOSED DUE TO WX. I CONTACTED DISPATCH AND TOLD THEM I WOULD NEED TO PROCEED TO MY ALTERNATE; MKE. I ADVISED ATC WE MUST PROCEED TO MKE AS WE WERE NOW DOWN TO DIVERT FUEL REQUIREMENTS. WE WERE VECTORED NW TO CIRCUMNAV THE WX LINE. I WAS MONITORING THE RADAR AND DETERMINED THAT RTE WOULD TAKE US TOO FAR N TO BE PRACTICAL. I ADVISED ATC THAT WE NEEDED TO PROCEED DIRECT TO MKE AND WAS GIVEN A TURN DIRECT TO MKE BUT THAT RTE; AS I COULD NOW SEE ON RADAR AFTER THE TURN; WAS COMPLETELY BLOCKED BY WX. I ADVISED ATC THAT WE MUST TURN SE; AWAY FROM THE WX AND WAS TOLD TO 'STAND BY.' I AGAIN TOLD ATC THAT WE MUST TURN SE AND I WAS AGAIN TOLD TO 'STAND BY.' ATC ADVISED THAT ORD WAS USING THE AIRSPACE S OF US FOR DEPS. WE WERE CLOSE TO THE WX LINE AT THIS TIME. I HAD THE OPTION TO PROCEED THROUGH WHAT I PERCEIVED AS A SQUALL LINE; WHICH WAS UNACCEPTABLE; AND NOW WITH MY FUEL SAFETY MARGIN DIMINISHING AND FOR THE SAFETY OF MY FLT DECLARED AN EMER AND TURNED SE; AWAY FROM THE WX LINE (MUCH RED ON RADAR) TO VFR WX. ATC ASKED IF I WOULD LIKE TO LAND AT ORD AND I ASKED 'ISN'T THE FIELD CLOSED?' ATC RPTED THAT THE FIELD WAS STILL CLOSED DUE TO WX BUT USING EMER AUTHORITY I COULD LAND AT ORD. MDW WAS CLR/WINDS LIGHT AND I DECIDED THAT LNDG AT MDW WAS THE SAFEST COURSE OF ACTION. WE PROBABLY LANDED WITH AROUND 6.5 GALLONS OF FUEL AND BLOCKED AFTER OBTAINING A GATE WITH 5.9 GALLONS OF FUEL. SUPPLEMENTAL INFO FROM ACN 245340: FLT WAS DISPATCHED WITH NO ALTERNATE BASED ON WX FORECASTS AVAILABLE AT DEP TIME. WHILE ENRTE; ALTERNATE OF MDW WAS ADDED. CHANGED ALTERNATE; DELETED MDW; AND ADDED RFD; BECAUSE MDW NOT AUTHORIZED AS LGT ALTERNATE. DUE TO INCREASING TSTMS NEAR CHICAGO ORD; A SECOND ALTERNATE OF MKE WAS ADDED. MDW WAS NOT FLT'S LEGAL ALTERNATE OR ALTERNATE AT TIME OF LNDG ON MDW. AFTER LNDG; FLT WAS FUELED AND REDISPATCHED TO ORD. FLTS BTWN MDW AND ORD IN LGT MODEL Y ACFT NOT IN OPS SPEC OF AIRLINE. SPECIAL PERMISSION FROM PRINCIPAL OPS INSPECTOR WAS NOT GIVEN; BUT WAS INTENDED TO HAVE BEEN GRANTED WHEN ASKED FOR. A GROSS WT FOR MDW ON RELEASE WAS OBTAINED FROM TABLE FOR RWY LENGTH AND ELEVATION AND TEMP DATA. ERROR IN NAMING MDW AS ALTERNATE WAS DUE TO MY THINKING OF AN LGT MODEL Y AS A NARROW BODY JET; SMALL ACFT; THUS USING AN MLG AND LGT X ALTERNATE. ONCE ALERTED TO MY ERROR WRONG ALTERNATE WAS CHANGED.