COMMUTER ACFT; DEVIATING WX; LANDS AT UNAUTH ARPT. DIVERSION TO ALTERNATE.

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CAPT X AND I DEPARTED CHICAGO'S (ORD) WITH DEST BEING GREEN BAY (GRB). WHILE ENRTE WE ENCOUNTERED TSTM ACTIVITY WHICH WE DEVIATED WELL E OF OUR INTENDED COURSE (TOWARD E SIDE OF HALLE; MICHIGAN) TO AVOID SUCH ACTIVITY. ONCE BREACHING OUT OF MOST WX WE PROCEEDED DIRECT GRB. WHILE ENRTE IT WAS OBVIOUS THAT SEVERE WX WAS MOVING RAPIDLY INTO THE GRB AREA BY USE OF THE AIRBORNE RADAR. WITH INTENTIONS OF TRYING TO OUTRUN THE TSTMS INTO THE GRB ARPT WE CONTINUED INBOUND. APPROX 25 NM OUT FROM GRB WE DECIDED TO ABORT THE ILS 3G APCH AS THE TSTMS HAD MOVED WITHIN 10 NM OF THE FIELD. WE OPTED TO HOLD SE OF GRB VOR WHICH WE DID FOR APPROX 30 MINS. MEANTIME DISCUSSING OUR OPTIONS CONSIDERING WX AND ALTERNATE ARPT AND FUEL WE CHANTED OUR ALTERNATE TO CENTRAL WISCONSIN'S ARPT (CWA). (ONE OF OUR SCHEDULED LNDG ARPTS.) AFTER SPEAKING WITH GRB FSS AND APPEARING THE TSTM ACTIVITY WAS MOVING S AND E TOWARD US; WE DECIDED TO CIRCUMNAV THE STORM TO THE W AND POSSIBLY THEN N WITH INTENTIONS OF LNDG GRB BY COMING IN FROM THE NW. AS WE PROCEEDED FURTHER W IT WAS OBVIOUS THE LINE OF WX EXTENDED WELL W OF GRB AND COULD SEE THIS WOULD NOT BE AN OPTION TO ATTEMPT GRB FROM ANY DIRECTION. AFTER DISCUSSING OUR SIT WITH CONSIDERATION OF THE WX FURTHER S TOWARD MILWAUKEE; CHICAGO; MADISON AND NOW W TOWARD CENTRAL WISCONSIN AND APPLETON IT WAS AGAIN OBVIOUS FOR THE SAFETY OF OUR PAX WE NEEDED TO LAND AT A SUITABLE ARPT. AT THIS TIME WAS WITHOUT QUESTION; OSHKOSH AS VISUAL WX CONDITIONS EXISTED. THIS DECISION BASED ON ATC AND THEIR ADVICE THAT TO ARRIVE CWA WE WOULD BE REQUIRED TO CIRCUMNAV A LINE OF TSTMS AS FAR S AS WISCONSIN DELLS VOR. W TO THE BACK SIDE OF THE STORM AND N TO CWA. AGAIN; ANOTHER CONVERSATION WITH DISPATCH AND DISCUSSING THE POSSIBILITY OF RHINELANDER (AS AN ALTERNATE) WHICH WOULD HAVE ONLY PUT US THROUGH THE WX WE DECIDED TO LAND OSHKOSH. PROCEEDING TO DO THIS WITHOUT DECLARING AN EMER IS PROBABLY PART OF WHAT COULD BE CONSIDERED CARELESS AND RECKLESS OP BEING WE DID NOT HAVE THE CORRECT PAPERWORK TO DO SO. THE ACTIONS TAKING PLACE IN THIS RPT ARE BASED ON TIME CRITICAL SITS DUE TO FUEL LOAD. IN OUR AIRLINES GENERAL OPS MANUAL WE COULD NOT CONDUCT SCHEDULED PAX OR CARGO OPS AT OSHKOSH WHICH WE HAD THIS KNOWLEDGE AND WERE TOLD BY DISPATCH. DUE TO THE WX CONDITIONS DEVELOPING AND BUILDING SO RAPIDLY ON THIS PARTICULAR EVENING MUTUAL DECISION BY CAPT AND MYSELF TO LAND AT AN ARPT IN VISUAL CONDITIONS WITH AN 8001 FT LNDG STRIP (RWY 36) SEEMED THE MOST SAFETY ORIENTED THING TO DO. OUR PROB LIES IN THE FACT IT'S NOT AN APPROVED ALTERNATE AND WE DID NOT HAVE THE APPROPRIATE LNDG DATA ONBOARD TO LAND AT OSH. SUPPLEMENTAL INFO FROM ACN 280877: UNFORTUNATELY WE DID NOT DECLARE AN EMER. IT WAS 13 1/2 HRS INTO MY DUTY DAY AND HAD FLOWN OVER 8 HRS AT THAT POINT. IN THE PREVIOUS 6 DAYS I HAD FLOWN 21 HRS AND 34 MINS. BY THE TIME I LANDED AT OSH I HAD FLOWN AROUND 30 HRS IN 7 DAYS. I WAS A LITTLE TIRED. I WAS ALSO THINKING ABOUT THE CRASHES THAT HAD OCCURRED DURING TSTMS IN THE LAST 6 MONTHS; I DID NOT WANT TO ADD MY NAME TO THAT LIST. I'VE LEARNED THAT I SHOULD HAVE DECLARED AN EMER. I ALSO LANDED WITHOUT PROPER DATA FOR OSHKOSH.