AN SA227 METRO III COMMUTER LANDS ON THE PARTLY PLOWED TXWY PARALLEL TO RWY 19; THINKING IT WAS THE RWY.

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ON AN NDB APCH TO RWY 19 AT SDY; THE ASOS WX RPTED VISIBILITY AT 1 1/4 MI AND CEILING 200 FT BROKEN 600 FT OVCST. WE INITIATED THE APCH PER DISPATCH REQUEST BECAUSE WE HAD THE REQUIRED VISIBILITY FOR APCH MINIMUMS OF 1 1/4 MI. ON APCH; WE BROKE OUT OF THE CLOUDS AT APPROX 1000 FT AGL WITH VISIBILITY APPEARING TO BE 1 1/4 TO 1 1/2 MI IN LIGHT SNOW. WE FIRST PICKED UP THE RWY AND ARPT ENVIRONMENT AT APPROX 1/4 - 1/2 MI AND ELECTED TO CIRCLE TO LAND RATHER THAN LAND STRAIGHT IN. THE RWYS HAD NOT BEEN PLOWED OF SNOW. IN THE CIRCLING APCH BACK TO RWY 19 WE IDENTED THE LNDG SURFACE AS PARTIALLY (APPROX 90%) SNOW COVERED AND INITIATED A NORMAL DSCNT TO LAND. UPON LNDG AT SDY; BOTH THE FO AND I DISCOVERED THAT WE HAD LANDED THE ACFT ON A PARALLEL TXWY ADJACENT TO RWY 19; WHICH WAS COMPLETELY SNOW COVERED. THE PROB AROSE BY US NOT BEING ABLE TO IDENT THE RWY AND LIGHTS DUE TO THE BRIGHT SNOW AND SNOW PILES AROUND THE RWY LIGHTS. CONTRIBUTING FACTORS WERE THE LOW VISIBILITY; UNPLOWED RWY AND BRIGHT SNOW. IT WAS DISCOVERED ONLY AFTER TOUCHDOWN AND DECELERATING THE ACFT BELOW 80 KTS TO 70 KTS; WHEN WE COULD SEE THE YELLOW TAXI LINE BTWN THE FINGER DRIFTS ACROSS THE TXWY. CORRECTIVE ACTION WAS TO IMMEDIATELY RPT THIS TO COMPANY DIRECTOR OF FLT OPS. OUR PERCEPTION WAS THAT WE HAD IDENTED THE LNDG SURFACE AND CONTINUED THE LNDG APCH. FACTORS CONTRIBUTING TO OUR MISPERCEPTION WERE NOT USING OTHER CUES TO AID IN IDENTING THAT THIS WAS INDEED THE PROPER RWY; BUT LOW VISIBILITY AND TIME WERE NOT IN OUR FAVOR TO AID US IN THIS DECISION. SUPPLEMENTAL INFO FROM ACN 424388: I; AS PNF; SPENT MUCH TIME MONITORING ALT; AIRSPD AND PWR SETTINGS AND HAVING NEVER FLOWN INTO SDY; DID NOT PROPERLY QUESTION THE LCL CAPT AS TO HIS CHOICE OF LNDG SURFACE. BETTER; MORE INTENSE ARPT LIGHTING AS WELL AS MORE STRINGENT SNOW REMOVAL WOULD HAVE HELPED AS WELL AS A PRECISION APCH. ON MY PART; INCREASED DILIGENCE IN CONFIRMING THE INTENTIONS OF CAPTS IS A MUST.