ACR CREW WAS CLRED TO LAND ON RWY 16L AT SEA ARPT HOWEVER LINED UP AND LANDED ON RWY 16R. FLC WAS INITIALLY CLRED FOR AN ILS ON RWY 16R BUT WERE DISTRACTED ON FINAL BY 8- 10 BLACK BALLOONS ON THE FLT PATH AND LANDED ON THE WRONG RWY.

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APCHING SEATTLE WE WERE VECTORED FOR AN ILS TO RWY 16R. THE ATIS SAID THEY WERE CONDUCTING ILS AND BAY VISUALS TO RWY 16R. WHILE TRACKING LOC INBOUND WE BROKE OUT OF THE CLOUDS AND RPTED THE ARPT. I WAS THE PF. AT THE SAME MOMENT TWR CLRED US FOR LNDG (APPROX 2 NM OUTSIDE MARKER) WE SAW WHAT APPEARED TO BE A SMALL PLANE AT OUR ALT APCHING US. WE STOPPED OUR DSCNT AND AS IT PASSED BELOW US WE SAW IT WAS A PACK OF 8-10 BLACK BALLOONS. WE WERE NOW SLIGHTLY HIGH AND I WAS BUSY GETTING US ON PROFILE. THE CAPT HAD REPEATED OUR LNDG CLRNC FOR RWY 16L BUT WE WERE BOTH DISTRACTED BY THE BALLOONS AND IT DID NOT REGISTER THAT THIS WAS A CHANGE IN RWYS AS THE RWY CONFIGN IS VERY CLOSE IN SEA. AS WE APCHED THE RWY THE CAPT TWICE RPTED THE BALLOONS TO ATC. WE ALSO SAW AN ACFT TAXI DOWN AND EXIT OUR RWY (16R). HE WAS SLOW IN CLRING; BUT WE HAD TO DO A GAR THE PREVIOUS WK IN SEA FOR ANOTHER ACFT ON THE RWY; SO DID NOT THINK THIS WAS UNUSUAL. THE ACFT DID CLR AND WE LANDED ON RWY 16R. WE ONLY REALIZED WE WERE CLRED FOR THE L WHEN TOLD BY TWR OF OUR MISTAKE. WE WERE DISTRACTED AT THE VERY MOMENT OUR LNDG CLRNC WAS GIVEN. BUT THE ACFT TAXIING ON OUR RWY DID LOOK STRANGE AND WE SHOULD HAVE CLARIFIED CLRED TO LAND WITH ATC. BUT A SIMILAR SIT HAD HAPPENED THE PREVIOUS WK. ALWAYS ASK IF SOMETHING DOESN'T SEEM RIGHT.