A320 FLT CREW ENCOUNTERS WINDSHEAR DURING FINAL APCH TO SEA.

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WE WERE ON FLT TO SEA. I WAS FLYING THE A319 ON THIS LEG AS FO AND EXPERIENCED WINDSHEAR WHILE ATTEMPTING TO LAND THE ACFT. ON DSCNT INTO SEA; THERE WERE NO WINDSHEAR ADVISORIES OR ANY OTHER TYPE OF HIGH WIND WARNINGS. ATIS RPTED THEY WERE LNDG RWYS 34L/R. WE BRIEFED THE APCH FOR FULL FLAPS AND AUTOBRAKES LOW. THE TWR CLRED US FOR AN APCH TO RWY 34R AND RPTED WINDS WERE WITHIN FLT MANUAL LIMITS AND VARIABLE FROM THE ENE WITH SOME GUSTS. ON APCH; I COMMENTED TO THE CAPT ON THE WINDS WHICH WERE APPROX 120 DEGS AT ANYWHERE FROM 20-30 KTS. WE CONFIGURED AND BEGAN OUR DSCNT PAST THE FINAL APCH FIX. THE RIDE THEN STARTED BECOMING BUMPY AND TURBULENT. MY CAPT SUGGESTED WE BUMP UP THE AIRSPD 5-10 KTS AND I AGREED. WINDS WERE STILL A R QUARTERING TAILWIND AND STRONG BUT TWR RPTED PREVIOUSLY WINDS WITHIN LIMITS. IT WAS SOMEWHERE INSIDE 100 FT THAT I STARTED TO INPUT XWIND CTLS AND THEN EXPERIENCED WINDSHEAR. THE ACFT WAS VERY DIFFICULT TO CTL AND WE INITIATED A GAR. THE ACFT RECOVERED AND MY CAPT RPTED A SEVERE WINDSHEAR TO SEA TWR. TWR RPTED TO US WHILE ON VECTORS FOR ANOTHER APCH THAT A GUST FRONT HAD COME THROUGH WITH WINDS GUSTING 25-30 KTS. WE THEN ASKED WHAT THE WINDS WERE AND THEY WERE WITHIN LIMITS. WE DECIDED TO DO AN APCH TO RWY 34L THIS TIME BECAUSE IT WAS FURTHER FROM THE ARPT AND MAY REDUCE THE CHANCE OF ANY STRUCTURAL INDUCED TURB FROM THE ARPT. WE CONFIGURED WITH FULL FLAPS AND AUTOBRAKES LOW WITH AN EXTRA 5-10 KTS ON FINAL. I LANDED THE ACFT AND WE TAXIED BACK TO THE GATE. THE CAPT TOLD TWR ON TAXI BACK THAT HE RECOMMENDED THEY SWITCH RWYS TO RWY 16L/R. SUPPLEMENTAL INFO FROM ACN 601802: AT ABOUT 100 FT AGL; THE FO APPLIED L RUDDER NORMALLY TO ALIGN THE LONGITUDINAL AXIS OF THE AIRPLANE FOR LNDG AND WAS IMMEDIATELY HIT BY A LARGE WIND GUST. THE ACFT ROLLED ABRUPTLY L AND BEGAN A VERY RAPID DSCNT ACCOMPANIED BY A RAPID CHANGE IN GND TRACK. WHEN THIS OCCURRED; THE CAPT SAID 'LET'S GET OUT OF HERE -- I HAVE THE AIRPLANE.' WE SELECTED MAX THRUST; AND THE CAPT ASSUMED THE PF ROLE. FULL AND SUSTAINED PITCH-UP AND R-ROLL CTL WERE NECESSARY TO PREVENT AN UNINTENDED GND IMPACT DURING THE EARLY STAGE OF THE RECOVERY.