AN A320 LANDED IN GUSTY WINDS FOLLOWING A WINDSHEAR ALERT AT 50 FT. THE ACFT REMAINED ON THE RWY IN DIFFICULT WINDS AFTER LNDG.

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LNDG IN EXTREMELY GUSTY CONDITIONS 330 DEGS AT 30 KT GUSTING TO 40 KTS ON RWY 1R IN ZZZ1. AT 50 FT; RA GOT A PARTIAL WINDSHEAR ALERT: 'WINDSHEAR' AND THEN A 'THIRTY' CALLOUT. I LANDED AND THE AIRPLANE BEGAN TO WEATHERVANE IN THE GUST. I CORRECTED BACK ONTO THE CTRLINE AND EXITED ABOUT 1/2 WAY DOWN THE RWY. THE ARPT WAS THEN CLOSED DUE TO WIND. POSTFLT WALKAROUND SHOWED NO SIGNS OF ACFT DAMAGE. BECAUSE OF THE WINDSHEAR ALERT; I SHOULD HAVE GONE AROUND AT 50 FT. I BECAME AWARE DURING THE DEBRIEF HOW TASK SATURATED I WAS TRYING TO LAND THE AIRPLANE. THERE WERE NO AIRSPD OR PWR EXCURSIONS TO INDICATED WINDSHEAR NOR ANY CONVECTIVE ACTIVITY. SUPPLEMENTAL INFO FROM ACN 774217: THIS WAS THE FIRST LEG OF A 4 DAY TRIP ASSIGNED TO ME ON RESERVE. I HAD NOT FLOWN WITH THE CAPT BEFORE. DURING THE PREFLT BRIEFING; THE CAPT AND I REVIEWED THE WX AND FLT PLANNING FOR THE SHORT FLT OVER TO ZZZ1. WE DISCUSSED THE GUSTY WINDS FORECAST FOR OUR ARR; AS WELL AS NUMEROUS PIREPS OF TURB ON THE ARRS AND DECIDED TO ADD EXTRA FUEL FOR HOLDING OR A DIVERSION. BELOW APPROX 12000 FT MSL; WE WERE ENCOUNTERING CONTINUOUS LIGHT; OCCASIONAL MODERATE TURB. WHILE TURNING BASE AT APPROX 2500-3000 FT AFE; THE CAPT DISCONNECTED THE AUTOPLT AND AUTOTHRUST. WE WERE CLRED FOR A VISUAL TO RWY XXR TURNING FINAL AT APPROX 2000 FT AFE; AND TO MAINTAIN VISUAL SEPARATION WITH ANOTHER ACR ON A VISUAL TO RWY XXL; WHOM WE HAD RPTED IN SIGHT. HANDED OFF TO THE TWR; WE WERE CLRED TO LAND ON RWY XXR. BY APPROX 1500 FT AFE WE WERE STABILIZED AND CONFIGURED GEAR DOWN; FLAPS FULL. THE CRAB ANGLE REQUIRED TO MAINTAIN CTRLINE WAS SIGNIFICANT. THE FMGC WAS RPTING WINDS OF OVER 50 KTS ON THE NAV DISPLAY. THE TURB WAS NOW BECOMING CONTINUOUS MODERATE. I WAS CONSTANTLY MONITORING THE AIRSPD; NOTING A RAPIDLY CHANGING TREND ARROW; BUT ACTUAL AIRSPD VARIATIONS OF ONLY 5-10 KTS AT MOST. THE MANAGED SPD BUG WAS IN THE VICINITY OF 150 KTS; WITH THE CAPT MAINTAINING A SPD OF CLOSER TO 140 KTS. I ASSUMED THAT HE FELT THE MINI-GND SPD PROTECTION TO BE EXCESSIVE. WE WERE WELL ABOVE VLS; AND OUR SPD WAS STABLE. OUR VERT PROFILE WAS ALSO NORMAL. AT SOME POINT I BECAME PREOCCUPIED MONITORING THE ACFT; BECAUSE AT ABOUT 300 FT AFE; I REALIZED I HAD FAILED TO MAKE THE 1000 FT AND 500 FT CALLS. AT ABOUT 200 FT; I CALLED '500 FT;' AND THE CAPT RESPONDED 'FINAL FLAPS FULL.' ALTHOUGH I COULD NOT SEE HIS STICK INPUTS; I COULD TELL THAT HE WAS EXTREMELY BUSY JUST FLYING THE AIRPLANE. SOMEWHERE BTWN 100-50 FT; I HAD TRANSITIONED MY GAZE OUTSIDE; WHEN WE ENCOUNTERED A SIGNIFICANT GUST. THE ACFT WENT THROUGH A RAPID PITCH CHANGE OF APPROX 5 DEGS. AT APPROX 50 FT THE REACTIVE WINDSHEAR SOUNDED 1 CYCLE OF 'WINDSHEAR! WINDSHEAR! WINDSHEAR!' I GLANCED INSIDE LONG ENOUGH TO SEE THE RED WINDSHEAR ANNUNCIATED ON THE PFD. LESS THAN 1 SECOND LATER; THE REACTIVE WINDSHEAR WARNING CEASED; AND WE HEARD THE AUTOMATED 'THIRTY' CALL. THE CAPT WAS IN THE FLARE AT THIS POINT WITH THE THRUST COMING TO IDLE. HE TOUCHED DOWN APPROX 2000 FT DOWN THE RWY; ON THE CTRLINE; AND IMMEDIATELY WENT INTO FULL REVERSE. SHORTLY AFTER TOUCHING DOWN; WE ENCOUNTERED ANOTHER SIGNIFICANT GUST AS WE WERE DECELERATING THROUGH APPROX 120 KTS. THE ACFT WEATHERVANED STRONGLY TO THE L; AND I WAS MOMENTARILY CONCERNED ABOUT THE CAPT'S ABILITY TO MAINTAIN DIRECTIONAL CTL. HOWEVER; AS WE SLOWED THROUGH 100 KTS; THE ACFT STRAIGHTENED OUT AND IT WAS CLR THE CAPT WAS ABLE TO KEEP THE ACFT ON THE CTRLINE. THE MAIN FEATURE OF THIS INCIDENT WAS THE FAILURE TO PROMPTLY EXECUTE THE WINDSHEAR RECOVERY MANEUVER THE INSTANT THE REACTIVE WINDSHEAR WARNING OCCURRED; WHICH LED TO PRESSING AN APCH THAT HAD BECOME UNSTABILIZED. THE CAPT AS THE PF SHOULD HAVE IMMEDIATELY REACTED TO THE WINDSHEAR WARNING; WITH OR WITHOUT PROMPTING FROM THE PNF TO EXECUTE A RECOVERY MANEUVER. AS THE PNF; I FAILED TO RECOGNIZE THE CAPT'S PREOCCUPATION AND FIXATION ON MAINTAINING BASIC CTL OF THE AIRPLANE; AND SHOULD HAVE BACKED HIM UP WITH A CLR DIRECTIVE; 'WINDSHEAR! GO AROUND!' A CONTRIBUTINGFACTOR WAS NOT BRIEFING THE FLT MANUAL WINDSHEAR PRECAUTIONS OR WINDSHEAR ESCAPE PROFILES. HAD WE DONE THAT; I THINK I WOULD HAVE BEEN LESS STUNNED BY THE WINDSHEAR WARNING AT SUCH LOW ALT; AND MORE PRIMED TO CALL FOR AN IMMEDIATE GAR. THE CAPT ALSO WOULD HAVE BEEN MORE IN THE MINDSET TO EXECUTE THE MANEUVER WITHOUT HESITATION. IT SHOULD BE NOTED THAT AT NO TIME DID WE RECEIVE A PIREP OF WINDSHEAR OR AIRSPD FLUCTUATIONS ON FINAL FROM PRECEDING ACFT; NOR WAS THERE AN LLWS ALERT.