DO-328 EXPERIENCES A RWY EXCURSION DURING LNDG XWIND LNDG.

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FO'S LEG. PORTLAND ATIS INDICATED THAT THE VISUAL WAS ACTIVE FOR RWY 10R. A PLUS OR MINUS 20 KTS WINDSHEAR WAS RPTED ON FINAL SO BUG SPDS WERE ADJUSTED TO A REF OF 131 (UP FROM 111). LIGHT AND MODERATE TURB WAS EXPERIENCED ON FINAL. WE WERE CLRED FOR THE ILS JUST OUTSIDE OF YORKY AND RWY 10R WAS IN SIGHT SHORTLY AFTER PASSAGE OF YORKY. A PLUS 20 KTS SHEER WAS EXPERIENCED FOLLOWED BY A DECREASE WHICH WAS ANTICIPATED AND COMPENSATED FOR BY INCREASING PWR LEVERS. ACFT WAS SLIGHTLY L OF CTRLINE AND THE CAPT CALLED 'CTRLINE .' I ACKNOWLEDGED AND CORRECTED. WE ALSO EXPERIENCED A BUFFET WHICH CAUSED A MOMENTARY BANK TO R OF APPROX 20 DEGS. THIS WAS COMPENSATED FOR. OVER THE RWY THRESHOLD TRANSITION WAS MADE FROM A CRAB TO A SLIP AND LNDG WAS MADE ON THE R MAIN WITH GOOD ALIGNMENT AND ON CTRLINE. AS THE NOSEWHEEL AND L MAIN TOUCHED DOWN; I CALLED FOR CONDITION LEVERS MAX AND THE CAPT COMPLIED. I IMMEDIATELY PULLED THE PWR LEVERS TO FULL REVERSE (RAPIDLY). AT THAT SAME MOMENT; THE L WING (DOWNWIND WING) LIFTED INTO THE AIR SUBSTANTIALLY. ALSO AT THIS MOMENT; THE ACFT SMOOTHLY; BUT QUICKLY STARTED TO VEER TO THE R OF CTRLINE. THE CAPT SAID 'MY AIRPLANE' AND TOOK CTL ABOUT HALFWAY BTWN CTRLINE AND THE EDGE OF THE RWY. HE MOVED THE PWR LEVERS FORWARD AND MAINTAINED MY CTL INPUTS ON THE RUDDERS AND AILERONS. THE L RUDDER WAS DEPRESSED TO FULL DEFLECTION FROM TOUCHDOWN; TO THE ACFT COMING TO A STOP. THE APPROX TIME IT TOOK FROM LEAVING THE CTRLINE TO LEAVING THE RWY EDGE TOOK APPROX 3 SECONDS. THE TRANSITION AND DECELERATION TO A STOP WAS SMOOTH; DUE TO THE MUDDY CONDITION. THE TIRE TRACKS IN THE MUD TO THE AIRPLANE WERE APPROX 210 FT LONG AND THE POS OF THE ACFT WAS ESTIMATED TO BE 45 FT FROM THE RWY. THE L MAIN WAS UP TO THE HUB IN MUD; WHILE THE R MAIN AND NOSEWHEEL SEEMED TO BE ON THE SURFACE OF THE GND. THE CAPT SHUT THE L ENG DOWN AND CHKED WITH PAX TO SEE IF THERE WERE INJURIES AND THEN PROCEEDED TO DO AN ACFT INSPECTION. WE THEN SHUT DOWN THE #2 ENG AND SECURED THE ACFT VIA OUR NORMAL CHKLISTS. THE CAPT DETERMINED THAT PAX WERE SAFER ON THE ACFT AFTER CONSULTING WITH EMER RESPONSE PERSONNEL. I BELIEVE WE EXPERIENCED A WIND SHIFT FROM A R FRONT QUARTERING XWIND OF 150 DEGS AT 25 KTS TO A REAR QUARTERING WIND OF 220 DEGS AT 25 KTS GUSTING 30 AT THE MOMENT I PULLED INTO REVERSE. THE ACFT 'WX VANED' INTO THE WIND AND MOMENTUM CARRIED US AWAY FROM CTRLINE 22 DEGS. I FEEL THAT EVASIVE ACTIONS SUCH AS DIFFERENTIAL PWR OR ENGAGING THE TILLER WOULD HAVE RESULTED IN POSSIBLE CATASTROPHE DUE TO THE 'WING HIGH' CONDITION. I HAVE THOUGHT THIS ONE OVER AND OVER IN MY HEAD AND FEEL THAT ANYTIME THE WIND COMPONENT IS 20 KTS OR GREATER; IT IS BEST TO CHOOSE ANOTHER RWY OR DELAY LNDG UNTIL THE WIND CALMS DOWN A BIT. THE MAX DEMONSTRATED CAPABILITY OF THE DO328 IS PLUS OR MINUS 20 KTS. I THINK ITS IMPORTANT TO NOTE THAT THE NEWER 'MOD 110' ACFT HAVE MORE RUDDER AUTH AND ALL DO328 ACFT WILL BE 'MOD 110' BY THE END OF THE YR. A GOOD MOVE ON BOTH THE MANUFACTURER'S AND THE OPERATOR'S PART!