PLT LOST CTL OF ACFT EXITING RWY ONTO TXWY DURING WIND RAINY CONDITIONS RESULTING IN AN EXCURSION FROM THE TXWY.

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ARRIVING AT PDX THAT EVENING; ATIS INFO WAS CURRENT: 19-20 KTS GUSTING TO 39 KTS; VISIBILITY 10 MI; 60 BROKEN 70 OVCST 8/4 ALTIMETER 29.94 LOC DME RWY 21. ON APPROX A 2 MI FINAL; THE TWR ADVISED WINDS 160-180 DEGS AT 38-40 KTS. AFTER WE HAD LANDED; THE TWR HAD ADVISED US TO TURN OFF AT TXWY C OR TXWY G. EVEN THOUGH I COULD HAVE MADE THE FIRST TURNOFF; TXWY C; I FELT THAT EXCESSIVE USE OF BRAKES WOULD BE NECESSARY. I CONTINUED TO ROLL OUT TO TXWY G. STRONG XWINDS PREVAILED AND IT TOOK CONSIDERABLE PWR TO GET THE ACFT TO TXWY G. DURING THIS PERIOD; RUDDER DEFLECTION OF UP TO 3/4 (OR MORE) WAS NECESSARY TO TRACK THE RWY CTRLINE. AS I STARTED TO TURN ONTO TXWY G; THE TURN APPEARED NORMAL. HOWEVER; WITHIN A COUPLE OF SECONDS; FULL R RUDDER WAS NECESSARY TO KEEP THE TURN GOING. AT THIS POINT; FULL R RUDDER WOULD NO LONGER KEEP THE TURN GOING. WHEN I REALIZED THAT; TO MY DISBELIEF; THAT INSTEAD OF TURNING THE ACFT WAS CONTINUING TOWARDS THE EDGE OF THE TXWY DIRECTLY IN FRONT OF ME; I STARTED DEPRESSING THE 'PARK BUTTON' FOR MORE NOSE STEERING AUTH. SINCE IT TAKES APPROX 7 SECONDS FOR THE ADDITIONAL STEERING AUTH; I REALIZED THAT GOING INTO THE GRASS WAS IMMINENT. EVEN THOUGH MY TAXI SPD WAS NOT EXCESSIVE; I FELT THAT APPLYING MAX BRAKING AT THIS POINT WAS NOT AN OPTION AND THAT MAINTAINING CTL STRAIGHT AHEAD WAS THE PRUDENT AND SAFEST COURSE OF ACTION. ONCE THE NOSEWHEEL WAS OFF THE TXWY; I STARTED TO APPLY LIGHT BRAKING; I WAS SURE THAT MY EXCURSION WOULD BE EXACERBATED BY EXCESSIVE USE OF BRAKING. THE ACFT CAME TO REST IN THE SOFT GRASS/MUD ON ABOUT A 45 DEG ANGLE FROM THE EDGE OF TXWY G. THE R MAIN LNDG GEAR STOPPED APPROX 6-10 FT FROM THE EDGE OF THE TXWY. DAMAGE: NONE -- CONFIRMED BY AN INSPECTION OF THE ALL LNDG GEAR BY COMPANY MECHS AND RETRACTION TEST. AS REQUESTED BY COMPANY MECHS; A NOSEWHEEL STEERING TEST WAS PERFORMED BY MYSELF TO CONFIRM THAT THE STEERING MECHANISMS WORKED PROPERLY AFTER TOWING THE ACFT OUT OF THE GRASS/MUD. CONTRIBUTING FACTORS: THE WX -- BLOWING RAIN; STRONG XWINDS IN EXCESS OF 40 KTS; RESULTING IN A STRONG WX VANING MOMENT. PLT -- MY DISBELIEF IN WHAT WAS OCCURRING. I COULD HAVE AVOIDED THE OUTCOME IN A COUPLE OF WAYS: FIRST; I SHOULD HAVE ACTUATED THE PARK BUTTON SOONER. THIS IN ITSELF WOULD HAVE GIVEN ME THE ADDED NOSEWHEEL AUTH TO NEGOTIATE THE TURN AND OVERCOME THE WX VANING MOMENT FROM THE WIND. SECOND; I FEEL NOW; ONCE I REALIZED MY IMMINENT SIT; I COULD HAVE APPLIED MORE BRAKE PRESSURE THAN I HAD ORIGINALLY THOUGHT.