A B737-200 TAXIED ONTO THE STOPWAY AFTER LNDG. DURING ROLLOUT THE FLC HAD RECEIVED A CLRNC TO CONTINUE TO THE END. SNOW COVERING THE THRESHOLD LIGHTS AND DETERIORATING CONDITIONS DISGUISED THE RWY END. RWY TURNOFF OVERSHOT.

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UPON LNDG IN DETERIORATING CONDITIONS RWY 34 RIC; WE WERE ADVISED TO 'ROLL TO THE END' OF RWY 34. WE HAD DISCUSSED EXIT PLAN AT TXWY C. THE CALL CAME AS WE WERE SLOWING THROUGH 100 KTS; IN REVERSE THRUST; USING CAUTIOUS BRAKING. VISIBILITY WAS POOR WITH BLOWING SNOW; SNOW SHOWERS AND SNOW DRIFTS. RWY MARKINGS OBSCURED WITH SOLID SNOW; AND ICE COVERED THE RAMPS; TXWYS; AND RWY. RWY AND STOPWAY HAD BEEN PLOWED AND THE OUTLINE OF THIS ENVIRONMENT WAS OUR VISUAL CUE. LIGHTS AND SURFACE MARKINGS WERE COMPLETELY OBSCURED AND TAXI MARKER 'Q' (OUR TURN OFF) WAS EITHER PLOWED UNDER OR DRIFTED OVER. IT WAS NOT IN VIEW. THE END OF RWY AND START OF STOPWAY WAS NOT DISCERNIBLE. THE WHITEOUT AND DRIFTS OF SNOW COVERING IDENTIFIABLE TAXI LANE 'Q' HARD TO IDENT AS WE PASSED IT. WE RECOGNIZED WE WERE ON STOPWAY APPROX 80 FT INTO IT. THE YELLOW CHEVRONS AND MARKINGS WERE NOT VISIBLE BUT THE END OF RWY AREA SEEMED DIFFERENT. AN ACR FLT HAD BEEN ON RADIO FREQ WITH US DURING APCH AND I WAS UNSURE OF HIS POS. I ADVISED TWR 'WE ARE ON STOPWAY; NEED 180 DEG TURN FOR TXWY Q.' TWR OKAYED 180 DEG. OTHER ACR NOT CLOSE TO ARPT; AND NO CONFLICT. WE NEVER LEFT HARD SURFACE; NO COMMENT WAS ISSUED FROM TWR AND TAXI SPD WAS SLOW. IF THERE IS A CHANGE OF RWY EXIT PLAN; WE WOULD RATHER BE ADVISED IN THE AIR. DURING THESE DIFFICULT CONDITIONS; A PLOW TO RWY SURFACE MARKING TO SHOW RWY END OR CLEARING SNOW FROM RWY END LIGHTS IS NEEDED TO DIFFERENTIATE STOPWAY.