SMT DRIFTS INTO SNOW BANK AT EDGE OF RWY AND LOSES CTL; DEVIATING FROM THE RWY.

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I HAD A COMPLETE WHITEOUT AT TOUCHDOWN ZONE; AND LOST COMPLETE RWY ENVIRONMENT. ACFT STRUCK 2 1/2 FT PLOWED SNOW BANK THAT WAS 15 FT OUT ON THE EDGE OF RWY WITH L PROP; AND WAS PULLED OFF RWY. WITH FULL REVERSE; HVY R BRAKING AND FULL R RUDDER; WAS ABLE TO KEEP ACFT CLOSE TO RWY EDGE. ACFT CAME TO FULL STOP AT TXWY INTXNS WITH NOSE OF ACFT POINTING PERPENDICULAR TO THE RWY. THE ACFT NOSEWHEEL WAS SITTING ON EDGE OF RWY. I EVACED THE ACFT AND ASSESSED PAX AND THERE WERE NO INJURIES TO CREW OR PAX. THE ATIS WAS 300 FT; 1/2 MI VISIBILITY AND FOG. SUPPLEMENTAL INFO FROM ACN 235470: AT THE OM TWR ADVISED VISIBILITY WAS 3/16 OF A MI. PLANE BEHIND ME LANDED APPROX 3-4 MINS LATER AND HIT A SNOW BANK WHILE LNDG. IF I HAD ENTERED THE AREA OF THICK FOG DURING FLARE; I WOULD NOT HAVE BEEN ABLE TO LAND. I CAN ONLY CONCLUDE THAT DURING THE LNDG ROLLOUT I ENTERED AN AREA OF THICK FOG TILL I TURNED OFF RWY. EVEN THOUGH I HAD REQUIRED VISIBILITY TO LAND IN LNDG ZONE OF RWY; PAST THAT VISIBILITY WAS WORSE. SLANT VISIBILITY FROM INFLT ALSO MAY HAVE MADE VISIBILITY DOWN RWY APPEAR BETTER THAN WHEN ACTUALLY BEING DOWN ON RWY DURING ROLLOUT.