CPR JET EXPERIENCES RWY EXCURSION DURING LNDG PROC WHEN VISIBILITY WAS REDUCED SUDDENLY DUE TO BLOWING SNOW AND RWY CONDITION ICE SNOW.

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AFTER DROPPING THE LAST OF OUR PAX OFF IN LEX; MYSELF AND SIC WERE RETURNING HOME FOR THE EVENING. WX WAS FORECAST TO BE MID-OVCST AND GOOD VISIBILITY WITH A CHANCE OF LIGHT SNOW SHOWERS. SIC CHKED ATIS APPROX 15 MIN OUT AND WX WAS RPTED AS 4000 OVCST; 10 MI VISIBILITY WITH WINDS 270 DEG AT 15 KTS. WHILE BEING VECTORED FOR AN ILS TO RWY 24L; APCH CTL ADVISED US THAT TWR VISIBILITY WAS DOWN TO 1/2 MI IN SNOW AND BLOWING SNOW AND WINDS HAD INCREASED TO 280 DEG AT 22 KTS. SIC ASKED FOR BREAKING RPTS AND WE WERE TOLD 'SEVERAL ACFT RPT BRAKING ON 24R AS FAIR; THE ONLY ACFT TO LAND ON 24L WAS AN SMT JET; HE DIDN'T SAY ANYTHING SO I ASSUME IT'S THE SAME OVER THERE.' MYSELF AND SIC ASSUMED THE SNOW HAD JUST BEGUN AND THEREFORE ACCUMULATION WOULD BE MINIMAL AND WOULDN'T BE A PROBLEM; SO WE CONTINUED FOR 24L. AFTER A NORMAL TOUCHDOWN ON CTRLINE; I CALLED FOR SPOILERS AND DEPLOYED THRUST REVERSERS. AS I LOWERED THE NOSE ALL I COULD SEE WAS WHITE DUE TO ABOUT AN INCH OF ACCUMULATION ON THE RWY AND THE FACT THAT IT WAS SNOWING VERY HARD AT THIS TIME. WE DEPARTED THE L SIDE OF THE RWY AT A HIGH RATE OF SPD AND CAME TO A STOP IN THE MUD AFTER A THRILLING RIDE. WHEN WE GOT OUT OF THE AIRPLANE WE NOTICED ABOUT 1/4 INCH OF ICE UNDERNEATH APPROX 1 INCH OF SNOW. LUCKILY ONLY MINOR FLAP DAMAGE OCCURRED CAUSED BY STRIKING 2 RWY LIGHTS AND NO ONE WAS INJURED. I NOW KNOW WHAT HAPPENS WHEN ONE ASSUMES TOO MUCH; HOWEVER AND HOPEFULLY WILL NOT ASSUME TOO MUCH IN THE FUTURE. SUPPLEMENTAL INFO FROM ACN 200242. I NOTICED THE RWY EDGE LIGHTS VERY CLOSE TO THE L SIDE OF THE ACFT; IT WAS NOT APPARENT STRAIGHT AHEAD; ALMOST A WHITEOUT CONDITION. THE PIC WAS NOT ABLE TO CORRECT THE ACFT'S DIRECTION AND WE LEFT THE RWY AT A HIGH RATE OF SPD. WE WERE NOT EXPECTING RWY CONDITIONS OR VISIBILITY TO BE SO BAD. POSSIBLY IF WE HAD ASKED FOR 24R WHICH HAD CTRLINE LIGHTS WE WOULD HAVE AVOIDED THE VISIBILITY PROBLEM AND IF THAT RWY WAS INDEED TO BE FAIR BRAKING ACTION WE WOULD HAVE BEEN ABLE TO KEEP THE ACFT STRAIGHT. THINGS ARE NOT ALWAYS AS THEY APPEAR.