EXPECTING THE RPTED 1 INCH OF PACKED SNOW ON RWY 1R AT KIAD; A CITATION PLT IS SURPRISED BY RUTTED CONDITIONS AND UP TO 4 INCHES OF LOOSE SNOW ON TOP OF THAT. ACFT CTL IS LOST ON LNDG; AND THE END OF THE FLT IS OFF THE RWY IN 4-5 FT OF SNOW.

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I WAS FLYING A CITATION EXCEL TO IAD. I WAS CAPT WITH A NEW HIRE FO; WE HAD A FULL WRITTEN RTE BRIEFING FROM OUR COMPANY DISPATCHER; HAD CONTACTED THE AFSS IN FLORIDA AND LEESBURG; VA; AND HAD CALLED THE CTL TWR PRIOR TO DEP AND HAD BEEN UNIFORMLY AND CONSISTENTLY GIVEN THE SAME RWY CONDITIONS AT IAD; 1 INCH PACKED SNOW ON THE RWY WITH BRAKING ACTION FAIR. AT APPROX XA00E (XA00Z) WE WERE ON FINAL APCH; AND AGAIN WERE TOLD THE BRAKING ACTION WAS FAIR. WHEN WE LANDED ON RWY 1R; IT WAS DARK; AND WE FOUND THAT ON TOP OF THE 1 INCH OF PACKED SNOW THERE WAS 3-4 INCHES OF LOOSE SNOW. FURTHER; AIRLINERS THAT HAD LANDED BEFORE US HAD LEFT RUTS; GROOVES; AND RIDGES IN THE SNOW; SOME RIDGES BEING 6 INCHES DEEP OR SO. AFTER TOUCHDOWN; OUR MAIN GEAR FELT AS IF IT CAUGHT A RUT OR RIDGE ON ONE SIDE OR THE OTHER; AND THE NOSE STARTED MOVING TO THE L. I STRAIGHTENED OUT THE NOSE; BUT THE MAINS BROKE TRACTION; AND THE AIRPLANE SLID OFF THE L EDGE OF THE RWY. THE L WING CAME TO REST ON TOP OF 4-5 INCHES OF SNOW; WITH THE R MAIN GEAR STILL ON THE PAVEMENT. WE SHUT DOWN THE ENGS; LEFT THE APU RUNNING UNTIL HELP ARRIVED; AT WHICH TIME; WE EVACED THROUGH THE EMER EXIT OVER THE R WING. WE HAD 2 PAX ON BOARD; NO ONE WAS INJURED AT ALL. I TOOK PHOTOGRAPHS OF THE RWY CONDITION; AND THE PEOPLE WHO DROVE OUT TO US WERE AMAZED THAT ACFT WERE BEING CLRED TO LAND ON A RWY WITH SO MUCH SNOW ON IT. I BELIEVE THE CAUSE OF THE ACCIDENT WAS THE CONTINUED INACCURATE RWY CONDITION RPTS WE RECEIVED. WE HAD PLENTY OF FUEL ON BOARD FOR HOLDING; AND HAD WE KNOWN THE TRUE CONDITIONS; WE WOULD HAVE HELD WHILE THE RWY WAS PLOWED; USED RWY 1L OR DIVERTED. IN THE FUTURE; MORE ACCURATE RWY CONDITIONS SHOULD BE DISSEMINATED.