If you haven't seen Yellowstone in winter then you haven't seen Yellowstone

2011-2012 WINTER SEASON

Winter Opening Dates are Snow Permitting and all dates are subject to change

December 15, 2011—Most roads open to oversnow travel (weather and snow conditions permitting).
December 22, 2011—East Entrance Road (between East Entrance and Fishing Bridge) opens to oversnow travel.
Only two park roads are plowed in the winter and remain open for wheeled-vehicle use: The road between the North Entrance to the Northeast Entrance at Cooke City, Montana. & From Mammoth Hot Springs to the parking area at the Upper Terraces (approximately 1/2 mi. of road) They are maintained only during daylight hours and may close during severe storms.

Oversnow travel ends in March. Roads close at 9 PM on the days listed:
March 1, 2012—East Entrance to Lake Butte (Sylvan Pass.)
March 4: Mammoth to Norris
March 6: Madison to Norris to Canyon
March 11: Canyon to Fishing Bridge
March 15, 2012—All other groomed roads in the park are closed.
Only commercially guided oversnow vehicles are allowed on other park roads.  For information on snowcoach tours and snowmobile rentals, call SeeYellowstone.com at 1-800-221-1151 or visit www.seeyellowstone.com