I have updated the area guides for the three currently online (SW, NW and Paradise, SE and NE coming in the spring) for the winter season (December through April). I've updated the overview guide (above link) also for the season. I won't update these monthly because you can get that information from the other news, access, conditions and monthly report Web pages.
The winter is probably the most consistent and the most dynamic of the seasons on and around Mt. Rainier, and it's the longest, usually from early-mid November through mid-late April. All the rest fit into the remaining months with the only exceptions for years where the spring is early from a low snowpack or the fall is later into November. And in some years winter will last into June at higher elevations.
The winter is the most consistent because of snow. Once into winter (late November to early December) the seasonal snow is almost present throughout the NP, the only excpetions will be at the lower elevations, about 2,000-2,400 feet near the entrances. But even snow will be there when major snowstorms drop the snow level to 1,500 feet elevation throughout the Puget Sound region.
The winter is also the most dynamic for weather, from clear, sunny days for a few days to a week to major rain-on-snow storms and flood events and major snowstorms. This is why you have to be prepared, meaning your vehicle, your photography and hiking gear and yourself. You also have to be flexible with your plans and schedule and always be prepared to change them during your visit.
Winter is also the time the NP is in winter operation mode where everything is closed save one entrance and two visitors areas. The only entrance open is the southwest, Nisqually, entrance along with the facilities at Longmire. The road from Longmire to Paradise is controlled at a gate east of Longmire. The road to Paradise is checked and cleared of snow before the gate is opened, usually about 9 am, meaning snowstorms will force the closure until after the storm and clearing. It closes at 4 pm.
And even then only the Jackson Visitors Center at Paradise is open, and only on weekends and holiday (10 am to 5 pm). Everything else there is closed. And all that is available are winter activities, snowshoeing, skiing, snowboarding and the snow play area for kids, the last two after at least 5 feet of snow is present.
That said, winter is a good time if you love winter, and especially snow. And you still have two entrance, southweste and Carbon River ones, where snow isn't always present and you can get some winter hiking in, at least to the snow where you can snowshoe from there if you want. These areas with the Mowich Lake entrance offer excellent snowshoe routes.