Advise for Ski Trip in mid of Dec

I will be travelling to Tokyo on the 13th of Dec and will be returning on the 20th. I would like to plan a 2-3 days ski trip in between my trip, however I realized that many of the ski resort opens after the 20th Dec.

Would anyone be able to advise a couple of possible ski/board place that I may visit at that time? Preferably somewhere accessible by train / bus, would be good if there is onsen.

Please advise, and thanks in advance.