This is right up the top of Nasu. There are so many good onsens in the area and its hard to compare, but this is a very nice onsen.
Yes, that is snow!
The notable thing about this onsen is the very large outdoor baths and the very elegant interior of the building its in. The men's outdoor bath is actually mixed. The women have their own private outdoor pools too. It costs 1000 yen per person to use the onsen so its a bit more expensive than other places. Its well worth the price though.
We liked it so much we book a night there on Christmas Eve at 15000 yen each for 2 people. So we'll have more details later.