Back from Seoul + Jeju ......overall good trip but weather was really too cold for me. Went for ski at Yongpyong resort, recommended!! the kids were very cool when they ski pass through me as I'm still newbies and fell few times, butt pains like hell.

YongPyong is 1 of the best in Korea, not sure about Jisan though.

Yes weather is very cold, wear thick gloves and leather boots if possible. Don't wear sports shoes if possible it's not suitable there as the wind will blow the cold wind into your feets 

Seoul is a bit unpredictable, and it won't have heavy snow like the rest of Korea. Even it snows it will be a while only.