Japan is in an easy stretch right now, November. Nothing extreme is running: pack for the temperature and stop worrying about the season.
Going in February
Cold seasonDecember to February
Cold and dry on the Pacific side, with heavy snow on the Sea of Japan side and in the mountains.
What this means for your bag
Layers, not one thick coat
Three thin layers beat one heavy one: they cover a wider range and pack smaller. A base layer, a fleece and a shell.
See the Winter Warmth Kit packSomething genuinely windproof
Wind is what makes the cold unbearable here, and an umbrella is useless in it. A hood that closes, and gloves.
See the Weather Protection pack
Packing for Japan?
Build a packing plan around the season you are actually travelling in.
Seasonal ranges are typical, not guarantees — a monsoon can arrive early and a dry season can break. Check the forecast before you seal the bag.