What to pack in mid-late October

Greetings,
I will be travelling to Japan this mid-late October, trying to pack as light as possible.

My travel will include visit to north Tohoku as well as to south Kyushu and the Tateyama Alpine route and some light "hiking" in the mountains (Kaimon, Daisen).

I would suggest a jacket (not sure what kind), fleece shirt if cold, t-shirts, pants and some light rain gear.

Should I bring some other kind of outer clothing? I heard Tateyama can be very cold (2-8 deg C) and not sure about the mountains either so maybe I travel too light?

Appreciate your help!

My advice is always to pack layers and one good jacket, a windbreaker/waterproof style is best. If you've got layers, then you can add and subtract depending on where you are. So, for example, you could pack a nice undershirt, a regular shirt or sweater to go over that, a fleece jacket/inner jacket for over that, and finally your main jacket. The fleece inner layer can go on or come off as needed, as can the jacket. That way you can be versatile for the weather.

I did buy some kind of Goretex jacket recently made by Tierra but not sure if it might be too thick or heavy really. The other jackets I have are one with lining I think its called and a simple rain jacket I use for small rain. Not sure if I should buy a new jacket again for the trip.