Amazing website and app for maps and current conditions! It has online and offline maps for pretty much every hike in Japan. The app is partly in Japanese, but enough of it is in English so it can be used without understanding Japanese. The website can be translated with Google Chrome.
People also use it like Instagram for hiking, so it's a great resource to check current trail conditions, snow levels, parking, etc.
Two maps a month are free to download and use offline. The app uses GPS and is very accurate. It's great to check your location while hiking, track elevation gain, and more.
Honestly, if you download one app for Japan, it should be this one!
Weather website that works well for Japanese mountains.
Japanese weather website, use it with chrome and use the Kanji of the mountains. (I add them to all my posts)
Most reliable website to check for trains and busses. You might have to use local public transportation websites sometimes.