The weather in the Japan Tokyo Mission for the most part is pleasant. There are four seasons. Spring and fall are very lovely. But it gets very humid in the summer. The Japanese say it's mushi atsui or "sticky hot" during the summer. Winters are relatively cold with very little snow. Japan has a rainy season. You will need an umbrella and good rain gear. As you move north of Tokyo, winters grow colder. Sweaters and dry socks are a must.
See the Japan Meterological Agency Average Weather comparison of Tokyo and other Japanese Cities.
Japan often suffers such serious natural disasters as flooding from typhoons and torrential rain, volcanic eruptions, and earthquakes, but tornadoes are rare.