Tokyo Weather - The city of Tokyo's climate is in large part influenced by the mountains and sea. The climate is sort of like the coastal regions in the Southeastern United States. So things will change from season to season and you need to be aware of that when planning your trip.
Summers are hot and humid. Some feel they are downright uncomfortable so if you travel during this time, be sure to bring lots of loose fitting clothing, preferably of cotton material. Also during June and into July the baiu, or rainy season takes hold. It is a month long rainy time in Tokyo. So bring your umbrella and raincoat. The average temperature in July is 76.5 degrees F, but the humidity is high so keep that in mind.
During the Winters the weather is cool, though the sky is clear and sunny with little or no snow. The average temperature in January is 38.5 degrees F. If you travel during this time you will need winter clothing and heavy outer wear, gloves, scarves and hats.
During the Fall there can be typhoons with rains that bring flooding to the area. You may wish to bring your umbrella during this time once again.
Spring is the most comfortable of all the seasons. You may wish to travel to the area during this time so that you can see as much of the area as possible. Bring comfortable shoes so you can do a lot of sight seeing in the area. For up to the minute weather information in Tokyo, check this website. Weather.com.