Want to know what the weather will be like during your stay ? 
Well.........

Forecasting the weather is difficult in any country, more so in a 
mountainous island nation like Japan.
As such,  weather forecasts in Japan should be considered  "for 
amusement purposes only" -  they are so often wrong, sometimes it 
seems that the forecasters guess randomly what the weather will be like.

For what it's worth, the official forecaster is the Japan Meteorological Agency.

A second opinion ?
See the weather section of the Japan Times website.

The links above show temperatures in Celsius.
To convert to Fahrenheit, see this temperature converter.