Nuclear Readout Japan
Online help
The Weather warning screen will, after the initial map of japan has been loaded,look like this:
 

 
At the top there is a dropdown control (Choose prefecture...). By hitting this control you can select a japaneese prefecture to get a more detailed view about current weather warnings in that area:
 

 
By pressing the back softkey on your cellphone or by touching the symbol for the weather warnings again you can close the weather warnings view and return to the main screen.
 
Further options are explained here when new versions are released.