As the API from Weather Channel has changed, the XBMC built-in weather forecast vanished from core and from 15th November onwards
it is fully addon based.
This feature requires a build from 15th November 2011 or newer and needs the following commit included:
In order to use the weather forecast you can specify which addon shall provide the weather information (by default Weather Undergound will be selected).
You start at specifying which addon shall provide the data:
2 Forecast for Unknown Locations
Weather forecasts for smaller cities/villages/communities locations outside of America/Canada, for example in Europe may or may not exist on the weather service you're using. The only thing you can do then is select the closest larger city/village/community.