Internal weather is pulled from 7Timer.com, NOAA or Yr.no. The default setting is automatic. I assume it picks the provider closest to your location. I haven't changed it, and it seems to work just fine. Once the internal forecast screen is up, it gives an option to go to Accuweather for an alternate forecast.
What's awesome about One More Clock is *any* weather app can be configured to open upon tapping an area of the widget. Any app at all can be selected.