IBM bought WeatherUnderground, and consequently pulled all 3rd party API support, so now, no other weather apps (E.g. Hello Weather, BeWeather, etc.) can use it as their source.
I'm aware of the reasons of the new owner of Weather Underground IBM which took down this API. I only put a small note at Weather Underground app about the BeWeather 2 use previously. Indeed it seems Hello Weather also had it as an API before which I didn't knew. I only started using that app since recently. It's my favorite alternative now with AerisWeather and the Weather Company being the best alternatives as far as weather source is concerned for my area.