Nice! That sky totally deserves to be the central piece. The scene does look good.
Could use a bit of fog to give some color to the last line of trees in the distance, so to make them meld with the sky a little bit. Seem them standing there like how they're now (instead of slowly fading into the distance) reminds me that there's a map edge, instead of making me believe there's a rolling landscape beyond those hills.
Other than that, looks great as I've said =) Good work!