the clouds seems detached from the rest of the picture, and do not create the feeling like you are under it.
can try a couple of things:
1. landscape oreintation: if u didnt take one then nothing u can do abt it.
2. make the clouds the single dominant element in the pic, maybe occupying 90% or more of the pic, so that it gives a completely overwhelming feeling, such as this example of mine.
3. wait a while, or change ur angle so less of the bright clear sky is seen.
as u can see most of the suggestions can only happen at the point of capture. u can tweaked the picture as it is but I dun think it can competely convey the impression u desire.
as for tweaking u can do the following:
1. darken the clear skies, that may help a little
2. clone out some of the bright sky: tough to pull off successfully
3. stretch the contrast in the clouds, give it more textures, but this is really secondary, it works only after the other elements are in place