The biggest area that stands out to me for improvement is your handling of values and understanding of light. They go hand in hand, but lets try to break it down.
I dont know if you used photo ref, or just went from imagination but something is not quite right with the lighting. Its very much looking like HDR, also know as high-dynamic-rendering. I have seen a lot of photos on google search that are manipulated to match this. Its essentially high contrast in both light and shadow but with detail in both. its very unnatural, and if you don't know what your looking for, its very easily to be mislead when working off ref pictures. Study light outside yourself is the only way of learning and avoiding this. Below I have provided an image of some HDR samples for you to see what I mean. These images are not mine and are from google search.
Your values are not really controlled. I have done a paint over to show you what can be achieved with understanding and learning values. The hill area in your image is very bright and drawing attention away. Big heavy clouds like the Cumulonimbus types are low to the ground and generate a large shadow area, with light often breaking through or diffused heavily in places. Of course you can have sunshine coming through, but I think with these clouds you want a dramatic scene so the viewer can feel that just before the storm feeling.
Colour wise, you have a very unflattering yellow tint to everything and those greens don't match up under the lighting conditions. You want to put more blues into the scene, especially your shadow areas and de-saturate your colour palette. I think, spend some time to study light and values, do as much plein air as you can, or simply get out and draw outside, it will help you immensely. Hope it helps and best of luck with it.