Stormy skies, painted wet on wet.
Wet with clear water just the sky area, then drop on a strong mix of ultramarine blue, a little alizarin crimson and yellow to make a deep blue/grey.
Then while that's wet, lift out an area for the cloud, and drop in some more of the blue/grey around the bottom edge of the cloud and leave well alone!
When that's dry, add the land area with a deep grey near the sky, then add water and wash that colour down.
If you want you can add even more deep grey between the sky and land, put a mount around all 4 sides, stand back back and admire!!