Looking pretty good! I especially like the cloud-shapes in the right-foreground.
However, the "holes" to the left look a bit strange and the centre of the hurricane is very very big, dense and therefore a bit flat looking.
You might try to feed your imagemap-shader through a colour-adjust shader and play with the settings to change the definition of your mask.
I suggest to increase the black point from 0 to ~0.1 to create more contrast. Reducing gamma (2nd tab) might work as well.
Cheers,
Martin