add in dark clouds. used a few different brushes form a cloud brush pack i have had for years.
step 5 of 7
using the same brush pack add in some lighter versions of the clouds to finish the illusion of distance.
step 6 of 7
add some lightning strikes. have a lightning brush in an assorted brush pack i had. but the pen tool would have worked just as well for me. warped and bent the bolts as needed and added highlights to the clouds and boat for detail of the strikes.
step 7 of 7
add some additional filters and overlays to achieve the desired result.