The essentials of this painting technique is many thanks to Ekaterina Smirnova who just recently posted a video on this.
So, the first step is to tape off the paper and lay down some lightning shapes using masking fluid. The masking fluid will help to retain the white of the paper for the most intense portion of the image. If you don't have masking fluid, you can use a scraping technique with an art knife or, you can always try using white or light yellow gouache instead.
For the masking fluid, I'm using the tinted Pebeo fluid which you can find in most stores. I prefer the tinted variety as it's much easier to see on the paper. It should be noted, you need to allow each step to dry before moving onto the next.
The second step is to do a light yellow wash, using raw sienna, over the forks of the lightning. This will help create some of the glow in later stages. Avoid harsh lines, you want to fade the glow into the paper.
This is where I started to go off script more... Here I used phthalo turquoise to lay in the initial wash of colour for the sea. I'll darken it later, this is just the start.
Now, going into the sky with payne's gray which is really more of a shadowy blue colour in my opinion. With this, you want to start with a really pale wash and then start to build the colour and depth of the stormy sky. The pale wash will give you a wet area to help avoid hard edges.
Once the sky is dry, we can go to work on the sea. In this case, you want to create the illusion of choppy waves as you would expect from wind-lashed water. I think this is the hard part...
Then, finally, we're done.
As an aside to all of this, I'm becoming much less enamoured of the Fabriano Artistico paper as I find it buckles a great deal. Anyways, fun to do, but I need more practice keeping edges softer. Maybe I should go to larger paper and use bigger brushes. Hmm.