To begin, I used a previous long exposure I had taken on the beach for the foreground. I then cropped out the sky.

In the second step I added the sky. I wanted a stormy feel so i went with a photo with cloud movement. I pasted the sky directly above the foreground.

I felt the sky wasn't dramatic enough, so I blended another sky on top or it from a different photo. I cropped out the part of the sky I wanted and pasted it over last step's. Then I used the "multiply" blend function to blend the two.

Again I wanted to add more to the sky, this time I was going after cloud movement. I added yet another shot using "multiply" layer blending.

To add the person, I cropped her out of another previous photo I had taken at the beach. I then carefully pasted and brushed the area around it, and eventually added a shadow.

To create the island's tree, I used the same photo as before and cut/pasted the object onto the island.

For all of the image steps, I changed the color photos to black and white. This way I could match the tones of the original foreground. I used lightroom to dodge and burn; in the end adding a slight vignette. I then added a few of my color / tone presets and exported to photoshop, where I added the lightning, adjusted curves, and added the rain (blur with noise). I highlighted the lightening using a brush and softened the edges to create a more realistic feel. Finally I smoothed out the horizon with a low opacity white brush.