Watercolor can create expressive, moody skies perfect for describing stormy skies. The wet on wet and stroke and bleed techniques create convincing effects. The wet on wet technique is presenting the paper surface so that subsequent strokes form soft edges. The stroke and bleed technique involves making a brush stroke of fairly heavy pigment on a wet surface, then tipping the paper so that gravity pulls the pigment downward, suggesting rain falling from a cloud. By rotating the paper, you can change the direction that the paint bleeds, suggesting rain blown at a diagonal by the wind.
Here is how these techniques might be applied in a watercolor of a rainstorm in Yosemite Valley. Click on the first image to start a step-by-step slide show.