Monitor the National Weather Service radar moving images of recent cloud and rain activity, and go where the storms will be at sunset.
The main exposure was 1/20th second at f/16, ISO 1250, 16mm focal length. A 2-stop graduated neutral density filter was used to darken the sky to match the reflection exposure. Five exposures captured one stop apart were adjusted in Adobe Lightroom, then combined in PhotomatixPro 6.
Storms were over two small mountain ranges in the Eastern Sierra a couple of nights ago. I picked the one with a lake that would let me catch a reflection of the storm at sunset.