When shooting a stormy scene, you don’t have to wait and hope that your local weather station would announce that lower pressure is building and there’ll be a coming thunderstorm. You can opt to shot even with the sun shining brightly and then later add CG effects to make the sunny weather a bad one.
The article “Making a CG Rainstorm with After Effects Plug-ins and Automated Rotoscoping” discusses the latter option. Based on a commercial for BMW, it shows how a scene shot in sunlight is transformed into a fierce rainstorm.
When making this commerical, several techniques were employed. Rain and falling stones are generated by the plugin Particular. One can use it to render 3D particle simulations directly in After Effects. To postprocess the filmed BMW the team used Motor, a popular rotoscoping software by Imagineers.