Bob's car has broken down, far from civilisation, and he is forced to walk to the nearest phone. As he takes his first steps, a thunderclap sounds over-head and it begins pouring with rain.
Often used as comedy, the instant rainstorm, almost always preceded by a thunder-clap, relies on Willing Suspension of Disbelief
. In reality, though thunder may coincidentally cue a rain storm, lightning (and thunder) are unrelated to the occurence of rain, and will more likely sound during a large storm.