What visual feature is the most reliable indicator that a tornado might be imminent?

Answer

The development of a wall cloud

The development of a wall cloud, an isolated, often abrupt lowering of the rain-free base, serves as the most reliable visual cue that a tornado may be forming.

What visual feature is the most reliable indicator that a tornado might be imminent?

#Videos

Everyone should know to look out for these clouds - YouTube

weatherskycloudtornado