What is the critical differentiator between a severe thunderstorm that will not produce a tornado and one that is capable of doing so?
Answer
Rotation
While darkness signals intensity, the presence of rotation around a vertical axis is the essential characteristic differentiating a severe storm from one capable of producing a tornado.

#Videos
Everyone should know to look out for these clouds - YouTube
Related Questions
What does the dark coloration of storm clouds primarily indicate about their composition?What visual feature is the most reliable indicator that a tornado might be imminent?What is the critical differentiator between a severe thunderstorm that will not produce a tornado and one that is capable of doing so?What specific precipitation characteristic defines a wall cloud?What weather phenomenon is signaled by a shelf cloud?What determines the actual, visible color of a mature tornado?What is the primary characteristic that distinguishes scud clouds from a developing wall cloud?If a storm cloud appears nearly black, what does this imply about the cloud structure?What does the overarching meteorological principle state regarding dark clouds and danger?Why is waiting for a visible funnel cloud to descend potentially dangerous?What physical feature are shelf clouds associated with?