I think it can be dangerous to treat dream symbols as predictors of future events, for example, as in saying this symbol means future disappointment, or future good luck.  This minimizes a symbol's personal importance by saying that it MUST mean the same thing to everybody, and not only that, it treats dreams as merely attempts at prophesying rather than as snapshots of what's going on in the human unconscious.

Tornadoes are basically violent cycling gusts of wind.  Terribly destructive and dangerous.  Even a reply or two posted here indicate that the dreamers tended to dream of this symbol in times of stress and upheaval.  Air is traditionally the element associated with thinking and the intellect, so to dream of a tornado might indicate that one's thoughts are in a state of disarray and upheaval and are beyond one's control.  One might fear that they will lose control, themselves.  A tornado is something beyond our control and often causes great destruction and confusion wherever it goes.

I used to dream of tornadoes frequently when little, not so much now.  Oddly I think my own tendency to dream of them was based more on my childhood fear of one striking our home (I live in the Midwest, where tornadoes are frequent).

Of course, everyone will have their own personal associations with tornadoes, so an attempt at defining a symbol can be seen as only that, an attempt, which is not always necessarily correct.  Someone might even view a tornado as a good sign based on personal experience.  For example, a tornado tears something down, which leaves room to build something anew.