Tornado
A "tornado watch" is given when the weather conditions look like a tornado could form. A 'PDS (Particularly Dangerous Situation)' watch is given when a likely tornado outbreak is to start, many strong tornadoes will form in the area, or an ongoing tornado outbreak is in the works in the area. A "tornado warning" is given if somebody has actually seen a tornado or if a tornado 'signature' (usually the storm has a 'hook' or 'U' echo) has shown up on radar. Tornado emergencies are issued in Special Weather Statements or Tornado Warnings saying that a powerful tornado is about to hit an area with a lot of people in it (especially cities in Tornado Alley), a tornado has been spotted, and the tornado is expected to cause deaths.