Tornadoes form when unstable air in a thunderstorm creates a horizontal rotation in the clouds and strong downdrafts draw that vortex down to the ground. Overlapping fronts can trigger the wind shear necessary to initiate a tornado's rotation, which is why meteorologists issue watches whenever severe thunderstorms threaten. Tornadoes can form with very little notice and are particularly unpredictable and dangerous weather events.
When weather fronts clash, sometimes warm and cool air layers overlap at the boundaries. This can create strong updrafts and downdrafts, unpredictable winds that carry moisture and warm air into the various layers of the atmosphere. The temperature differences between these layers help trigger air movement, setting up the horizontal rotation high in a storm that can transform itself into a tornado.
Often, the only warning of a tornado is a sudden increase in wind shear. Meteorologists use Doppler radar systems to detect this wind shear, looking for particular signatures that can indicate the beginning of a tornado. The most common sign of a tornado's formation is a "hook echo," a spiral-shaped radar return that indicates clouds being wrapped around a vortex. In many cases, the National Weather Service announces a tornado warning based on one of these echoes before anyone has visually spotted the tornado.