Tornadoes can form extremely quickly under the right conditions, forming and touching the ground within minutes. It may take some time for the proper rotation to form high in the thunderstorm, but a funnel cloud can drop from the sky with very little warning.
Because tornadoes can form so quickly, meteorologists issue watches whenever conditions are favorable for tornado formation. They also use Doppler radar systems to watch wind shear in particularly strong storms, looking for the sharply divergent winds that can presage a tornado's formation.
Tornadoes develop a distinctive radar signature as they form, creating a hook-shaped echo of clouds wrapping around the central vortex. The mere presence of a hook echo is often enough for the National Weather Service to issue a tornado warning.