According to the National Oceanographic and Atmospheric Administration, the conditions that can spawn a tornado occur so suddenly that accurately predicting a tornado more than a few minutes ahead of time is extremely difficult. The National Weather Service issues tornado watches when conditions are conducive to tornado formation in order to keep citizens alert and listening to weather broadcasts for storm warnings.
Any time severe weather systems become powerful enough, they have the potential to spawn tornadoes. The key factor that turns a strong thunderstorm into a tornadic storm is rotation — that is, when the convection of hot air sets up a motion in the clouds that builds into a powerful funnel cloud. Unfortunately, this rotation may begin only moments before a devastating tornado descends from the clouds, so when severe weather threatens, meteorologists must pay close attention to their instruments. A "hook echo" on Doppler radar, indicating a sharp divergence in wind direction over a small area, is one of the biggest warning signs of an impending tornado. While the National Weather Service only issues tornado warnings when a funnel is visually spotted or detected clearly on radar, the mere presence of a hook echo is usually enough for meteorologists to warn citizens in the affected area to take cover immediately.