The Glossary of Meteorology (AMS 2000) defines a tornado as a violently rotating air column beneath or descending from a cumuliform cloud which may or may not be visible as a funnel cloud. The most deadly and damaging of these storms form with rotating thunderstorms called supercells or mesocyclones.
Tornado forecasts in the US. are issued exclusively by the National Weather Service (NWS).
In Canada, tornado forecasts are managed by the Meteorological Service of Canada.
The National Weather Service’s Storm Prediction Center issues tornado watch areas. They are defined by the coordinates of a parallelogram within which conditions are favorable for the development of tornadoes.
A watch does not indicate that a tornado is imminent, but is intended to alert the populace to be prepared for a tornado warning.
Response to A Tornado Watch
The NWS issues tornado watches so that people become attentive to the weather conditions where they are. The Tornado Project, a company that gathers, compiles and disseminates tornado information, lists indicators of an impending tornado as a greenish tint to the sky; hail in combination with a tornado watch; an unusual quiet occurring within a thunderstorm; fast-moving or rotating clouds; a roar that has been likened to a waterfall, a jet or a train; and debris being pulled upward.
The local office of the NWS issues tornado warnings. A warning means that storm spotters have seen a tornado or that Doppler radar has indicated circulation that can spawn a tornado.
You should take immediate safety precautions when a warning is issued for your area.
Response to A Tornado Warning
When a tornado warning is issued you should move to the most sheltered location available. If you have a storm shelter, you should go to it immediately.
If you have a basement you should go there, and in any case you should be on the lowest level you can reach. If there is heavy furniture that might protect you from falling walls and flying debris, you should get under it.
If you have planned ahead and have easy access to mattresses or quilts, use them as a barrier to flying debris, but do not waste time searching for them. If you have no storm shelter or basement, go to the innermost, preferably windowless room like a walk-in closet or bathroom on the lowest floor of the building.
Bathrooms often have windows, but the pipes and the extra framing due to the plumbing add strength to the walls.
According to the NWS Online Tornado FAQ page, mobile homes are generally more susceptible to tornado damage than well-built houses or office buildings. If you are in a mobile home you should try to find a more sturdy building when a watch is issued.
If you are in a vehicle when a tornado is in your area you should leave the vehicle for sturdy shelter or drive out of the storm’s path. If you can do neither of these you should get out of the vehicle and lie flat in a low spot as far from the road as possible.