A tornado can form with very little warning in any severe thunderstorm which has sufficient wind shear. If you live in a tornado-prone area, always be prepared for severe weather conditions. Pay close attention to local weather forecasts, learn the characteristic warning signs of an approaching tornado, and be prepared to react fast. When a funnel cloud is coming your way, seconds count.
Signs of an impending tornado
= Sight =
If you are some distance away from the main part of the thunderstorm and rain is not blocking your view, you may be able to see the early signs of a possible tornado. A tornadic thunderstorm is usually a towering cumulonimbus cloud with an anvil. Nearly all tornadic thunderstorms have a rear flank downdraft which is completely free of rain. The sky may be green or yellow.
A wall cloud may form in the area of strongest updraft just ahead of this rain-free section. It may sink down from the main cloud or join to the main cloud from surrounding low-level scud clouds. Some wall clouds have a trailing tail cloud which usually points toward the front of the storm, where the heaviest rain is.
Roughly half of all wall clouds produce funnel clouds, which sink down from the wall cloud immediately or up to an hour after the wall cloud's formation. Wall clouds which rotate or tighten are more likely to produce tornadoes, but wall cloud rotation is not a prerequisite to producing a tornado. The funnel may also drop down from the tail cloud instead of from the wall cloud. A sinking funnel cloud becomes a tornado when it reaches the ground.
Do not count on being able to see a tornado approaching. A tornado which approaches at night may be nearly invisible. Many severe thunderstorms are so thick and dark that they turn day into night. Some tornadoes have no visible funnels. Tornadic thunderstorms also produce heavy rain and hail which can completely obscure the view. Large hail should always be considered a warning sign of a potentially tornadic storm.
= Hearing =
Approaching tornadoes are loud. Some tornadoes sound like the roar of an oncoming train. Others make a loud hissing noise. The sound of a tornado can be distinguished from thunder because it is ongoing at a steady or increasingly louder volume.
Depending on where you live, you may also hear a tornado siren if an approaching tornado has been spotted. However, most towns and cities outside the Great Plains don't have tornado sirens.
= Weather report =
A tornado watch is issued when conditions are right for tornadoes to form. A tornado warning indicates that a tornado has been seen, or that Doppler radar shows the characteristic hook shape of a tornado. In some cases, the National Weather Service may issue a Particularly Dangerous Situation statement about imminent, particularly dangerous tornadoes expected within the next 24 hours.
What to do when a tornado is approaching
Take immediate cover. Ideally, this means a basement or storm cellar. If you don't have that, get into an interior room on the lowest floor with no outside windows and nothing to fall on you. Have a battery-operated radio at hand, if possible. Do not leave until you hear an all-clear.
Do not take shelter under overpasses. These are subject to exceptionally high winds and offer even less protection than being in the open.
If you can see a distant tornado on the ground, check to see if it is drifting left or right. If it is doing neither, it could be coming right towards you. Find shelter fast.