Generally, warm air rises, but when the two fronts meet, the cold air can trap the warm air beneath it.
Tornadoes forming over warm water are known as waterspouts, and are most common in southern states and along the Gulf Coast. Tornado wind and debris cause most of the structural damage suffered, but nearly half of the injuries from such disasters occur after the tornado has left, during rescue work and cleanup.
Tornadoes can occur at any time during the year, but they form most frequently in the spring and summer, depending upon your location.
If your television or radio announces a tornado watch, this means that conditions are favorable for tornado.
If a tornado strikes and you are inside a sturdy building, go to the lowest floor, such as the basement or storm cellar. There are many good websites with tornado information, but also, many inaccurate and unreliable ones. Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. There were multiple tornadoes with only short separation, but the survey erroneously classified them as one tornado. That is more unusual than it seems, because most video that seems to show tornadoes merging actually involves either one tornado, or one among multiple subvortices, going behind another. To oversimplify this a bit, a tornado (or any other atmospheric vortex) is the most efficient way to move air from one part of the atmosphere to another on its size and time scale.
The Enhanced F-scale (simple table or detailed 95-page PDF) is a much more precise and robust way to assess tornado damage than the original. F1 through F5 were used in practice, with F0 attached for tornadoes of winds weaker than hurricane force. A tornado is considered "significant" if it was rated EF2 or greater on the Enhanced F scale, or at least F2 on the old F-scale.
Grazulis (1993) also included killer tornadoes of any damage rating in his significant tornado database.

It is important to know that those definitions are arbitrary, mainly for parsing out more intense tornadoes in scientific research. It may sound trite; but by far, the most important software in the tornado forecast process is within the human brain. Doppler radar signatures can tell warning meteorologists a great deal about a thunderstorm's structure, but usually can't see the tornado itself.
A few hours later, despite the tiny odds of a repeat, the second tornado in five days directly hit the base. By most measures, SPC (formerly SELS, NSSFC) has improved its tornado forecasting over the past few decades. There are many ways to objectively gauge forecast performance--for example, verifying tornado watches with tornado reports and both watch types by all severe reports.
The general trend from 1985 onward has been for a greater percentage of tornado watches to contain tornadoes, and for more significant (EF2-5) tornadoes to occur in watches and outlooks. Killer tornadoes, those striking densely populated areas, or those generating reports of exceptional damage are given highest priority for ground surveys. Most of the time, this happens either with multiple-vortex tornadoes or very small, intense single-vortex tornadoes.
Hurricanes tend to cause much more overall destruction than tornadoes because of their much larger size, longer duration and their greater variety of ways to damage property. We have some public domain images of typical examples of F0 through F5 tornado damage linked from this FAQ's F-scale page.
Otherwise, public-domain tornado damage pictures are scattered among the tornado-related historic news items of various National Weather Service websites.
Even when strongly gripping the girders (if they exist), people may be blown loose, out from under the bridge and into the open--possibly well up into the tornado itself. Whether or not the tornado hits, parking on traffic lanes is illegal and dangerous to yourself and others.
An average of 800 tornadoes are reported each year, resulting in 80 deaths and 1,500 injuries.

Tornadoes can last for a minute or an hour, and they can tear a damage path up to 10 miles long. FEMA encourages families to have a disaster plan in place for all contingencies, not only for tornadoes; this not only can keep you safe, but can allow for some peace of mind when families are separated. If the building has no rooms beneath the ground, get to the lowest level possible and find an interior room, putting as many walls as possible between you and the tornado.
She combines her degrees in English and Astrophysics to write about science, with an emphasis on all things space-related. There is a statistical trend (as documented by NSSL's Harold Brooks) toward wide tornadoes having higher damage ratings. In 1987, a tornado tore through Yellowstone National Park in Wyoming at elevations reaching up to 10,000 feet, making it the highest altitude violent tornado in the United States. An EF0 tornado may damage trees but not buildings, with winds ranging up to 85 miles per hour.
If a tornado warning is announced, it means a tornado has been sighted or indicated by radar. Watch for flying debris, which causes the most fatalities and injuries while the tornado is in process. An EF5 tornado is devastating; winds exceed 200 miles an hour, and buildings can be annihilated. The benefits of this extend into many areas, including: improving building codes for resistance against most tornadoes (since most are weak anyway), the insurance and re-insurance industries, construction designs and practices, and comparisons of tornado damage with their weather situations and radar signatures (for improved watches and warnings). Worth has compiled a very detailed study of several such violent tornado disaster scenarios in the Metroplex, which could be adapted to other major metro areas as well.

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