Found this link which has some explanation as to how tornadoes are formed.
http://www.geocities.com/joefurr2/torform.html
Tornado Formation
The Ingredients
To have a tornado you need a thunderstorm. To have a thunderstorm you need three things:
. instability
. an uplifting mechanism
. moisture in the mid to lower levels of the atmosphere
An unstable air mass is warm and moist near the ground and relatively cold and dry in the upper atmosphere. If an air mass is unstable, air that is pushed upward will continue upward.
Lift is the mechanism that pushes the air upward. Sources of lift can be cold fronts, mountains, converging air (as in Florida), or differential heating. The most common of these is differential heating. Differential heating takes place when portions of the earth's surface warm more readily than other nearby areas. These "warm pockets" are less dense then the surrounding air and will then rise.
The final ingredient is moisture. As air rises in a thunderstorm updraft, moisture condenses into small water drops, which form clouds. When the moisture condenses, heat is released into the air, making it warmer and less dense than its surroundings. This lets the updraft continue rising.
The Life of a Thunderstorm
There are three stages in a thunderstorm's life. They are the cumulus stage, mature stage, and dissipating stage.
The cumulus stage is the development stage. During this stage there is only an updraft and no precipitation reaching the ground. As the storm develops, precipitation is produced in the upper parts of the storm. This starts the downdraft.
The mature stage is the height of the storm's strength. During this stage there is both an updraft and downdraft, precipitation, and possible severe weather. As the precipitation spreads throughout the storm the downdraft grows. This is the stage in which tornadoes form.
The dissipating stage is dominated by the downdraft. As the downdraft strengthens, it cuts off the updraft and stops the inflow of moist air. This leaves the storm with nothing else to do but rain itself out.
Thunderstorm Types
The Single Cell Storm
Single cell thunderstorms have lifespans of 20-30 minutes. They are usually not strong enough to produce severe weather. A true single cell storm is actually quite rare.
Photo of a Single Cell Storm
More Information on Single Cell Storms
The Multicell Cluster Storm
This is the most common type pf thunderstorm. It consists of a group of cells, moving as one unit, with each cell in a different phase of the thunderstorm life cycle. The mature cell is usually found at the center. Although each cell may last only 20 minutes, the cluster may last several hours. These can produce heavy rainfall, downbursts, moderate-sized hail, and occasional weak tornadoes.
Photo of a Multicell Cluster Storm
More Information on Multicell Cluster Storms
The Multicell Line Storm or "Squall Line"
This consists of a line of storms with a continuous, well-developed gust front at the leading edge of the line. This line of storms can be solid or it can have gaps. The main threats with these storms are golfball-sized hail, heavy rainfall, and weak tornadoes. However, they are best known for their downbursts.
Photo of a Squall Line
More Information on Squall Lines
The Supercell Storm
This is a highly organized thunderstorm. Although these are rare, they pose a great threat to life and property. This is like a single cell storm in that it has one updraft. However, the supercell updraft is extremely strong. This storm has a rotating updraft, or mesocyclone, that is the key to its ability to produce severe weather. This storm can produce large hail, strong downburst, and strong to violent tornadoes.
Photo of a Supercell
More Information on Supercells
Convective Variables
Research has found that if the environment (uplifting, instability, or moisture) of a storm has changed then the type of storm favored to exist may also change.
The amount of vertical wind shear in a storm's environment is critical in determining the type of storm that will form. Vertical wind shear is defined as a change in wind direction or speed with height. If the vertical wind shear is weak, the multicellular storms with short-lived updrafts will be favored. If the vertical wind shear is stronger than the updraft, then storms with longer-lived updrafts will develop.
Photo of a thunderstorm in an extremely high shear environment
Closely related to the concept of vertical wind shear is veering. Veering is defined as a clockwise turning of the wind direction as we move up through the atmosphere. If there are two layers of clouds in the lower levels of the atmosphere and the direction turns clockwise between the lower and upper layers, then veering is present.
Computer simulations and observational studies have suggested that veering of the low-level wind is instrumental in the production of storm rotation. Once this vertical rotation is established, a mesocyclone can develop which may produce a tornado or other significant severe weather.
The amount of moisture in the air has an effect on storms too. If the amount of moisture is low, then the storms tend to have high bases. If the amount of moisture is high, the storms tend to have low bases. The higher the cloud base, the better the chance for microbursts. The lower the cloud base, the better the chance for flash flood-producing rains.
The Birth of a Tornado
As the mesocyclone strengthens it extends further downwards. At the same time, it is becoming more compact which is causing it to spin faster and faster. If this process continues, then the mesocyclone will reach to the ground, spawning a tornado.
the areas that need the rain there could be between 200-300mm of rain in some areas from 5am Saturday morning till 11pm Sunday this wekend.
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