Thunderstorms are a pretty common occurrence in North America. Even northern portions of Alaska see a few storms each year. There are four main types of thunderstorms: ordinary cellular storms, multi-cell clusters, squall lines, and supercells. Supercells are the rarest and the most intense of the four types. They cause the majority of severe weather events including hail, strong winds, flash flooding, and tornadoes.
A supercell is a unique kind of cellular thunderstorm that is very well structured and organized. The main characteristic that sets them apart from other storms is a rotating updraft. Like all thunderstorms, supercells are formed when warm, moisture-rich air masses collide with cooler, drier air masses. However, supercells are much more likely to form in environments where the winds veer, or turn clockwise with height. The veering winds lead to the rotational updrafts, also known as mesocyclones. These mesocyclones act as their own miniature low pressure system. This makes it possible for a supercell to last several hours, much longer than the lifespan of an ordinary thunderstorm.
In an ordinary thunderstorm, warm, moist air is lifted up into the storm (the updraft) where it eventually condenses and forms rain. The falling rain causes the surrounding air to cool and become more dense, which then gets pushed down (the downdraft) to the ground. The cooler and drier air of the downdraft eventually chokes off the supply of warm, moist air of the updraft, causing the storm to weaken and then dissipate.
In a supercell, however, the rotating updraft means there is a constant supply of warm, moist air. The rotational nature steers the downdraft(s) away from the updraft; either on the back end (rear-flanking) or the front/side (forward-flanking) of the storm. This prevents a supercell's downdraft from stabilizing, and thus weakening the storm. The storm will continue on until it encounters a more stable air mass.
Rotating updrafts not only increase the lifespan of supercells, but they also increase the likelihood of severe weather within those cells. A strong updraft will take the storm's precipitation and push it higher into the atmosphere, where it freezes and becomes hail. The longer it stays at high altitude (or the stronger the updraft), the larger the hail becomes. Large hail stones are thus much more likely to form in supercells than garden variety thunderstorms.
Severe wind gusts (>58mph) are also more likely to occur in a supercell than in an ordinary thunderstorm. The enhanced winds produced by thunderstorms are directly related to the strength of their downdrafts. The faster the downdraft, the stronger the winds. Because supercells have such strong downdrafts, the winds they produce can often do considerable damage, even with no tornado present.
With their unique structure and amplified dynamics, most supercells display the same shape on a radar screen (image below). This shape appears in the form of a comma or hook echo return. The familiar hook signature on the bottom left is formed by the rear downdraft. The updraft is located in the notch just above and slightly to the right of the hook in the precipitation-free zone. This notch also happens to be the area where tornadoes form.
There is no definitive way to say how tornadoes form, even though they are one of the most-studied phenomena in meteorology. However, the rotational nature of a supercell often leads to tornadic development.
As discussed earlier, when a mesocyclone forms, it acts like its own mini low pressure system. As the rotating updraft of the mesocyclone intensifies, this low pressure area contracts and becomes very well defined. When the low contracts, the winds of the updraft begin to speed up, much like a figure skater pulling in her arms when going into a spin. This tightening of the winds often forms a circulation in the form of a funnel cloud. Initially, this circulation is horizontal to the ground, but if the updraft is strong enough, it will tilt the funnel cloud more vertically. When the circulation forms close enough to the rear downdraft, it will get "pushed" down towards the ground. If the downdraft forces the circulation all the way to the surface, it officially becomes a tornado.
As you can see, a tornado needs the unique characteristics of a supercell in order to take shape. Not all tornadoes form within supercells and not all supercells spawn tornadoes. Nevertheless, supercells do make up the vast majority of tornadic thunderstorms. Because of their dynamics, they also make up a large majority of severe weather in general, despite the fact that they are the rarest of the four types of thunderstorms.