What kinds of thunderstorms produce tornadoes?
There are three main types of thunderstorms that can generate a tornado.
The first is a supercell thunderstorm, a long-lived storm with an intense, rotating updraft (known as a mesocyclone) that can often be detected by Doppler radar. The tornado develops near the right-rear flank of the storm beneath a rotating wall cloud. Because the thunderstorm's updraft can be so powerful, most significant tornadoes (EF2+) and all violent tornadoes (EF4-5) develop with supercells.
The second type is a thunderstorm complex organized roughly along a line, known as a Quasi-Linear Convective System or QLCS. Here, the storm updraft is located along the leading edge of rain-cooled storm downdrafts that often push out ahead of the storm. Tornadoes that develop with updrafts along this gust front are generally weaker than those with supercells but can be strong (EF2-3).
Third, a thunderstorm can move over or develop along a local air mass boundary that has pre-existing pockets of vertically oriented rotation along it. The storm's updraft stretches the rotation into the vertical and strengthens the vortex. These tornadoes, often called 'landspout' tornadoes, tend to be weak (EF0-1) and brief, and develop as the storm is intensifying.
Sometimes the storm that produces a tornado is a hybrid that does not fit nicely into one of these categories. Note that all of these storm types can produce a weak (EF0-1) tornado, or a tornado over water (a waterspout).