Supercell

Supercell thunderstorms are among the most dangerous thunderstorms that occur on the planet, and almost always produce severe weather of one kind or another. Ordinary thunderstorms can pass by with no more than a few rumbles of thunder and a heavy, but brief, deluge of rain. The supercell, though, is a factory for many of the most damaging weather types, ranging from large hail to vast wind gusts to numerous lightning strikes to some of the most powerful tornados recorded.

On the surface, a supercell looks much like any other thunderstorm. A rounded, shelf-like anvil projects from the storm's summit, sometimes with bulging mammatus clouds hanging from its underside, but this alone is not proof of a supercell.

The true sign of a supercell is a cloud dome poking up through the flat top of the anvil at the southwest corner of the storm -- the visible sign of the mesocyclone updraft that makes the dangerous weather characteristic of supercells. If you see a thunderhead with such a cloud dome rising from the flat summit, then this is probably a supercell; however, if the thunderstorm is moderately close before you can see it, then the cloud dome will probably be invisible, because you will only be able to see the underside of the anvil from the angle you are looking at the storm from.

Supercell thunderstorms are often isolated from other weather, and on radar they can be easily distinguished by their hook echo, which is caused the main updraft and rear flank downdraft curling around the mesocyclone and its tornado. For this reason, although it may not be possible for a ground observer to tell whether a storm is a supercell, meteorologists can quickly pinpoint such a storm and issue appropriate warnings.

Supercell development and traits

A supercell usually forms from convection in a very hot, moist environment, rather than as part of a frontal system. The sun heats already hot air and causes it to rise powerfully. The high humidity in the air condenses out into a towering cumulus, then into a cumulonimbus (thunderhead). The anvil marks the point where the rising air meets dry upper level air and spreads out. The updrafts in an isolated thunderstorm like this can reach between 150 and 175 miles per hour, and extremely powerful downdrafts occur nearby.

A supercell almost always has a mesocyclone as well. A mesocyclone is a rotating updraft like a huge tube in the thunderstorm. The mesocyclone is created when a whirling roll of air made by horizontal wind shear is tilted upright by the main updraft of the supercell and combines with it, creating a huge circular updraft at the southwest edge of the storm. This updraft is marked by a rotating wall cloud under the storm, and a cloud dome atop it. The south edge of a supercell usually has no visible precipitation falling from it because the updrafts here are very strong -- most visible rain and hail falls in the northern half of the storm. This rain-free base is one of the unique characteristics of a supercell.

Supercells last a few hours, typically, and during that time can produce a variety of damaging weather types. Tornados are the most destructive, and occur in about 30% of supercells. Supercell tornados are often much stronger than frontal tornados, and are usually produced from the mesocyclone's base. Very large hail of two to three inches or more, downburst winds of up to a hundred miles an hour, and other large wind gusts also contribute to the damaging effects of supercell thunderstorms. Some supercells can produce lightning strikes at a rate of 15,000 per hour, which is far in excess of the usual rate in thunderstorms.

There are three basic kinds of supercell thunderstorms -- high precipitation and low precipitation.

• High precipitation supercells have much more rain that classic or low precipitation supercells and may have no rain-free base. This can hide tornadoes from observers, making it much more difficult to know that you are in danger from a tornado and resulting in more deaths and injuries. These supercells are also very powerful and are those which produce the most lightning strikes. Their rain may be strong enough to cause flooding in the areas they pass over, which can in turn cause more damage and fatalities.
• Low precipitation supercells have no visible rain and can therefore be more dangerous to people if they are producing hail. Large hail may begin falling without warning and cause injuries or even deaths. Lightning is fairly infrequent in these supercells, and tornados are weaker most of the time if they occur at all.
• Classic supercells are more or less middle of the road supercells -- with the classic heavy rain in the north, rain-free base in the south, and typical chance for producing dangerously strong tornadoes and other violent weather.