There are three types of storms: single-cell, multi-cell (with two subtypes: clusters and lines), and supercell.
Single cell thunderstorms are the most common type of storm. These storms are short-lived because they “self-destruct” very quickly in that their downdraft quickly cuts off their updraft and the supply of warm air necessary to keep the storm alive. While most are weak, some can be stronger and produce hail, torrential rain, and microbursts, but these are the exception, not the rule, and are most storms still do not last much longer than 30 minutes. It is very hard to predict the precise location of individual single-cell storms, so cut your weatherman a little slack if you get unexpectedly smacked by one of these storms.
Multicell clusters consist of a group of cells moving along. Most are short lived, but since these cells have formed in a cluster, the region in which they formed must have been conducive to thunderstorm formation. As such, individual cells will constantly form up and dissipate, but the cluster as a group will retain the same general characteristics. Multicell storms are usually more potent than single cell storms but are generally not severe.
Multi-cellular systems can stick around for a good deal of time. This is because these storms often have a gust front, and the outflow from the gust front lifts air ahead of the gust front, allowing new cells to continually form. As a result, a “conveyor belt” of storms can form and sustain itself for several hours.
Multi-cell lines, also called squall lines, consist of a more-or-less continuous line of storms that form at or ahead of a cold front with a gust front at the leading edge. These storms often have breathtaking shelf clouds and have the heaviest rain and hail just to the west of (behind) the updraft. Squall lines can produce golf ball sized hail, weak tornadoes, and, of course, torrential rain, but they are best known for their powerful downbursts and resulting straight-line winds.
Some downbursts are so strong that they can accelerate parts of the squall line ahead of others, forming what is called a bow echo, and I was lucky enough to observe one on radar tonight (August 14). In cases with sustained winds over 58 mph, storms that take the form of a bow echo are called derechos and can cause severe damage for hundreds of miles. The June 2012 North American Derecho killed 28 people and caused 2.9 billion dollars in damage as it tracked from the Midwest to the Atlantic Coast.
Supercells garner the most attention from everybody. They are extremely prominent and beautiful thunderstorms, but underneath that mask of beauty lies a ferocious beast that can cause massive hail, powerful downbursts, and most of all, tornadoes.
The anatomy of a supercell is similar to a typical strong thunderstorm. The difference is that a regular thunderstorm does not rotate, while a supercell does. This is evidenced by a wall cloud and a tornado in the supercell diagram while nothing of that sort exists in a typical thunderstorm.
To form a rotating updraft, you must first have wind shear, and lots of it. When there is enough wind shear, a vortex of air parallel to the ground is formed. This vortex is then lifted into an upright column by an updraft in the unstable atmosphere, and this rotating column of air becomes the rotating updraft.
When the updraft lifts this vortex up, we now have two vortices in the storm. The high pressure prefers to be on the upshear side of the low pressure, so the storm actually ends up splitting into a “left-mover” and a “right-mover” so that this preference is conserved.
The right mover is generally the “favored” mover in the US, meaning it is more likely to retain its strength or further strengthen. I give myself a headache trying to visualize and explain it, so I’ll just give you the diagrams and let you do the dirty work yourselves. The vertical pressure gradient helps to reorganize the storm so that the updraft is on the right and the downdraft is on the left once the vortex of spinning air has been lifted upright by the initial updraft.
Tornadic storms with a powerful mesocyclone often have what is called a “hook echo” on radar. The hook echo provides a good estimate of where the mesocyclone (and the tornado it may spawn) is.
Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017