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Not all thunderstorms are created equal. Thunderstorms come in multiple shapes, sizes, and strengths.

For the purposes of this lesson, we’re categorizing storms into four classifications:

  • Single cells
  • Multicells
  • Supercells
  • Squall Lines (these are considered a form of multicells as well)

Each of these have the potential to produce severe weather. The types of severe weather each tend to produce are slightly different from one another on average. Still, each can be dangerous in their own right.

Single Cells

Single cells are where we start our storm type journey. Typically, single cells form on days with weak wind shear and their life spans are relatively short — less than an hour on average.

Single cell storms are relatively less likely to produce severe weather compared to other storm types, but that is not always the case. These storms can produce very damaging winds, hail, and flash flooding — along with dangerous lightning.

Brief and weak tornadoes can occur with these but they are rare.

Another note too, true ‘single cell’ storms are actually rare. Even the storms we tend to classify as ‘pulse’ single cell storms tend to be somewhat multicellular. Still, these storms will have a basic and straightforward life-cycle:

  • Development: The updraft is dominant and little precipitation is noted.
  • Maturity: The updraft and downdraft are equal strength.
  • Death: The downdraft becomes dominant and rains into the updraft, killing the storm.

Multicells

A multicell storm is a group of individual storms in a cluster moving as a unit, with each storm at a different stage of its life-cycle.

As multicells move along their path, storms take turns being the dominant cell in the cluster. Multicells can be dangerous severe weather producers.

Typically, multicells can be responsible for very large hail as well as strong downburst winds. Weak tornadoes are rare, but not unheard of.

Multicell clusters tend to form on days with weak or modest wind shear and/or days with weak capping. They are much more common than supercells, and tend to form on days that are less conducive overall.

Squall Lines

A squall line is simply, a line of storms.

A squall line consists of storms situated along a gust front, or the leading edge of the complex’s outflow. The gust front is typically denoted by a shelf cloud, which acts as the complex’s updraft. The shelf cloud occurs at the interface of the outflow rushing out and away from the storm’s core and the warm/moist air being driven upwards.

Lines of storms can persist for hours or even days as they move across the landscape.

The highest severe weather threat for these storms is typically just behind the gust front with the highest threat being damaging winds with these storms. Still, hail and tornadoes do occur along squall lines. Flash flooding can be a major threat too, especially if the line is moving slowly with individual cells moving over the same area.

Supercells

A supercell is a thunderstorm with a deep, persistently rotating updraft.

Supercells are the least common form of thunderstorm yet they are potentially the most violent. Large hail of greater than baseball size, strong damaging winds, and tornadoes can accompany these storms.

To storm chasers, supercells are the grand catch.

What are the Parts of a Supercell?

Supercells are made up of several different parts. At the most basic level, a supercell features a deep/rotating updraft and a strong downdraft. An updraft is where warm and moist air rises and condenses within the storm and thanks to strong wind shear, the updraft and the downdraft are typically in different places, which allows the storm to thrive as it can “breathe”.

Underneath the updraft at it’s strongest point is oftentimes a wall cloud. A persistent and rotating wall cloud indicates the potential for tornado formation and should always be watched closely.

What are the types of supercells?

There are three main types of supercells.

  • Classic Supercells: The rear flank downdraft (RFD) oftentimes is pretty strong but doesn’t have much precip, which results in the updraft and updraft base being visible next to a heavy and opaque downdraft/storm core.These are the storms which most commonly produce the most tornadoes, as the RFD isn’t so strong as to overwhelm the storms updraft/area of rotation.
  • HP Supercells: These storms are a nightmare to chase as the RFD is strong and wet, which results in much of the updraft base being obscured in heavy rain. When a storm is HP, tornadoes are oftentimes buried in rain and invisible as they approach. The cores on these storms can oftentimes look incredibly mean as heavy rain and hail accompany them.
  • LP Supercells: LP supercells are almost always incredibly beautiful. They often feature weak RFDs and the downdrafts/hail cores are oftentimes transparent. These storms are oftentimes big hail producers and can sometimes produce spectacular and photogenic tornadoes. To a storm chaser, an LP supercell is an amazing catch. They are most common in the high plains of the US.

How do supercells form?

A supercell requires several very unique factors coming together in order to see them form. All thunderstorms require three ingredients to form: moisture, instability, and lift.

Supercells, on the other hand, require all three of those plus an additional factor: wind shear.

  • Moisture:¬†Supercells require adequate moisture to be present in order to form. On the Plains, the base line for good supercell thunderstorms is usually 50F. For supercells to be tornadic, the base line figures are usually dewpoints of 55F on the High Plains and 60F for the lower plains. These vary setup to setup though.
  • Instability and Lift:¬†Unstable air is air that tends to rise when it is lifted. For air to rise, it has to be hotter than the air surrounding it, think about the old saying “hot air rises”. That saying is true for the atmosphere as well. Unstable air occurs when the air at the surface is warmed beneath cooler temps aloft. The faster the air tends to rise, the more unstable the atmosphere is.
  • Wind Shear:¬†Supercells need to have the updraft rotating, this is accomplished through wind shear or wind which veers (turns westward) and speeds up with height. This wind shear creates horizontal vorticity which is then titled upwards by a strong thunderstorm updraft, creating the deep/rotating updraft supercells require.
Back to: The Basics of Storm Season

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