Supercell Thunderstorms

Supercell Thunderstorms

A beautiful reddish to orange to yellow to off-white colored thunderstorm (from bottom to top respectively) grows into a bluish-purple sky over farm land with patchy forest of deciduous trees. The updraft is fleecy/woolly about half as wide as the feather-edged, otherwise crisp Frisbee-shaped anvil.

Supercell thunderstorms produce some of the most violent weather on earth. They are a unique form of single-celled convection that is defined by a rotating updraft.

Idealized radar reflectivity showing area that supercells are most commonly found within a low pressure system

Supercell thunderstorms usually develop just ahead of cold fronts and along the warm front close the center of low pressure.

Idealized radar reflectivity showing area that supercells are most commonly found within a low pressure system

They can also form along drylines. They typically last several hours, are prolific producers of large hail and tornadoes, and occasionally, damaging straight-line winds.

Radar reflectivity animation showing a distinct low-reflectivity fine line oriented from southwest to northeast. Several intense thunderstorms develop a short distance ahead of the line.

Supercells usually consist of a single cell of intense convection, usually about 30-60 km (20 to 40 mi) across the widest portion of the storm on radar. There are two hallmark signatures that characterize most supercell thunderstorms:

Idealized radar reflectivity of a supercell thunderstorm and its hook echo. Idealized signature of rotation on radial velocity within a supercell thunderstorm.
  1. A mesocyclone, which is a rotating updraft that displays as a rotational couplet and
  2. A hook echo, which is created by precipitation echoes wrapping around the mesocyclone.

Reflectivity

Base reflectivity showing a pronounced, wide hook echo on a supercell thunderstorm

Velocity

Base reflectivity showing a pronounced, large mesocyclonic circulation

In this example, the hook echo is rather broad, and the mesocyclone couplet is clear.

Reflectivity

Base reflectivity image showing a large, round supercell with a small appendage of high reflectivities that protrudes southward. This is sometimes called a "pendant" echo

Velocity

Base velocity showing a coupling of strong inbound velocities and strong outbound velocities indicating a counterclockwise rotating mesocyclone

This hook echo is rather narrow and does not curve as much as a typical hook echo. Sometimes, hooks that look more like small appendages sticking out from the main convective core are referred to as "pendant" echoes. The mesocyclone couplet in this case is easy to discern and contains a significant amount of convergence as well.

Rotational signatures such as these may occur within some cells that comprise lines of convection, such as those that make up the leading portions of squall lines and some mesoscale convective systems. There, the rotation is usually smaller and weaker.

To summarize, supercells usually possess the following characteristics:

  • Long-lived, lasting several hours
  • Consist of a large, single cell of very strong convection that has a characteristic hook shape on radar reflectivity
  • Have a rotating updraft that manifests as a rotational couplet on radial velocity