Chris Dolce
Published: April 9,2020
The term supercell is used by meteorologists to describe a breed of long-lasting thunderstorms that rotate and are accompanied by dangerous weather conditions, including large hail, damaging winds and sometimes tornadoes.
Classic supercells appear as individual storms like the one in the radar image below, but sometimes can be embedded within a line or cluster of thunderstorms. When viewed in an open area, classic supercells have the "mothership" appearance often photographed in the Plains states.
Radar image of a supercell in southwestern Oklahoma on Nov. 7, 2011. The hook echo and hail core are labeled.
Supercells have a distinct appearance on Doppler radar, which often features a so-called hook echo on the lower-left portion of the storm. That hook echo appendage extends southward from the thunderstorm's main core, where large hail and heavy rain are typically located. When tornadoes are spawned by supercells, they are near the hook echo on radar.
A classic supercell photographed near Sanford, Kansas, on May 7, 2013.
The National Severe Storms Laboratory (NSSL) said fewer than 20% of all supercell thunderstorms produce tornadoes. However, supercells are responsible for spawning the vast majority of all tornadoes – violent ones in particular – NSSL added. More research is underway to discover why some of these rotating thunderstorms are accompanied by tornadoes while others are not.
Supercell thunderstorms are sometimes referred to as low precipitation (LP) or high precipitation (HP).
As the name implies, an LP supercell is accompanied by lighter amounts of precipitation – rain or hail. This type of supercell is easy to spot in the Plains because there is a clear view of the structure.
HP supercells contain heavier concentrations of rain and hail. The precipitation can sometimes obscure the storm's structure, including wall clouds and tornadoes. Tornadoes obscured by heavy precipitation are typically known as "rain-wrapped."
When supercells develop in a volatile atmospheric environment, they can survive for dozens or even hundreds of miles over multiple hours. The best-organized supercells may spawn multiple tornadoes, some of which can stay on the ground for a lengthy period of time.
Radar snapshots every 30 minutes of a classic long-lived supercell from central Alabama to North Georgia during the April 27, 2011, superoutbreak.
A somewhat recent example is a supercell in southwestern Georgia on Jan. 22, 2017, which spawned a tornado in progress for 72 minutes along a 70-mile-long path.
Low-pressure systems in various regions of the world can spawn supercell thunderstorms when atmospheric conditions are ripe. They can also occur in landfalling tropical storms and hurricanes.
The Science Behind How Supercells Form
A supercell thunderstorm is characterized by a sustained and powerful rotating updraft (rising air). These storms originate in unstable air accompanied by a particular type of changing wind direction at various altitudes in the atmosphere known as wind shear. A common combination supportive of supercells often found in the Plains states is a southerly or southeasterly wind near ground level and a southwesterly or westerly wind higher in the atmosphere.
This combination of changing wind directions and speeds creates a horizontal rolling motion in the lower atmosphere. The same rapidly rising air motions that form the thunderstorm turn this horizontal rotation into a vertical rotation, which can then be spectacularly evident in the circular striations, or layers, visible in the cloud structure of the supercell.
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