On the thunderstorm spectrum, supercells are the least common type of thunderstorm, but they have a high propensity to produce severe weather, including damaging winds, very large hail, and sometimes weak to violent tornadoes. What makes a supercell unique from all other thunderstorm types is that it contains a deep and persistent rotating updraft called a mesocyclone. If the environment is favorable, supercell thunderstorms can last for several hours.
Although supercells require some degree of buoyancy, moderate to strong speed and directional wind shear between the surface and about 20,000 feet is the most critical factor. Wind shear not only creates the mesocyclone, but it also allows the storm to be tilted, which is important for maintaining a separate updraft and downdraft region. A separate updraft and downdraft allows the supercell to be long-lived because it reduces the likelihood that too much rain-cooled, stable air from the downdraft region will be ingested into the updraft, causing the storm to weaken.
Supercells are responsible for nearly all of the significant tornadoes produced in the U.S. and for most of the hailstones larger than golf ball size. Supercells are also known to produce extreme winds and flash flooding.
They are highly organized storms characterized by updrafts that can attain speeds over 100 mph (160 km/h) and are able to produce giant hail with strong or even violent tornadoes. Downdrafts produced by these storms can produce downbursts/outflow winds in excess of 100 mph (160 km/h), posing a high threat to life and property.
The most ideal conditions for supercells occur when the winds are veering or turning clockwise with height. For example, in a veering wind situation the winds may be from the south at the surface and from the west at 15,000 feet (4,500 meters). This change in wind speed and direction produces storm-scale rotation, meaning the entire cloud rotates, which may give a striated or corkscrew appearance to the storm’s updraft.
Dynamically, all supercells are fundamentally similar. However, they often appear quite different visually from one storm to another depending on the amount of precipitation accompanying the storm and whether precipitation falls adjacent to, or is removed from, the storm”s updraft.
Based on their visual appearance, supercells are often divided into three groups;
- Rear Flank Supercell – Low precipitation (LP),
- Classic (CL), or
- Front Flank Supercell – High precipitation (HP).
In low precipitation supercells the updraft is on the rear flank of the storm providing a barber pole or corkscrew appearance to the cloud. Precipitation is sparse or well removed from the updraft and/or often is transparent.
Also, large hail is often difficult to discern visually. With the lack of precipitation no “hook” seen on Doppler radar.
The majority of supercells fall in the “classic” category. The classic supercell will have a large, flat updraft base with striations or banding seen around the periphery of the updraft. Heavy precipitation falls adjacent to the updraft with large hail likely and has the potential for strong, long-lived tornadoes.
High precipitation supercells will have…
- the updraft on the front flank of the storm
- precipitation that almost surrounds updraft at times
- the likelihood of a wall cloud (but it may be obscured by the heavy precipitation)
- tornadoes that are potentially wrapped by rain (and therefore difficult to see), and
- extremely heavy precipitation with flash flooding.
Beneath the supercell, the rotation of the storm is often visible as well. The is visible as a lowered, rotating cloud, called a Wall Cloud, forms below the rain-free base and/or below the main storm tower updraft. Wall clouds are often located on the trailing flank of the precipitation.
The wall cloud is sometimes a precursor to a tornado. If a tornado were to form, it would usually do so within the wall cloud.
With some storms, such as high precipitation supercells, the wall cloud area may be obscured by precipitation or located on the leading flank of the storm.
Wall clouds associated with potentially severe storms can:
- Be a persistent feature that lasts for 10 minutes or more
- Have visible rotation
- Appear with lots of rising or sinking motion within and around the wall cloud
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