101: Supercell Thunderstorms

Introduction to Supercells

Few meteorological phenomenon are as intense, dramatic, and dangerous as a supercell thunderstorm. What makes these storms "super" is their unique structure. While ordinary storm cells consist of an updraft and downdraft that essentially occupy the same air column, a supercell's updraft is entirely separate from a downdraft. When a downdraft occupies the same space as a storm's updraft, the downdraft will weaken the updraft until it collapses, resulting in the death of the storm cell. Without a downdraft to weaken it, a supercell's core updraft can maintain itself for a very long long time. Thus, while a typical thunderstorm might last 45 to 90 minutes, a supercell can exist for six to eight hours (or more).

Supercells occur across the world, wherever sufficient instability and wind shear are present. However, several hot spots include Bangladesh, Argentina, Japan, and most famously, North America east of the Rocky Mountains. All of the locations feature areas of intense daytime heating, unstable low-level air flow, relatively dry air flow aloft, and a significant amount of wind shear.

Because supercells are best studied using weather radar (Figure 1), this post will focus on storms in the US, since it is the only country that provides full radar data access to the public. In principle, most of this material can be applied worldwide (although some of the structure described may need to the mirrored in the Southern Hemisphere).

Figure 1. This 1953 radar image was the first case in which a supercell's structural feature was directly associated with an observed tornado.

Anatomy of a Supercell

To understand supercell thunderstorms, it helps to understand their key structural components. Below is an excellent diagram of an idealized classic Central Plains supercell as seen from a distance, looking towards the northwest (Figure 2), although real supercells (Figure 3) often deviate from this form to some degree. The order that those features are presented here vaguely follow the path that air flows through the storm.

Figure 2. This idealized diagram depicts what a supercell might look like when facing to the northwest.

Figure 3. A panoramic photograph displays most of the features depicted in Figure 2.

Inflow
The inflow supporting the main updraft of a supercell typically consists of warm, moist air moving northward (Figures 4 and 5). A key ingredient to this air is a decent amount of horizontal vorticity; which can be thought of as the inherent "spin" to the air with an axis parallel to the ground. In special circumstances, this can be seen on radar images as initially parallel lines that feed into the core of the storm.

Figure 4. On May 20, 2013, a line of supercells formed along a boundary in the southern Great Plains. While this storm was relatively benign, further north was the supercell that spawned the devestating EF-5 tornado in Moore, OK. The inflow of the storm (orange) displays the signature parallel lines of small cumulus clouds.

Figure 6. Striations along the underside of this supercell are due to inflow approaching the main updraft of the storm (in the distant background)

Updraft
The updraft of a supercell is the main engine that drives the storm (Figure 6). It is an area of intense upward motion so powerful, that it prevents all but the heaviest precipitation from falling through it. At upper levels, the distinctness of the updraft forces air from the surrounding environment to split. The result is a 'V' shape on radar and (sometimes) satellite imagery as precipitation and cloud material carried further downstream in the strong split flow, than in the weakened flow directly behind the updraft. The other function of the updraft is to tilt the horizontal vorticity into the vertical, thus providing the rotation that is at the heart of the supercell and one of its most defining characteristics.

Figure 6. The updraft region is highlighted by the small red area. On low elevation radar, the updraft of a mature supercell will often appear as a notch of low reflectivity. 

Overshooting Top
The updraft of a supercell is strong enough that it carries the condensed air (cloud) above the inversion that is capping the storm (Figures 7 and 8). The convection of a supercell is so deep and vigorous that it often extends through the entire depth of the troposphere. In those cases, the overshooting top actually punches into the typically cloud-free stratosphere.

Figure 7. The overshooting tops can often be seen on visible satellite imagery as a small rough area surrounded by comparatively smooth cloud tops. Note that this feature lies directly above the updraft indicated on radar.

Figure 8. The very top of the overshooting top can be seen in this photo of a distant supercell.

Anvil
While the overshooting top punches through the capping inversion, the rest of the storm's growth is impeded by it and spreads out horizontally instead. The anvil at the top of a supercell is generally comprised of thick cirrus that spreads out and downstream of the updraft (Figure 9). While anvils are common amongst thunderstorms, those produced by supercells are gigantic and can extend hundreds of miles downstream.

Figure 9. The anvil appears much smoother than the overshooting top and extends mainly in the direction of the upper-level flow.

Forward Flank Downdraft (FFD)
After ascending the updraft, precipitation laden air falls in two distinct downdrafts. The largest descends the downstream (or front) side of the supercell and is thus referred to as the forward flank downdraft (FFD) and typically extends roughly toward the northeast (Figure 10). The precipitation associated with the FFD is delineated by weight, with large hail falling close to the updraft and light rain falling furthest from the updraft. Some of the moist air produced by evaporated precipitation in the downdraft is pulled back into the updraft and may condense, creating what is often called a tail cloud. After the evaporatively cooled air reaches the surface, it spreads out in a gust front that can trigger new convection.

Figure 10. The FFD is usually seen on radar as a vast area of precipitation with the strongest echoes forming a vague 'V' shape due to the split in winds caused the the updraft.

Rear Flank Downdraft (RFD)
The other downdraft of a supercell essentially wraps around the updraft on the upstream (of back) of the storm, and is thus called the rear flank downdraft (RFD). Compared to the FFD, the RFD is much smaller, but is much more intense and dynamic (Figure 11). There is often little precipitation associated with the RFD, since it tends to evaporate quickly in the rapidly descending air. When the RFD reaches the ground it creates a powerful gust front that can contain dangerous straight-line winds (Figure 12). Like the FFD, some moist RFD air may get pulled back into the updraft, creating a low cloud in the process.

Figure 11. The RFD, indicated by the blue outline, wraps around the updraft. In this case, its gust front has triggered new convection.

Figure 12. RFD winds are made visible here by the dust that is being lofted; some of which is even being pulled in the the updraft (to the right).

Flanking Line
In many cases, the RFD's gust front triggers the formation of convection immediately upstream of the supercell (Figure 13). These convective towers are known as the flanking line and are sometimes deep enough to produce their own precipitation (Figures 14 and 15).

Figure 13. The supercell featured in most of the previous diagrams is seen here near the bottom of the image, while the Moore tornado producing storm is the cell furthest north. What appears to be flanking line clouds extend immediately to the west of storm.

Figure 14. This supercell near Wichita, KS in 2012 occured at the same time as an inversion that allowed the radar to pick up low level features. The flanking line convection is evident in center of the reflectivity image (left) by the dark blue line. Striations caused by the inflow can be seen to the south of the storm, particularly in the radial velocity image (right), with  a few lines wrapping into the storm's core

Figure 15. An radar isosurface of the same storm as in the previous figure looking toward the northeast with the flanking line in the foreground, the RFD to the left (mostly invisible on radar), and the FFD in the background.

Mesocyclone
The mesocyclone is the rotating core of a supercell and their single most defining feature (Figure 16). The full structure and evolution of a mesocyclone is not completely understood and beyond the scope of this post. Essentially, though, it is co-located with the updraft to the extent that supercells are sometimes said to have a rotating updraft. The rotation is associated with the vorticity that the updraft ingested. As the air rapidly ascends the vorticity is said to be stretched, increasing the rotation in order to adhere to the conservation of angular momentum. In cases where there is relatively little cloud material associated with the supercell, corkscrew-like striations can be seen spiraling up the main core of the storm (Figure 17).

Figure 16. The yellow circle indicated the estimated location of the mesocyclone. Since the rradar is to the northwest of the storm cyclonic (counter-clockwise) rotation is evident by outgoing (red) velocities to the southwest of incoming (green) velocities.

Figure 17. This supercell near Northfield, TX on May 23, 2016 had little cloud material, allowing the striations caused by the rotating mesocyclone to be easily observed.

Wall Cloud
Sometimes a portion of the supercell's cloud base will descend below the mesocyclone (Figure 18). This rotating cloud feature is known as a wall cloud and is a feature storm chasers look for, as it often means the formation of a tornado is imminent.

Figure 18. While this wall cloud did not produce a tornado, a slight horizontal extension on its right indicates inflowing moisture from the FFD (distant far right). There might be hints of the RFD in the form of a sharp boundary seen near the surface on the left side of the image.  

Tornado
A tornado is the most violent manifestation of a supercell, as well as the most dangerous (Figure 19). Most supercell-spawned tornadoes descend from the wall cloud. As the winds inside the tornado increase, the pressure decreases, causing condensation and the formation of the iconic funnel cloud.

Figure 19. This high resolution image shows the 2013 Moore tornado (centered on the small red dot) just before it caused its first area of EF-5 damage. Much of the reflectivity (left) near the tornado is actually debris, while the sharp change in wind direction associated with the rotation is evident in the velocity image (right).

Lifecycle of a Supercell

Pre-Storm Environment
In order for supercells to form, several meteorological conditions must exist. First, the environment should be characterized as being conditionally unstable. This means that a low capping inversion prevents air parcels from ascending to any significant degree, but above that temperature decreases rapidly with height (ideally all the way up to the tropopause). Thus, if something were to erode the inversion, air parcels could rise to a considerable height.
Second, considerable wind shear must be present, with the most intense supercells forming when wind direction changes in both speed and direction with height (Figure 20). In the US, this condition is usually meet by steady low-level flow from the south to southeast and upper-level winds from the southwest to west. Additionally, the low-level flow should be considerably warm and moist.
Finally, the low-level flow should have considerable horizontal vorticity. Often this is provided by long parallel lines of small cumulus clouds that form early in the day called cloud streets (Figure 21). Vorticity is generated in these streets because the rising air in the cumulus updrafts is paired with the descending air between the streets to form. The tops of these cumulus correspond to the bottom of the inversion layer and that height is usually about a third the distance between the streets (Figure 22).

Figure 20. Earlier in the day of the Moore tornado, the wind profile showed considerable turning with height. Near the surface winds were from the south-southwest, while at upper levels they were from the west. 

Figure 21. Roughly three hours before the Moore tornado, cloud streets dominated the region. Note that the first bit of deep convection is just starting to form.

Figure 22. This diagram explains how vorticity is generated by the updrafts and downdrafts of adjacent cloud streets.

Storm Formation
Thunderstorms will begin to develop when something removes the capping inversion that is preventing air parcels from creating deep convection. Often, this mechanism is daytime heating. When the layer near the surface becomes very warm due to being in close proximity to the heated ground, near surface air parcels become extremely buoyant and begin pushing into the inversion and mixing in. If enough of this air mixes with the air in the inversion, the vertical temperature profile will shift until the inversion ceases to exist. It is because of this mechanism that most supercells form in the late afternoon or early evening.
Another common mechanism is forced lifting due to some boundary. This boundary might be outflow from a distant storm complex or a synoptic feature such as a front or dryline. When the warm low-level air meets the boundary, it is forced to rise (even through the inversion). Then, like the daytime heating mechanism, this forcibly lifted air mixes with the inversion layer until it dissipates.
Once the capping inversion is removed, convection often begins building extremely rapidly. Sometimes turning a nearly clear sky into one full of intense thunderstorms over the course of just an hour. It is because of this that an area is said to 'erupt' in activity (Figure 23).

Figure 23. This animation of the day of the Moore tornado shows how quickly deep convection forms once the capping inversion has been lifted.

Splitting Storms
Storm splitting is a key step in the formation of supercells, yet a full explanation is beyond the scope of this post, thus a simplified description will be given here.
As storms grow, their updrafts lift the inherent vorticity into the vertical (Figure 24). This can be envisioned as a rotating tube whose middle has been lifted up. Once lifted, the tube would have two vertical portions, spinning in opposite directions. In a thunderstorm, these vertical sections become the rotating cores of supercells. Through a series of complex dynamic processes, new updrafts form on either side of the storm (paired with the rotations). Meanwhile, precipitation associated with the original thunderstorm cell creates downward motion between the two new cores as well as a pool of cool air at the surface beneath it. The central downdraft works to sever the connection between the cores, while the expanding cold pool forces the new updrafts apart from one another. The end result is two storms with rotating updrafts that are separated from their respective downdrafts (portions of the original downdraft).
These two new supercells will move roughly 30 degrees to the left and right of the original storm's motion (Figure 25). However, when the vertical wind shear changes direction with height, dynamic interactions will cause one of the supercells to further develop and slow down while the other dissipates. In the US, the right moving cell nearly always dominates. For this reason, these supercells tend to have a greater eastward component to their motion compared to non-supercellular storms in the same environment (Figure 26). The remainder of this post will assume a right moving storm.

Figure 24. Vertical rotation is created when an updraft lifts horizontal rotation into the vertical (top). Opposing rotating cores and a central downdraft cause the storm to split in two (bottom).

Figure 25. Here, two supercells are formed by splitting. Notice on the right side of the image that the left mover has moved farther than the right cell and is somewhat smaller.

Figure 26. This animation shows the formation of a right moving supercell near Big Springs, TX on May 22, 2016. Notice how the storm makes a sharp turn to the right and slows down considerably (relative to the other storms to its north).

Weak Echo Region (WER)
As the storm intensifies, the angle at which the updraft ascends becomes increasingly more vertical. At the same time, the separation of the updraft from the downdraft (the FFD or its precursor) results in precipitation forming in the middle and upper levels of the updraft, but falling down-shear of it. Down-shear simply means downstream in the direction of the vertical wind shear that is driving the storm.
While the precipitation free portion of the updraft will appear visually as a rain-free portion of the storm cloud, on radar this region will have little or no reflectivity. A radar cross-section will depict the storm having an overhang, with high reflectivity located almost directly above a region of little to no reflectivity (Figure 27). Therefore this region is referred to as a weak echo region (WER). A storm with this signature feature should be closely monitored for severe weather and further development.

Figure 27. This cross-section through the supercell shown in Figure 17 displays a clear overhang with high reflectivity values (red and purple) located above much lower values (blue and green).

Hook Echo
Eventually, the RFD will form and begin to curve around the back side of the storm, carrying with it precipitation. When the precipitation associated with the RFD begins to make its way towards the south side of the storm, the supercell's radar signature will appear to form a pendant or hook. This is the infamous "hook echo" that indicates an intense supercell that likely has a robust mesocyclone core and has the potential for tornadic development (Figure 28).

Figure 28. The Moore supercell displayed a clear hook echo shortly before the tornado touched down. Note that this image is from the same time as the satellite image in Figure 13.

Bounded Weak Echo Region (BWER)
At its peak intensity, the supercell's updraft will become nearly vertical. This, combined with the precipitation wrapping around the updraft will result in cross-sections through the storm's core to appear to have a hole extending up into it (Figure 29). This is know as the bounded weak echo region (BWER) and can sort of be thought of as a portion of the WER that has been enclosed on all sides, versus just one (Figure 30).
The presence of a BWER is an indication that a large amount of mass (rain and hail) is being held aloft. Like the hook echo, a supercell with a BWER should be considered extremely intense and is likely to produce severe weather (if it hasn't begun to already). It is at this time that the largest hail reaches the ground.

Figure 29. The Big Springs supercell exhibited a classic BWER radar signature.

Figure 30. This is the same radar isosurface as Figure 15, but looking toward the northwest. The little "cave" to the right of the center of the image shows where inflow is entering the BWER region.

BWER Collapse
As the supercell ages, and the RFD continues to further limit the inflow of low-level air into the updraft, the BWER often appears to begin to shrink. This BWER collapse phase is accompanied by an overall decrease in the storm's reflectivity and its apparent height. However, it is at this time that the supercell is at its most dangerous. During the BWER collapse is when tornadoes are most likely to form and when they will be their strongest.


Mesocyclone Occlusion
Eventually, the RFD wraps completely around the updraft, cutting the updraft off from the inflow in a process known as mesocyclone occlusion. On radar, it will appear that the mesocyclone is moving from the edge of the storm towards its center. Often tornadoes, which are fixed to the mesocyclone, will appear to track slightly to the left of their original heading. At this time a new updraft and mesocyclone is forming on the edge of the supercell and will replace the old one as the new core. Supercells will often undergo this cycle many times throughout their lives (Figure 31).

Figure 31. The Moore supercell's full lifetime is shown in this animation. The estimated location of the updraft is in the center of the white circle, which is 30 miles in diameter. The EF-5 tornado dissipates shortly before the storm passes very near the radar unit. Throughout the animation, several occlusion cycles occur, as indicated by the formation and disappearance of several hook echoes.

Supercell Varieties
Meteorologists often classify supercells into different modes, mostly based on the relative strength of the inflow into the updraft compared to the outflow from the downdrafts. The spectrum ranges from inflow dominant low precipitation supercells to outflow dominant high precipitation supercells. Often storms will switch between modes as they evolve and undergo internal changes such as mesocyclone occlusions.


Classic Supercells (CL)
Supercells whose inflow and outflow are roughly in balance are referred to as classic supercells (CL). These storms are the textbook type often seen across the great plains of the US. They often exhibit well defined hook echoes and tend to produce the strongest tornadoes (Figure 32).

Figure 32. The Big Springs supercell at a time it had a classic CL structure; complete with a hook echo and a 'V' shape of the highest reflectivities.

Low Precipitation Supercells (LP)
In drier air masses, such as is often observed in eastern New Mexico and western Texas, supercells tend to be dominated by their updraft and have little precipitation associated with them. The relative lack of precipitation and reduced amount of cloud material often allow many of the core structures of the storms to be visible. For the same reason, they have a very small radar signature and can be difficult to identify (Figure 33). The main hazard from these storms is very large hail, although tornadoes do occasionally occur. These storms have a tendency to transition to a more CL structure as they move eastward where there is often more moisture.

Figure 33. This Kansas LP supercell displays most of the primary features of supercells, although they are not well pronounced. The radial velocity image (right) does hint at the location of a mesocyclone.

High Precipitation Supercells (HP)
The most commonly observed supercell mode it the outflow dominant high precipitation supercell (HP). These are especially common in the southeast US during winter. HP supercells are particularly dangerous for several reasons. First, the intense amount of precipitation often obscures many of the key structures that help supercells be identified on radar (Figure 34). Second, tornadoes spawned from these storms are often hidden inside heavy precipitation. Finally, they are commonly active at night, further reducing the chance of noticing an approaching tornado. CL supercells will often take on HP characteristics temporarily after a mesocyclone occlusion.

Figure 34. A line of storms in central Texas contained a HP supercell. Many of the classic features of supercells are either absent or hard to detect in this storm. However, the radial velocity (right) clearly shows a rotating core. 

Miniature / Low-Topped Supercells
Under certain conditions, supercells can form far out of season or in places that otherwise do not see many supercells. These storms are typically much smaller than the other varieties and generally have much lower tops (Figure 35). Mini supercells are still considered dangerous as they can still produce hail and tornadoes (Figure 36). The fact that they are so small often makes them hard to identify on radar due to the radar's ability to discern their structure. Furthermore, since they form in regions unfamiliar to supercells, local meteorologists might not be able to readily identify them.

Figure 35. This mini supercell north of Seattle, WA on October 20, 2012 was covered in a previous post. A classic supercell shape is faintly recognizable, but the reflectivity values (left) are very low compared to typical supercells. Although there does appear to be rotation (right), it is not particularly robust.

Figure 36. Despite being small and weak, the supercell did produce a well defined wall cloud and at least one waterspout (possibly three).