Supercell

A supercell is a thunderstorm characterized by the presence of a mesocyclone, a deep, persistently rotating updraft.^[1] Due to this, these storms are sometimes referred to as rotating thunderstorms.^[2] Of the four classifications of thunderstorms (supercell, squall line, multi-cell, and single-cell), supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local weather up to 32 kilometres (20 mi) away. They tend to last 2–4 hours.

Supercells are often put into three classification types: classic (normal precipitation level), low-precipitation (LP), and high-precipitation (HP). LP supercells are usually found in climates that are more arid, such as the high plains of the United States, and HP supercells are most often found in moist climates. Supercells can occur anywhere in the world under the right pre-existing weather conditions, but they are most common in the Great Plains of the United States in an area known as Tornado Alley. A high number of supercells are seen in many parts of Europe as well as in the Tornado Corridor (es) of Argentina, Uruguay and southern Brazil.

Supercells are usually found isolated from other thunderstorms, although they can sometimes be embedded in a squall line. Typically, supercells are found in the warm sector of a low pressure system propagating generally in a north easterly direction^[globalize] in line with the cold front of the low pressure system. Because they can last for hours, they are known as quasi-steady-state storms. Supercells have the capability to deviate from the mean wind. If they track to the right or left of the mean wind (relative to the vertical wind shear), they are said to be "right-movers" or "left-movers," respectively. Supercells can sometimes develop two separate updrafts with opposing rotations, which splits the storm into two supercells: one left-mover and one right-mover.

Supercells can be any size – large or small, low or high topped. They usually produce copious amounts of hail, torrential rainfall, strong winds, and substantial downbursts. Supercells are one of the few types of clouds that typically spawn tornadoes within the mesocyclone, although only 30% or fewer do so.^[3]

Supercells can occur anywhere in the world under the right weather conditions. The first storm to be identified as the supercell type was the

The current conceptual model of a supercell was described in Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis by Leslie R. Lemon and Charles A. Doswell III (see

Supercells derive their rotation through the tilting of horizontal vorticity, which is caused by wind shear imparting rotation upon a rising air parcel by differential forces. Strong updrafts lift the air turning about a horizontal axis and cause this air to turn about a vertical axis. This forms a deep rotating updraft, the mesocyclone.

updraft (blue) 'bends' the spinning air upwards.

A cap or

As the cooler but drier air circulates into the warm, moisture laden inflow, the cloud base will frequently form a wall, and the cloud base often experiences a lowering, which, in extreme cases, are where tornadoes are formed. This creates a warmer, moister layer below a cooler layer, which is increasingly unstable (because warm air is less dense and tends to rise). When the cap weakens or moves, explosive development follows.

In North America, supercells usually show up on Doppler weather radar as starting at a point or hook shape on the southwestern side, fanning out to the northeast. The heaviest precipitation is usually on the southwest side, ending abruptly short of the rain-free updraft base or main updraft (not visible to radar). The rear flank downdraft, or RFD, carries precipitation counterclockwise around the north and northwest side of the updraft base, producing a "hook echo" that indicates the presence of a mesocyclone.

This "dome" feature appears above the strongest updraft location on the anvil of the storm. It is a result of an updraft powerful enough to break through the upper levels of the troposphere into the lower stratosphere.^[7][8] An observer at ground level and close to the storm may be unable to see the overshooting top because the anvil blocks the sight of this feature. The overshooting is visible from satellite images as a "bubbling" amidst the otherwise smooth upper surface of the anvil cloud.

An anvil forms when the storm's updraft collides with the upper levels of the lowest layer of the atmosphere, or the tropopause, and has nowhere else to go due to the laws of fluid dynamics- specifically pressure, humidity, and density, in simple terms, the packet of air has lost its buoyancy and cannot rise higher. The anvil is very cold (-30°C) and virtually precipitation-free even though virga can be seen falling from the forward sheared anvil. Since there is so little moisture in the anvil, winds can move freely. The clouds take on their anvil shape when the rising air reaches 15,200–21,300 metres (50,000–70,000 ft) or more. The anvil's distinguishing feature is that it juts out in front of the storm like a shelf. In some cases, it can even shear backwards, called a backsheared anvil, another sign of a very strong updraft.

This area, typically on the southern side of the storm in North America, is relatively precipitation-free. This is located beneath the main updraft, and is the main area of inflow. While no precipitation may be visible to an observer, large hail may be falling from this area. A region of this area is called the Vault. It is more accurately called the main updraft area.

The wall cloud forms near the downdraft/updraft interface. This "interface" is the area between the precipitation area and the precipitation-free base. Wall clouds form when rain-cooled air from the downdraft is pulled into the updraft. This wet, cold air quickly saturates as it is lifted by the updraft, forming a cloud that seems to "descend" from the precipitation-free base. Wall clouds are common and are not exclusive to supercells; only a small percentage actually produce a tornado, but if a storm does produce a tornado, it usually exhibits wall clouds that persist for more than ten minutes. Wall clouds that seem to move violently up or down, and violent movements of cloud fragments (scud or fractus) near the wall cloud, are indications that a tornado could form.

Mammatus (Mamma, Mammatocumulus) are bulbous or pillow-like cloud formations extending from beneath the anvil of a thunderstorm. These clouds form as cold air in the anvil region of a storm sinks into warmer air beneath it. Mammatus are most apparent when they are lit from one side or below and are therefore at their most impressive near sunset or shortly after sunrise when the sun is low in the sky. Mammatus are not exclusive to supercells and can be associated with developed thunderstorms and cumulonimbus.

This is generally the area of heaviest and most widespread precipitation. For most supercells, the precipitation core is bounded on its leading edge by a

The rear flank downdraft of a supercell is a very complex and not yet fully understood feature. RFDs mainly occur within classic and HP supercells although RFDs have been observed within LP supercells. The RFD of a supercell is believed to play a large part in tornadogenesis by tightening existing rotation within the surface mesocyclone. RFDs are caused by mid-level steering winds of a supercell colliding with the updraft tower and moving around it in all directions; specifically, the flow that is redirected downward is referred to as the RFD. This downward surge of relatively cool mid-level air, due to interactions between dew points, humidity, and condensation of the converging of air masses, can reach very high speeds and is known to cause widespread wind damage. The radar signature of an RFD is a hook-like structure where sinking air has brought with it precipitation.

A flanking line is a line of smaller

Hook echo (or pendant)

The "hook echo" is the area of confluence between the main updraft and the rear flank downdraft (RFD). This indicates the position of the mesocyclone and probably a tornado.

Bounded weak echo region (or BWER)

This is a region of low radar reflectivity bounded above by an area of higher radar reflectivity with an untilted updraft, also called a vault. It is not observed with all supercells but it is at the edge of a very high precipitation echos with a very sharp gradient perpendicular to the RFD. This is evidence of a strong updraft and often the presence of a tornado. To an observer on the ground, it could be experienced as a zone free of precipitation but usually containing large hail.

Inflow notch

A "notch" of weak reflectivity on the inflow side of the cell. This is not a V-Notch.

V Notch

A V-shaped notch on the leading edge of the cell, opening away from the main downdraft. This is an indication of divergent flow around a powerful updraft.

Hail spike

This

three body scatter spike is a region of weak echoes found radially behind the main reflectivity core at higher elevations when large hail is present.^[9]

Supercell thunderstorms are sometimes classified by meteorologists and storm spotters into three categories; however, not all supercells, being hybrid storms, fit neatly into any one category, and many supercells may fall into different categories during different periods of their lifetimes. The standard definition given above is referred to as the Classic supercell. All types of supercells typically produce severe weather.

LP supercells contain a small and relatively light precipitation (rain/hail) core that is well separated from the updraft. The updraft is intense, and LPs are inflow dominant storms. The updraft tower is typically more strongly tilted and the deviant rightward motion less than for other supercell types. The forward flank downdraft (FFD) is noticeably weaker than for other supercell types, and the rear-flank downdraft (RFD) is much weaker—even visually absent in many cases. Like classic supercells, LP supercells tend to form within stronger mid-to-upper level storm-relative wind shear;^[10] however, the atmospheric environment leading to their formation is not well understood. The moisture profile of the atmosphere, particularly the depth of the elevated dry layer, also appears to be important,^[11] and the low-to-mid level shear may also be important.^[12]

This type of supercell may be easily identifiable with "sculpted" cloud striations in the updraft base or even a "corkscrewed" or "

LP supercells rarely spawn tornadoes, and those that form tend to be weak, small, and high-based tornadoes, but strong tornadoes have been observed. These storms, although generating lesser precipitation amounts and producing smaller precipitation cores, can generate huge hail. LPs may produce hail larger than baseballs in clear air where no rainfall is visible.^[16] LPs are thus hazardous to people and animals caught outside as well as to storm chasers and spotters. Due to the lack of a heavy precipitation core, LP supercells often exhibit relatively weak radar reflectivity without clear evidence of a hook echo, when in fact they are producing a tornado at the time. LP supercells may not even be recognized as supercells in reflectivity data unless one is trained or experienced on their radar characteristics.^[17] This is where observations by storm spotter and storm chasers may be of vital importance in addition to Doppler velocity (and polarimetric) radar data.

LP supercells are quite sought after by storm chasers because the limited amount of precipitation makes sighting tornadoes at a safe distance much less difficult than with a classic or HP supercell and more so because of the unobscured storm structure unveiled. During spring and early summer, areas in which LP supercells are readily spotted include southwestern Oklahoma and northwestern Texas, among other parts of the western Great Plains.^{[citation needed]}

The HP supercell has a much heavier precipitation core that can wrap all the way around the mesocyclone. These are especially dangerous storms, since the mesocyclone is wrapped with rain and can hide a tornado (if present) from view. These storms also cause flooding due to heavy rain, damaging downbursts, and weak tornadoes, although they are also known to produce strong to violent tornadoes. They have a lower potential for damaging hail than Classic and LP supercells, although damaging hail is possible. It has been observed by some spotters that they tend to produce more cloud-to-ground and intracloud lightning than the other types. Also, unlike the LP and Classic types, severe events usually occur at the front (southeast) of the storm. The HP supercell is the most common type of supercell in the United States east of Interstate 35, in the southern parts of the provinces of Ontario and Quebec in Canada, in France, Germany and the Po Valley in north Italy and in the central portions of Argentina and Uruguay.

Whereas classic, HP, and LP refer to different precipitation regimes and mesoscale frontal structures, another variation was identified in the early 1990s by Jon Davies.^[18] These smaller storms were initially called mini-supercells^[19] but are now commonly referred to as low-topped supercells. These are also subdivided into Classic, HP and LP types.

Supercells can produce hailstones averaging as large as two inches (5.1 cm) in diameter, winds over 70 miles per hour (110 km/h)^{[clarification needed]}, tornadoes of as strong as EF3 to EF5 intensity (if wind shear and atmospheric instability are able to support the development of stronger tornadoes), flooding, frequent-to-continuous lightning, and very heavy rain. Many tornado outbreaks come from clusters of supercells. Large supercells may spawn multiple long-tracked and deadly tornadoes, with notable examples in the 2011 Super Outbreak.

Severe events associated with a supercell almost always occur in the area of the updraft/downdraft interface. In the Northern Hemisphere, this is most often the rear flank (southwest side) of the precipitation area in LP and classic supercells, but sometimes the leading edge (southeast side) of HP supercells.

Part of a series on
Weather
Temperate and polar seasons Winter Spring Summer Autumn
Tropical seasons Dry season Harmattan Wet season
Storms Cloud Cumulonimbus cloud Arcus cloud Downburst Microburst Heat burst Derecho Lightning Volcanic lightning Thunderstorm Air-mass thunderstorm Thundersnow Dry thunderstorm Mesocyclone Supercell Tornado Anticyclonic tornado Landspout Waterspout Dust devil Fire whirl Anticyclone Cyclone Polar low Extratropical cyclone European windstorm Nor'easter Subtropical cyclone Tropical cyclone Atlantic hurricane Typhoon Storm surge Dust storm Simoom Haboob Monsoon Amihan Gale Sirocco Firestorm Winter storm Ice storm Blizzard Ground blizzard Snow squall
Precipitation Drizzle Freezing Graupel Hail Megacryometeor Ice pellets Diamond dust Rain Freezing Cloudburst Snow Rain and snow mixed Snow grains Snow roller Slush
Topics Air pollution Atmosphere Chemistry Convection Physics River Climate Cloud Physics Fog Mist Season Cold wave Heat wave Jet stream Meteorology Severe weather List Extreme Severe weather terminology Canada Japan United States Weather forecasting Weather modification
Glossaries Meteorology Climate change Tornado terms Tropical cyclone terms
Weather portal
v t e

Some reports suggest that the

Supercells occur commonly from March to May in Bangladesh, West Bengal, and the bordering northeastern Indian states including Tripura. Supercells that produce very high winds with hail and occasional tornadoes are observed in these regions. They also occur along the Northern Plains of India and Pakistan. On March 23, 2013, a massive tornado ripped through Brahmanbaria district in Bangladesh, killing 20 and injuring 200.[21]

On New Year's Day 1947 a supercell hit

On April 14, 1999, a severe storm later classified as a supercell hit the east coast of New South Wales. It is estimated that the storm dropped 500,000 tonnes (490,000 long tons; 550,000 short tons) worth of hailstones during its course. At the time it was the most costly disaster in Australia's insurance history, causing an approximated A$2.3 billion worth of damage, of which A$1.7 billion was covered by insurance.

On February 27, 2007, a supercell hit

South America

This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources in this section. Unsourced material may be challenged and removed. (December 2022) (Learn how and when to remove this message)

An area in South America known as the Tornado Corridor is considered to be the second most frequent location for severe weather, after Tornado Alley in the United States.^{[citation needed]} The region, which covers portions of Argentina, Uruguay, Paraguay, and Brazil during the spring and summer, often experiences strong thunderstorms which may include tornadoes. One of the first known South American supercell thunderstorms to include tornadoes occurred on September 16, 1816, and destroyed the town of Rojas (240 kilometres (150 mi) west of the city of Buenos Aires).^{[citation needed]}

On September 20, 1926, an F4 tornado struck the city of Encarnación (Paraguay), killing over 300 people and making it the second deadliest tornado in South America. On 21 April 1970, the town of Fray Marcos in the Department of Florida, Uruguay experienced an F4 tornado that killed 11, the strongest in the history of the nation. January 10, 1973, saw the most severe tornado in the history of South America: The

The December 26, 2003, Tornado F3 happened in Cordoba, with winds exceeding 300 km/h, which hit Córdoba Capital, just 6 km from the city center, in the area known as CPC Route 20, especially neighborhoods of San Roque and Villa Fabric, killing 5 people and injuring hundreds. The tornado that hit the State of São Paulo in 2004 was one of the most destructive in the state, destroying several industrial buildings, 400 houses, killing one and wounding 11. The tornado was rated EF3, but many claim it was a tornado EF4.^{[citation needed]} In November 2009, four tornadoes, rated F1 and F2 reached the town of Posadas (capital of the province of Misiones, Argentina), generating serious damage in the city. Three of the tornadoes affected the airport area, causing damage in Barrio Belén. On April 4, 2012, the Gran Buenos Aires was hit by the storm Buenos Aires, with intensities F1 and F2, which left nearly 30 dead in various locations.

On February 21, 2014, in Berazategui (province of Buenos Aires), a tornado of intensity F1 caused material damage including a car was, with two occupants inside, which was elevated a few feet off the ground and flipped over asphalt, both the driver and his passenger were slightly injured. The tornado caused no fatalities. The severe weather that occurred on Tuesday 8/11 had features rarely seen in such magnitude in Argentina. In many towns of La Pampa, San Luis, Buenos Aires and Cordoba, intense hail stones fell up to 6 cm in diameter. On Sunday December 8, 2013, severe storms took place in the center and the coast. The most affected province was Córdoba, storms and supercells type "bow echos" also developed in Santa Fe and San Luis.

Europe has its own hotspots for tornadoes and severe weather. Especially in the summer months damaging supercells occur frequently and parts of France, Germany and north Italy are experiencing a number of strong and violent tornadoes every decade.

During the evening of August 3, 2008, a supercell formed over northern France. It spawned an F4 tornado in the Val de Sambre area, about 90 kilometers east of Lille, which impacted nearby cities such as Maubeuge and Hautmont. This same supercell later went on to generate other tornadoes in the Netherlands and Germany.

In 2009, on the night of Monday May 25, a supercell formed over Belgium. It was described by Belgian meteorologist Frank Deboosere as "one of the worst storms in recent years" and caused much damage in Belgium – mainly in the provinces of East Flanders (around Ghent), Flemish Brabant (around Brussels) and Antwerp. The storm occurred between about 1:00 am and 4:00 am local time. An incredible 30,000 lightning flashes were recorded in 2 hours – including 10,000 cloud-to-ground strikes. Hailstones up to 6 centimetres (2.4 in) across were observed in some places and wind gusts over 90 km/h (56 mph); in Melle near Ghent a gust of 101 km/h (63 mph) was reported. Trees were uprooted and blown onto several motorways. In Lillo (east of Antwerp) a loaded goods train was blown from the rail tracks.^[30][31]

On May 24, 2010, an intense supercell left behind a trail of destruction spanning across three different states in eastern Germany. It produced multiple strong downbursts, damaging hail and at least four tornadoes, most notably an F3 wedge tornado which struck the town of Großenhain, killing one person.^[32]

On August 18, 2011, the rock festival Pukkelpop in Kiewit, Hasselt (Belgium) may have been impacted by a supercell with mesocyclone around 18:15. Tornado-like winds were reported, trees of over 30 centimetres (12 in) diameter were felled and tents came down. Severe hail scourged the campus. Six people reportedly died and over 140 people were injured. The event was suspended. Buses and trains were mobilised to bring people home.

On June 28, 2012, three supercells affected England. Two of them formed over the Midlands, producing hailstones reported to be larger than golf balls, with conglomerate stones up to 10 cm across. Burbage in Leicestershire saw some of the most severe hail. Another supercell produced a tornado near Sleaford, in Lincolnshire.

A third supercell affected the

On July 28, 2013, an exceptionally long-lived supercell tracked along an almost 400 km long path across parts of Baden-Württemberg and Bavaria in southern Germany, before falling apart in Czechia. The storm had a lifespan of around 7 hours and produced large hail of up to 8 cm in diameter. The city of Reutlingen was hit the hardest, houses and cars were severely damaged, dozens of people injured.^[33] With roughly 3.6 billion euros worth of damage, it was by far the costliest thunderstorm event ever documented in Germany.^[34]

Throughout June 2014,

On 25 July 2019 a supercell thunderstorm affected northern England and parts of Northumberland. Large hail, frequent lightning and rotation were reported by many people. On 24 September 2020 a similar event affected parts of West Yorkshire.[35]

On the morning of June 19, 2021, a MCS developed over the French Atlantic coast. While progressing to the north, the system gained supercellular aspects and spawned a F2 tornado 60 kilometers west of

Only 5 days after that on June 24, 2021, a supercell produced an F4 tornado in south Moravia, Czech Republic. This tornado caused 6 deaths and left more than 200 people injured. With roughly $700 million of damage it was one of the costliest tornadoes to occur outside of the United States.

In Europe, the mini-supercell, or low-topped supercell, is very common, especially when showers and thunderstorms develop in cooler polar air masses with a strong jet stream above, especially in the left exit-region of a jetstreak. ^{[citation needed]}

The

Gainesville, Georgia was the site of the fifth deadliest tornado in U.S. history in 1936, where Gainesville was devastated and 203 people were killed.^[36]

The 1980 Grand Island tornado outbreak affected the city of Grand Island, Nebraska on June 3, 1980. Seven tornadoes touched down in or near the city that night, killing 5 and injuring 200.^[37]

The Elie, Manitoba tornado was an F5 that struck the town of Elie, Manitoba on June 22, 2007. While several houses were leveled, no one was injured or killed by the tornado.^[38][39][40]

A massive tornado outbreak on May 3, 1999 spawned an F5 tornado in the area of Oklahoma City that had the highest recorded winds on Earth.^[41] This outbreak spawned over 66 tornadoes in Oklahoma alone. On this day throughout the area of Oklahoma, Kansas, and Texas, over 141 tornadoes were produced. This outbreak resulted in 50 fatalities and 895 injuries.^{[citation needed]}

A series of tornadoes, which occurred in May 2013, caused severe devastation to Oklahoma City in general. The first tornado outbreaks occurred on May 18 to May 21 when a series of tornadoes hit. From one of the storms developed a tornado which was later rated EF5, which traveled across parts of the Oklahoma City area, causing a severe amount of damage. This tornado was first spotted in Newcastle. It touched the ground for 39 minutes, crossing through a heavily populated section of Moore.^{[citation needed]} Winds with this tornado peaked at 210 miles per hour (340 km/h).^[42] Twenty-three fatalities and 377 injuries were caused by the tornado.^[43][44] Sixty-one other tornadoes were confirmed during the storm period. Later on in the same month, on the night of May 31, 2013, another eight deaths were confirmed from what became the widest tornado on record which hit El Reno, Oklahoma, one of a series of tornadoes and funnel clouds which hit nearby areas.^[45]

South Africa witnesses several supercell thunderstorms each year with the inclusion of isolated tornadoes. On most occasions these tornadoes occur in open farmlands and rarely cause damage to property, as such many of the tornadoes which do occur in South Africa are not reported. The majority of supercells develop in the central, northern, and north eastern parts of the country. The Free State, Gauteng, and Kwazulu Natal are typically the provinces where these storms are most commonly experienced, though supercell activity is not limited to these provinces. On occasion, hail reaches sizes in excess of golf balls, and tornadoes, though rare, also occur.

On 6 May 2009, a well-defined hook echo was noticed on local South African radars, along with satellite imagery this supported the presence of a strong supercell storm. Reports from the area indicated heavy rains, winds and large hail.^[46]

On October 2, 2011, two devastating tornadoes tore through two separate parts of South Africa on the same day, hours apart from each other. The first, classified as an EF2 hit Meqheleng, the informal settlement outside Ficksburg, Free State which devastated shacks and homes, uprooted trees, and killed one small child. The second, which hit the informal settlement of Duduza, Nigel in the Gauteng province, also classified as EF2 hit hours apart from the one that struck Ficksburg. This tornado completely devastated parts of the informal settlement and killed two children, destroying shacks and RDP homes.^[47][48]

• 
  Arcus in Poland
• 
  Supercell in Oklahoma
• 
  Supercell in Norman, Oklahoma
• 
  Rotating thunderstorm updraft
• 
  Chaparral, New Mexico, supercell
• 
  2013 Moore tornado in Oklahoma
• 
  Supercell in Needmore, Texas

• Convective storm detection
• Pseudo-cold front
• Pseudo-warm front

External links

Wikimedia Commons has media related to Supercell.

Look up supercell in Wiktionary, the free dictionary.

• Structure and Dynamics of Supercell Thunderstorms
• Lemon, Leslie R., (1998) On the Mesocyclone "Dry Intrusion" and Tornadogenesis
• Electronic Journal of Severe Storms Meteorology