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2018 South America tornado outbreak
Tornado outbreak in Brazil and Argentina From Wikipedia, the free encyclopedia
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A destructive regional tornado outbreak spawned 11 tornadoes across the Brazilian state of Rio Grande do Sul and the Argentine province of Misiones from the night of 11 June to the early morning of 12 June 2018. It is one of the best documented and most impactful tornado outbreaks in the history of South America, spawning the strongest tornado on the continent in nearly a decade and having had a partial damage survey conducted in its wake.
The most impactful tornado of the outbreak was a long-tracked, deadly tornado that tracked through northern Rio Grande do Sul, particularly devastating the municipality of Coxilha. The intensity of this tornado is debated: it was rated F3 or F4 by different organizations.
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Meteorological synopsis
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Atmospheric setup
This outbreak took place after a three-day period of unseasonably warm, moist air being advected into the area by an intense low-level jet stream; dew points in southern Brazil reached 18 °C (64 °F), signaling a persistently unstable atmosphere. A shortwave trough over the region helped maintain this due to an strong horizontal pressure gradient between a low-pressure area over northwestern Argentina and a high-pressure area over the Atlantic Ocean. An upper-level trough also contributed to steep mid-level lapse rates.
The aforementioned strong jet also created strong low-level wind shear, which supported the development of supercell thunderstorms. Low lifting condensation levels combined with weak convective inhibition made it easier for supercell thunderstorms to develop, and heightened their potential to produce tornadoes. The environment was conducive to tornadoes by the afternoon, and storms started rapidly developing in the warm sector by the mid- to late-afternoon on 11 June; most developed over northeastern Argentina or southern Paraguay before moving into the outbreak area over southern Brazil.[1]: 12–15 [3][2]
Coxilha tornado family
The most prolific storm of the outbreak, which would go on to produce its strongest tornadoes,[1]: 35 began as a small storm cell over southern Paraguay, moving southeast into Misiones Province in Argentina and eventually into Rio Grande do Sul State in Brazil, where it would begin to produce tornadoes.[1]: 15, 17 After producing its first tornado near Dois Irmãos das Missões, it underwent a merger with a weaker storm cell. Half an hour later, it began a phase of active cyclic tornadogenesis, where it would produce several strong and destructive tornadoes.[1]: 19, 21
The first tornado produced in this cyclic phase was a strong F3 tornado that impacted the Sarandi area, killing one person. As it occluded, it took a leftward turn towards the storm's inflow sector, which is not an occlusion behavior commonly seen during cyclic tornadogenesis. As this tornado was still ongoing, however, another mesocyclone developed and intensified just north of it, producing another simultaneous tornado. After this, a third, albeit much weaker, simultaneous tornado was produced by the storm. Having multiple simultaneous tornadoes from the same cyclic parent supercell is rare during a tornado outbreak; in fact, this is the first documented case of this phenomenon ever occurring in South America. Shortly after the last of these simultaneous tornadoes dissipated, yet another pair of simultaneous tornadoes touched down.[1]: 19–26 The final tornado produced by this supercell was also the strongest—a violent, long-tracked tornado that was on the ground for nearly 40 minutes.[4][1]: 17
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Confirmed tornadoes
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- Two tornadoes were not rated; they are included in the total number but not elsewhere in the ratings table.
11 June event
12 June event
Coxilha–Vila Lângaro–Água Santa–Ciríaco, Rio Grande do Sul
A final tornado was produced by the cyclic supercell, which would become the strongest and longest-tracked of the outbreak. This tornado started in rural fields and forests, initially causing just vegetation damage. Soon after its formation, it rapidly intensified as it crossed the RS-463 highway between Coxilha and Tapejara. Here, three trucks were overturned on the highway and one 14-ton truck was tossed several meters into a nearby field at up to F4 intensity.[4][9] Additionally, nearby trees were downed or debarked and roofs were ripped off. After crossing the highway, a grain bin silo was partially collapsed and more roofs were ripped off. Debris was wind-rowed[10] and made into "debris missiles" that penetrated concrete walls. Continuing southeast near Vila Lângaro, the external concrete walls of a house were collapsed at F3 intensity and many weak wooden houses were completely destroyed. Small objects in this area were found 70 km (43 mi) away, an indication of a strong tornado. Approaching Água Santa, the tornado became rain-wrapped and weakened to F2 intensity. However, it soon re-intensified to F3 strength and began to debark trees and destroy many structures, including a masonry home, two chicken farms, and two transmission towers. Ten aviaries were destroyed, resulting in the death of over 220,000 chickens.[11] From here, the tornado rapidly shrunk and weakened to just F1 intensity before impacting Ciríaco directly, where roofs were partially removed and a grain silo was partially collapsed. Despite its weaker intensity here, a 54-year-old man was killed[5] when an exterior wall of his home collapsed. The tornado dissipated shortly after exiting the town's urban area.[4][1]: 17, 26–31
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Aftermath
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This tornado outbreak is notable for its size and intensity, which is very rare for South America. It is also unique in its response, being one of the few tornado events on the continent to receive a post-storm damage survey.[1]: 34 The Coxilha tornado, having one of the longest and widest damage paths ever recorded on the continent,[1]: 2 was described by the PREVOTS group as "one of the most intense tornadoes in RS in recent years."[3]
Damage
Overall, the tornadoes killed three people—two in Brazil and one in Argentina—and injured many. 21 municipalities were affected by the storms,[12] and six declared states of emergency due to the weather conditions associated with the storms, which included wind and hail in addition to tornadoes. Large hail of over 6 cm (2.4 in) was observed in Porto Xavier, a city close to the Brazil-Argentina border, and again in Giruá.[1]: 32 [3] Both cities declared states of emergency due to its effects.[13] Additionally, a state of emergency was declared in Sarandi for strong winds and in Vila Lângaro, Coxilha, and Água Santa for tornadoes.[13] Widespread wind damage in Santa Rosa damaged the roofs of at least 50 people, leading to Civil Defense delivering tarps, tiles, and sweaters.[12][11] Wind and hail damaged at least 400 buildings in Tupanciretã.[12] Hundreds of thousands of chickens were killed in Água Santa after at least 10 large aviaries were struck by the Coxilha tornado,[12][11] causing losses in the millions of reals. Community efforts were made to recover wheat and soybean crops which were stored in grain bins affected by the tornado.[14]
Surveys
Damage surveys were necessary in order to establish a cause for the damage, as the nocturnal nature of the outbreak resulted in a lack of visual confirmation of tornadoes. However, surveying the damage proved a challenge due to limited resources and the remote areas which were affected. Nonetheless, surveyors from the Federal University of Santa Maria (UFSM) and Brazil Civil Defense jointly surveyed many areas of damage. Most attention was given to the long-tracked Coxilha tornado, as it was more difficult to survey shorter and weaker tornadoes due to the aforementioned difficulties. The choice to survey damage using the original Fujita Scale rather than the newer Enhanced Fujita Scale was made because of the lack of adaptations to the latter based on Brazilian building codes. Due to limitations with ground surveying, aerial surveying and analysis of satellite imagery were useful in assessing damage and path size. Deforestation detection programs were also helpful in this regard.[1]: 6–10
Based on their partial damage survey, UFSM researchers assessed the intensity of the Coxilha at F3 strength. However, they warned that their damage analysis may have underestimated the actual intensity of the tornadoes, and even suggested that the Coxilha and Sarandi tornadoes may have been capable of producing violent (F4+) damage.[1]: 18, 35 Meanwhile, separate damage analyses by other meteorological organizations, including MetSul Meteorologia, resulted in F4 ratings for the Coxilha tornado.[4][15][b]
Research
Learning algorithms were used to assess forest damage and identify 10 different tornado tracks across Argentina and Brazil (one tornado's path was not detectable due to its brevity and low intensity; it was identified only by video evidence).[1]: 8–10 Many tornadoes were observed on Doppler weather radar, allowing for the observation of phenomena such as tornado debris signatures. Furthermore, this was the first ever cyclic supercell and first ever case of multiple simultaneous supercell tornadoes in South America on record.[1]: 2
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See also
Notes
- All dates are based on the local time zone where the tornado touched down; however, all times are in Coordinated Universal Time for consistency.
- Although UFSM researchers rated the Coxilha tornado F3, they explicitly noted that its intensity may have been underrated by their analysis and that the observed tornado debris signature suggested F4+ intensity.[1]: 18, 35 Other meteorological organizations rated the tornado F4,[4][15] thus the F4 rating is used in this article.
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References
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