Case study: 07 June 2018

In the afternoon of 07 June 2018, severe storms affected the eastern São Paulo state and southern Minas Gerais state. The storms formed under a very intense upper-level jet, which caused deep-layer shear of more than 25 m/s in some locations. The thermodynamic instability was not high, but sufficient to promote convective updrafts along a stationary front, which in turn became severe due to the very high wind shear.

The GFS MUCAPE fields are shown below. Eastern São Paulo had nearly 1000 J/kg of CAPE associated with 25+ m/s of 1000-500-hPa wind shear magnitude. The São Paulo soundings of 07/06 1200 UTC and 08/06 0000 UTC show very strong winds from low to upper levels of the troposphere, and moderate instability in the midlevels. The storm-relative helicity values below -100 m²/s² are very favorable for rotating, long-lived storms, as the ones observed in the region.

 

The storm that moved across São Luiz do Paraitinga-SP, causing 2+-cm hail and high winds is shown below. The 35-dBZ echo top peaked at more than 15 km height, which is an extreme value.

 

Case study: 19 May 2018

A strong cold front was responsible for the formation of a squall line over São Paulo state on the morning of 19 May 2018. This system caused severe winds in the city of São Paulo which were accompanied by fallen trees and power lines (https://g1.globo.com/sp/sao-paulo/noticia/bombeiros-recebem-mais-de-30-chamados-para-quedas-de-arvores-em-sp-apos-chuva-forte-com-rajada-de-vento.ghtml). The strongest wind gust was 93 km/h in the Guarulhos airport.

The strong cold front approaching São Paulo state on the morning of 19 caused strong low-level northwesterly flow over the area, which contributed to relatively high wind shear and transport of warm/moist air ahead of the squall line. The GFS analysis show that CAPE was not very high, but sufficient to produce severe weather given the wind shear greater than 20 m/s.

The 19 May 2018 12 UTC sounding of São Paulo depicts a relatively unstable lower troposphere, with warm air under relatively colder air in the midtroposphere. The winds were strong as well, with 40 kt just above the surface. This pattern of strong instability and high winds in the boundary layer favored severe wind gusts.

The radar imagery is shown below. A classic squall line is observed passing over São Paulo. The line caused very little precipitation.

Case study: 28-31 March 2018

Thunderstorms stroke São Paulo city in five consecutive days in the end of March 2018. Multiple flash floods and fallen trees were reported throughout the city (https://g1.globo.com/sp/sao-paulo/noticia/chuvas-causam-queda-de-89-arvores-em-sao-paulo-no-feriado.ghtml), causing chaos in the traffic.

A stationary trough was located over the Andes of northwestern Argentina and northern Chile. A low-pressure system was located over Paraná state and provided continuous northwesterly flow from the Amazon Basin towards São Paulo state.  In the afternoons, the moisture-rich environment was suitable for deep convection.

The sounding from 28 March through 31 March at 12 Z are shown below. The soundings are very similar, corroborating the stationary regime in which the storms occurred in these consecutive days. All soundings show very weak winds in the middle and lower troposphere, and a nearly moist-adiabatic profile. 

Some radar images are shown below to exemplify the storm modes observed in the region. The convection was not very intense nor organized, but the fact that it occurred in a city with poor drainage was the key factor.

Case study: 26 March 2018

The city of Jacareí, in the Paraíba Valley, reported flash floods on 26 March 2018. Several streets were flooded due to the high precipitation rates (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/temporal-causa-alagamentos-e-agua-invade-creche-em-jacarei-sp.ghtml).

The São Paulo-SP 12 Z sounding depicts a moist layer at low levels and a temperature inversion at nearly 600 hPa. Above the inversion, the southerly winds are relatively strong. After strong radiative heating, the instability of this profile increased significantly.

The storm is shown below. It persisted over Jacareí for about 1:30h, and this slow movement was determinant in the flash flood. VIL values over Jacareí were greater than 40 mm for more than one hour.

VIL at 1950 Z 26/03/2018, São Roque radar. 

Reflectivity at 1950 Z 26/03/2018, São Roque radar.

Case study: 20 March 2018

The afternoon and night of 20 March 2018 were marked by heavy precipitation in the eastern half of São Paulo sate. Several cities reported damage due to fallen trees, flash flooding, and huge amounts of lightning. São José dos Campos reported small hail (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/sao-jose-dos-campos-tem-chuva-de-granizo-na-primeira-tarde-do-outono.ghtml), Guaratinguetá had flash flooding (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/temporal-provoca-alagamentos-em-casas-e-ruas-em-guaratingueta.ghtml) and in São Paulo 95 trees fell and 2 people died, along with the impressive amount of 14 thousand lightning strikes over the city (https://g1.globo.com/sp/sao-paulo/noticia/cerca-de-mil-pessoas-estao-fora-de-casa-apos-chuva-em-sp-62-arvores-e-14-mil-raios-cairam.ghtml).

The synoptic-scale environment was characterized by a trough in southern Brazil with attendant cold front over the coast of Paraná state. The cold front was not moving too fast north. However, there was an abrupt change in the lo-level wind direction over Paraná and São Paulo states, which directly increased low-level convergence over São Paulo. Below are the 18Z and 00Z 925-hPa specific humidity and winds, showing the change in wind direction over São Paulo state during the precipitation event.

 18Z:

 00Z:

Another feature that probably played a significant role was the upper-level jet located over southern Brazil, which moved about 200 km to the north along with the upper-level trough and surface front (below). São Paulo state was located in the equatorward entrance region of the jet, where ascent is enhanced by the ageostrophic circulations of the jet.

The radar imagery depicts de formation of several storms during the afternoon, and upscale growth in early night as ascent increased over the area. Some squall lines accompanied the precipitation area.

18Z:

20Z:

21Z:

Case study: 14 March 2018

Several cities in eastern São Paulo state were affected by heavy precipitation and severe winds on the afternoon of 14 March 2018. In Campinas, winds greater than 85 km/h were reported, resulting in fallen trees and damage to buildings (https://g1.globo.com/sp/campinas-regiao/noticia/temporal-em-campinas-e-valinhos-tem-vento-a-85-kmh-e-queda-de-arvores.ghtml).

The synoptic-scale flow in the mid/upper troposphere was characterized by a ridge north of São Paulo state, and strong zonal flow in the southern part of the state (500-hPa field below). There was no synoptic-scale forcing for ascent, but the strong zonal flow was important to the relatively high wind shear in the area.

 Fig: 500-hPa cyclonic vorticity (10^-5 s^-1, shaded), geopotential height (dam, black contours), temperature (°C, blue contours) and winds at 18Z 14 March 2018.

Moisture was not a problem in the area, where dewpoints reached more than 20 °C in several locations. The 850-hPa flow (below) shows moderate northwesterly flow in the region in association with specific humidity in the lowest 100 hPa of more than 15 g/kg, which characterizes a very moist (and buoyant) planetary boundary layer.

Fig: Specific humidity in the lowest 100 hPa (g/kg, shaded) and 850-hPa streamlines at 18Z 14 March 2018.

The 12Z São Paulo sounding (below) depicts a very unstable profile for the morning. With the expected diurnal heating, temperatures rose to more than 33°C, and CAPE increased to more than 2500 J/kg (second figure below). Also, the winds were relatively intense in the lower and mid troposphere, which favored severe storms.

Fig: Most unstable CAPE (J/kg, shaded) and 1000-500-hPa wind shear at 18Z 14 March 2018.

The storm that affected Campinas is shown below. The storm formed over the Campinas area at 00:40 UTC (first figure below) and caused intense precipitation. It soon evolved to a bow echo at 01:30 UTC (second figure below), which is generally associated with high winds.

Fig: São Roque reflectivity (dBZ) at 00:40Z and 01:30Z 15 March 2018.

The VIL was also high (> 40 mm) over Campinas as soon as the storm formed, and indicated a high-precipitation storm. The high precipitation was likely responsible for the bow echo formation as the cold pool spread over the surface.

Fig: São Roque VIL (mm) at 00:40Z 15 March 2018.

Case study: 05 March 2018

Some cities in the Paraiba river valley experienced strong winds in the afternoon of 05 March 2018. In São José dos Campos, the wind gusts peaked at 75 km/h, and nearly 100 trees fell, causing trouble to the traffic (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/temporal-com-ventos-de-mais-de-75-kmh-derruba-cerca-de-100-arvores-e-deixa-bairros-sem-energia-em-sao-jose.ghtml).

The lower and middle troposphere had very weak winds on te 05/03 afternoon, with low-level winds from the southwest at no more than 10 m/s. The precipitable water ranged from 40 to 45 mm, which is typical at this time of the year. This very moist environment helped in the high precipitation caused by some storms in the region. The 925 hPa winds and specific humidity in the lowest 100 hPa (figure below) show the very moist air mass over the area and weak 925-hPa winds. However, the sea breeze at this time caused inland-directed winds, and enhanced low-level convergence over the Paraiba river valley at this time.

Specific humidity in the lowest 100 hPa and 925-hPa winds at 1800 UTC 05/03/2018.

The reflectivity core that passed over São José dos Campos is shown below. The highest reflectivity was nearly 55 dBZ at 1.0° elevation, which characterizes high amounts of hydrometeors. The most affected area was the south of the city, exactly where the convective core is located at 1920 UTC.

Refletividade (dBZ) no PPI de 1.0° às 1920 UTC do dia 05/03/2018.

O VIL mostra valores altíssimos (>50mm) sobre a região. Esses valores sõ condizentes com alta quantidade de hidrometeoros na tempestade. Devido aos fracos ventos na troposfera, pode-se concluir que a descida da precipitação nas correntes ascendentes foi o que mais contribuiu para causar ventos fortes em superfície.

 

Vertically Integrated Liquid (VIL; mm) no CAPPI de 3km às 1920 UTC do dia 05/03/2018.

Abaixo está o Echo-top de 35 dBZ, que mostra que refletividades de 35 dBZ foram observadas pelo radar em mais de 12 km de altura, ou seja, tratava-se de grande quantidade de gelo na alta troposfera. Essa é uma assinatura de severidade da tempestade.

 

 Echo-top de 35 dBZ (km) às 1920 UTC do dia 05/03/2018.