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: