Case study: 26 February 2018

On 26 February 2018, the city of São Paulo was affected by heavy rain, which causes flash floods and traffic jams all over the metropolitan region. Nearly 70 mm of precipitation fell in a few hours (https://g1.globo.com/sp/sao-paulo/noticia/chuva-deixa-zona-leste-de-sp-em-estado-de-atencao-para-alagamentos.ghtml).

Below is the specific humidity averaged over the first 100 hPa with 850-hPa streamlines. Convergence is observed along a windshift (trough) that extends from western São Paulo state towards the Atlantic Ocean, where convergence occurs. The 100-hPa specific humidity is maximized along the trough axis, where the highest precipitation occurred.



The 12Z sounding at São Paulo depicted a warming boundary layer overrunned by several small inversions and steep lapse rate layers. The wind profile was relatively weak, but some anticyclonic turning with height was observed in the lowest levels.



The radar imagery between 1800 and 2000 UTC is shown below. Some thunderstorms formed over São Paulo near a southeast-northwest-oriented band of precipitation farther north, and causes intense precipitation at 1800 UTC. Then, the precipitation stalled over the city causing the high rainfall totals and hazardous weather.

1800 UTC:

 
1900 UTC:



 
2000 UTC:





The accumulated precipitation estimated by radar is shown below. Some points of São Paulo had 60-70 mm according to radar estimates, which nicely matches the observed precipitation.