In the afternoon of 18 December 2017, a severe thunderstorm caused flash flooding, hail and high winds in Taubaté-SP (https://g1.globo.com/sp/vale-do-paraiba-regiao/noticia/chuva-em-taubate-alaga-ruas-derruba-arvores-e-deixa-familias-desalojadas.ghtml).
The 1800 UTC 18 December 2017 250-hPa fields depict an anticyclonic circulation over Paraguay and the subtropical jet over central Argentina. A trough is located over the Atlantic Ocean at nearly 30°S, too far from SP state to cause lifting over the region where the storm occurred. The SP state locates in a ridge environment. At surface, a stationary front locates over southern Brazil, and a trough is observed over SP state extending towards the Atlantic Ocean. This trough is associated with low-level convergence and lifting.
Figure 1: (a) 250-hPa and (b) 850-hPa synoptic charts at 1800 UTC 18 December 2017 (CPTEC/INPE).
The São Paulo-SP sounding at 1200 UTC shows an unstable lower troposphere, with a deep moist layer. At this time, surface heating was ongoing due to clear skies, and CAPE was increasing. The lack of high winds below 400 hPa also indcate that the thermodynamic instability was the main source of energy of the thunderstorms in the region. Also, the quiescent winds favored slow storm motion, which is an important factor in flash flood events.
Figure 2: São Paulo 1200 UTC 18 December 2017 sounding.
The thunderstorms formed over the Mantiqueira mountains and moved over the Paraiba Valley, where Taubaté is located. The figure below shows the high reflectivity associated with the storms over Taubaté at 1840 UTC. The storms were predominantly stationary.
Figure 3: São Roque radar reflectivity (dBZ) at 1840 UTC 18 December 2017.