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.