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.