Case study: 25 December 2017

In the Christmas afternoon, a squall line (Fig. 1) moved through central São Paulo State and caused intense precipitation over Campinas-SP region (https://g1.globo.com/sp/campinas-regiao/noticia/campinas-tem-temporal-com-ventos-de-mais-de-80-kmh-e-alagamentos.ghtml). Several parts of Campinas were rapidly flooded due to high precipitation rates. According to Cepagri, the measured precipitation rate was nearly 20 mm/h, which is sufficient to cause flash flooding. The accumulated precipitation is shown in Fig. 2. The precipitation over Campinas (20-25 mm) occurred in only 1-2 hours.

 Fig. 1:  Reflectivity at 1730 UTC 25 December 2017, X-band SOS Chuva radar at Campinas-SP.

 Fig. 2: Accumulated precipitation in the last 6 h at 2100 UTC 25 December 2017, estimated by the X-band SOS Chuva radar at Campinas-SP.

The synoptic-scale environment at 1800 UTC 25 December 2017 at 700 hPa (Fig. 3) was characterized by a high precipitable water corridor extending from the Amazon towards São Paulo State, in association with northwesterly flow in the lower levels. The precipitable water over the Campinas region ranged between 50 and 55 mm. Confluence was observed over São Paulo state between the 700-hPa ridge over Minas Gerais state and the cold-frontal trough over the Atlantic Ocean farther south.

 Fig. 3: GFS analysis of precipitable water (mm, shaded) and 700-hPa winds, geopotential height (dam, black contours) and temperture (°C, red contours) at 1800 UTC 25 December 2017.

 The 1200 UTC sounding at São Paulo-SP is shown in Fig. 4. The temperature profile was nearly moist adiabatic in the entire troposphere, but the radiative heating noted at surface increased the surface temperature and thermodynamic instability in the afternoon. CAPE values were around 1000 J/kg according to the GFS analysis. This setup of moderate instability, large-scale confluence, high precipitable water and moderate low-level winds favored the formation of moderately organized convection capable of heavy rain in the area.

 Fig. 4: 1200 UTC 25 December 2017 São Paulo-SP sounding.

Figure 5 shows the Vertically Integrated Liquid (VIL) before the squall line hit Campinas. VIL values were peaking at 20 mm in some areas within the squall line, which indicated the high potential for flash flooding approaching the Campinas area. This figure elucidates how a nowcasting system can be used to forecast high-impact events with several hours of leading time.

Fig. 5: Vertically integrated liquid at 1630 UTC 25 December 2017 generated by the X-band SOS Chuva radar at Campinas-SP.