An extratropical cyclone forming over southern Brazil has placed nine states across the country's South, Southeast and Center-West regions under storm warnings on Thursday (Sept 10), with gusts that could top 100 km/h and a risk of tornadoes in Paraná, in southern and southwestern São Paulo state and in far southern Mato Grosso do Sul, according to Inmet, Brazil's national weather institute, and the private forecaster Climatempo.
Where the danger is highest
Inmet's warnings cover Rio Grande do Sul, Santa Catarina, Paraná, São Paulo, Rio de Janeiro, Minas Gerais, Goiás, Mato Grosso and Mato Grosso do Sul. The institute's highest level, great danger, applies to far northern Rio Grande do Sul, central-western Santa Catarina and Paraná and far southern Mato Grosso do Sul, where more than 100 mm of rain per day is forecast, with winds above 100 km/h and a risk of flash flooding and falling trees. In Paraná, news outlet g1 reported that 265 municipalities were under a red storm alert.
In São Paulo state, Inmet kept an orange danger alert for the coast at least until the end of Thursday morning, with rainfall of 30 to 60 mm per hour or 50 to 100 mm per day and a risk of flooding, landslides and overflowing rivers, according to the newspaper Folha de S.Paulo. In Greater São Paulo, the CGE, the city's emergency management weather center, forecast heavy cloud cover and rain through the day.
How the system works
Climatempo says the instability comes from the combination of a trough, an elongated area of low pressure whose winds help storm clouds form, and a low-pressure system over inland South America linked to the extratropical cyclone taking shape in the country's South. Heavily loaded clouds are expected to organize into supercells over parts of Paraná, far southern Mato Grosso do Sul and southern and southwestern São Paulo between Thursday and Friday (Sept 11), when a new cold front approaches the region and could intensify the storms. In Paraná, the tornado risk is highest in the north and west of the state.
Josélia Pegorim, a meteorologist at Climatempo, explained the phenomenon to Folha:
"Supercells are special cumulonimbus clouds that can produce tornadoes, microbursts and intense wind gusts above 90 km/h. A supercell is called a mesocyclone when it acquires a rotating motion."
MetSul, a meteorology company based in Rio Grande do Sul, had already flagged the week as high risk, with a storm wave and an intense cyclone, and estimates that rainfall in São Paulo could reach three times the September average. The system arrives days after Rio Grande do Sul recorded four straight days of below-zero cold, including the year's lowest temperature, near -7 degrees, on Sept 7. Inmet continued to update its advisories through Thursday.