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Cold front forming increases the risk of extreme rain in the South and Southeast, with nearly 200 mm in Rio Grande do Sul and a flood warning
Nearly 200 mm of rain may hit Rio Grande do Sul on Labor Day holiday, as the cold front forming this Friday (1) advances through Brazil with a risk of storms in the South and Southeast.
Rio Grande do Sul holds the highest alert
The system gains strength between Friday (1) and Sunday (3), with episodes of extreme rain in the South and Southeast. The main concern is Rio Grande do Sul, where an intense atmospheric river is active.
This moisture corridor transports a large volume of water vapor and can cause accumulations of nearly 200 mm during the holiday, increasing the risk of disruptions in areas exposed to heavy rain.
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Throughout this Friday (1), severe storms are expected in the state of Rio Grande do Sul. There is a possibility of hail and intense wind gusts, posing a potential danger to the population and infrastructure.
Cold front advances during the weekend
On Saturday (2), as the cold front moves, storms are expected to hit the northern half of Rio Grande do Sul, Santa Catarina, and Paraná, with greater intensity in western Santa Catarina and Paraná.
On Sunday (3), there is still a condition for storms in the central-west of Paraná, although with less intensity. At the same time, the system reaches the Southeast and regains strength.
The eastern strip of São Paulo and Rio de Janeiro are on alert, with accumulations between 50 mm and 80 mm throughout Sunday (3). The city of Rio de Janeiro is also at risk.
Index indicates unusual or extreme rain
The alert is reinforced by the extreme forecast index, known as EFI, from the ECMWF model. The indicator assesses the possibility of unusual or extreme precipitation in each region.
The EFI uses the 99th quantile as a reference, a statistical measure associated with extreme events. In practice, this limit represents rain exceeded in only 1% of cases in the local history.
When the forecast indicates accumulations above this level, the expected daily rain becomes one of the rarest ever recorded for the area, raising concern about high-impact episodes.
Warmed ocean reinforces instability in the Southeast
On Sunday (3), the warmed waters of the Atlantic Ocean between the South and Southeast coasts favor convection in the eastern strip of the Southeast, especially between São Paulo and Rio de Janeiro.
With the ocean warmer, evaporation increases, and more water vapor is released into the atmosphere. This process strengthens heavy clouds and contributes to intense rains over urban areas.
Precipitations are expected to be strong in the eastern strip of São Paulo and, especially, in Rio de Janeiro. The population needs to be alert to floods, inundations, and stream overflows in vulnerable areas.
Accumulations may cause disruptions
The most recent round of the ECMWF model, considered reliable by Meteored, indicates accumulations exceeding 150 mm in the western half of Rio Grande do Sul between Friday (1) and Sunday (3).
Most of this volume is expected to fall this Friday (1). As a result, floods, inundations, and flooding are expected in areas affected by persistent rain and associated storms.
In the east of São Paulo and Rio de Janeiro, the predicted volumes range between 40 mm and 80 mm. Although smaller than those projected for Rio Grande do Sul, they are also concerning.
There are two main reasons. The volume may occur in 24 hours, characterizing intense rain with the potential for disruptions. Additionally, the region is the most urbanized in the country, with impermeable soil.
This condition hinders water infiltration and enhances flooding. As these areas are among the most populous, the event may affect many people until Sunday (3).
With information from Tempo.
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