An intense extratropical cyclone is forming on Tuesday, December 9, 2025, over Rio Grande do Sul, bringing widespread rainfall, severe thunderstorms, and damaging winds across southern Brazil.
Satellite image acquired at 12:00 UTC on December 9, 2025. Credit: NOAA/GOES-East, RAMMB/CIRA, The Watchers
A deep low-pressure system over northeastern Argentina will evolve into an intense extratropical cyclone on Tuesday morning, December 9, in western Rio Grande do Sul, near Uruguaiana, with a central pressure near 994 hPa. As the system deepens, the center will track eastward through the afternoon and evening, reaching the region near Lagoa dos Patos, close to Porto Alegre.
Widespread rainfall is forecast across all three southern states of Brazil, extending into parts of the Central-West, São Paulo, and Minas Gerais. The combination of low pressure and warm, humid tropical air will create highly favorable conditions for locally heavy to intense rainfall and severe thunderstorms.
Convective storms that began on Monday under the influence of the low-pressure system have already caused severe weather in several regions.
Video credit: Tropical Tidbit São Paulo experienced a strong storm in the early afternoon with disruptions, while in Rio Grande do Sul, deep convection generated heavy rainfall and destructive winds across the northern half of the state. The Serra Gaúcha region, particularly Flores da Cunha, reported a violent windstorm.
Satellite imagery from Monday afternoon showed widespread instability over Central-Southern Brazil due to the circulation associated with the Argentine low-pressure area. The same system is expected to intensify as it enters southern Brazil, producing further rounds of strong convection and severe thunderstorms on Tuesday.
Satellite image acquired at 20:50 UTC on December 8, 2025. Credit: NOAA/GOES-East, Zoom Earth, The Watchers Satellite image acquired at 09:50 UTC on December 9, 2025. Credit: NOAA/GOES-East, Zoom Earth, The Watchers Rainfall totals may be locally very high, with episodes of torrential downpours likely to produce flash floods and localized flooding. According to the WRF model, 72-hour accumulations between Monday 9:00 LT and Thursday 9:00 LT may exceed typical monthly averages in many areas.
On Wednesday, the cyclone will continue to influence the region but with less widespread rainfall compared to Tuesday. Intervals of sunshine will occur as drier air moves in from northeastern Argentina and Paraguay. However, strong to intense winds are expected to become the main hazard.
Wind gusts of 50–80 km/h (31–50 mph) are forecast across much of southern Brazil, increasing to 80–100 km/h (50–62 mph) in southern and eastern Rio Grande do Sul. Along the southern coast and Lagoa dos Patos, gusts could reach 100–130 km/h (62–81 mph). Similar wind intensities are possible along the Serra coastal wall.
Eastern Santa Catarina, Paraná, and São Paulo– as well as parts of southern Minas Gerais–may also experience strong winds on Wednesday as the cyclone remains near the Rio Grande do Sul coast.
1 Cyclone to bring heavy rain and strong winds to southern Brazil – MetSul Meteorologia – December 8, 2025
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