Extratropical cyclone brings severe storms, hail risk to eight Brazilian states

An extratropical cyclone is forecast to form between late Monday night and early Tuesday morning (September 29) across southern Brazil, triggering severe weather across at least eight states. The system will bring intense rainfall, wind gusts exceeding 90 km/h, hail risk, and potential flooding. Brazil's National Meteorology Institute (Inmet) has issued orange-level alerts (danger) for Rio Grande do Sul and Santa Catarina, with yellow-level alerts (potential danger) extending across much of the southern and southeastern regions.

Rainfall projections are substantial. Rio Grande do Sul's Civil Defense reports accumulated precipitation between Sunday (27th) and Tuesday (29th) could range from 100–200 mm across the state, with some areas in the Campanha and Centro regions potentially exceeding 270 mm. Specific municipalities face severe risk: Dom Pedrito is forecast to receive around 115 mm on Tuesday alone, while Santana do Livramento and Rosário do Sul are each expected to receive approximately 105 and 63 mm respectively by Monday evening. Hourly rainfall rates could reach 30–60 mm/h in some areas, with 12-hour accumulations potentially surpassing 120 mm and 24-hour totals exceeding 170 mm in the most affected regions.

The cyclone formation is linked to the convergence of a warm front (warm, humid air mass) and a cold front advancing from lower latitudes. Meteorologists attribute the intense activity to an area of low pressure currently positioned north of Argentina, which is intensifying instability across the region. Wind gusts are expected to reach 90–100 km/h in affected areas, with the strongest conditions concentrated in the Fronteira Oeste, Campanha, Missões, Noroeste, Centro, and Nordeste regions of Rio Grande do Sul, as well as parts of Santa Catarina and Paraná.

Hydrological monitoring shows elevated risk for nine major rivers across Rio Grande do Sul. The Defesa Civil (State Civil Defense) has flagged inundation risks for the Caí, Quaraí, Ibicuí, Ibirapuitã, Jacuí, Santa Maria, São Sepé, Taquari, and Vacacaí rivers, with some—including the Jacuí and Rio dos Sinos—already at or above flood thresholds. Additional rainfall is expected to raise water levels further, increasing the probability of localized flooding, flash flooding in smaller waterways, and potential embankment overflow.

Damage from preceding weekend storms has already been documented. As of late Sunday (27th), Rio Grande do Sul's Civil Defense reported 9,517 people affected, 2,200 homes damaged, one injury, and no confirmed deaths. Santiago and Alegrete were the hardest hit, with approximately 5,000 and 2,000 residents affected respectively. Hail accumulation in some areas was so intense that ground surfaces appeared covered in ice.

The severe weather is forecast to persist through Tuesday morning and gradually weaken by Wednesday (30th). Conditions are expected to stabilize from Thursday (October 1st) onward, with predominantly clear skies and cooler temperatures across the region.