A cold front paired with an upper-level trough is set to bring three straight days of severe storms to southern Brazil, with rainfall totals that could top 200 millimeters in the Planalto Sul highlands and the southern coast of Santa Catarina state between this Friday (28) and Sunday (30). The forecast, from Meteored meteorologist Ana Maria Pereira Nunes, also includes a risk of hail, lightning and damaging wind gusts across Rio Grande do Sul, Santa Catarina and parts of Paraná, based on the extreme forecast index of the European weather model (ECMWF).
Brazil's National Institute of Meteorology (Inmet) placed 518 cities in the South region under a danger alert this Friday and Saturday, forecasting rain of 30 to 60 mm per hour, winds of 60 to 100 km/h and hail. The warning covers areas such as the Santa Catarina highlands and the Itajaí Valley, the Porto Alegre metro area and northeastern Rio Grande do Sul, and central, southwestern and southeastern Paraná. The institute lists risks of power cuts, crop damage, falling trees and flooding. In Greater Florianópolis, the Grande Oeste and the Itajaí Valley, Santa Catarina's Civil Defense expects 100 to 150 mm of rain, with a risk of street flooding and flash floods.
Rivers at risk of flooding
MetSul Meteorologia, a private weather service based in Rio Grande do Sul, reinforced a special high flood risk alert for Rio Grande do Sul and Santa Catarina this Friday, valid through Monday (31). According to the bulletin, the most threatened rivers in Rio Grande do Sul are the Uruguai, Jacuí, Taquari and Caí, because the heaviest rain is expected over the Campos de Cima da Serra plateau, where those basins begin. In the state capital Porto Alegre, the Guaíba, the lake-like waterway at the heart of the city, is highly likely to rise past its alert level of 2.50 meters next week, flooding islands and possibly forcing safety power shutdowns in the Arquipélago neighborhood. MetSul also warns that a cold air mass with strong southerly winds, forecast for September 4 and 5, could dam the Guaíba's outflow just as its level peaks.
The episode is fed by an atmospheric river, a corridor channeling moisture from the Amazon region toward the South, which explains the unusually long duration of the instability, according to MetSul. Rio Grande do Sul is still recovering from the historic floods of May 2024, which left dozens dead and entire cities under water, so any forecast of excessive rain triggers heightened attention from authorities. This Friday, neighboring Uruguay, which shares the same river basin, already recorded large hail near the Brazilian border, MetSul reported.
Heat and dry air across the rest of the country
While the South faces storms, the Southeast and Central-West are entering a stretch of heat and low humidity, according to g1. São Paulo's state Civil Defense forecasts intense heat through Monday, with highs of up to 40°C inland and 36°C in the state capital. Inmet, however, does not classify the period as a heat wave, because its criteria require temperatures at least 5°C above average for five consecutive days, and a new cold front is expected to push temperatures down from Tuesday (September 1).
In the Central-West, Cuiabá should reach around 40°C this Friday, with a chance of hitting 41°C in the coming days. Air humidity may drop to between 12% and 20% in Mato Grosso do Sul, Goiás, the Federal District and parts of Mato Grosso during the hottest hours, a condition that favors the spread of wildfires, warns Climatempo. Santa Catarina's Civil Defense advises residents to avoid sheltering near trees and power poles during windstorms, never cross flooded streets or submerged bridges, and seek protected locations away from windows during severe thunderstorms.