Extratropical cyclone brings intense cold and frost to the South

An extratropical cyclone brought a polar air mass to southern Brazil, causing sub‑zero temperatures, frosts and even snow in September.

Cold air mass invaded the southern part of the country

Last week, MetSul Meteorology reported the arrival of a cold air mass that intensified over the weekend, bringing snow in the middle of September to Rio Grande do Sul and Santa Catarina.

The third consecutive day with sub‑zero minima in Rio Grande do Sul was marked by a low of -6.8 °C in São José dos Ausentes, the lowest temperature recorded in the state in 2026, surpassing the previous record of -6.4 °C in Pinheiro Machado in June.

Frost and temperature drop in other regions

In Santa Catarina, Monday began with minima down to -6 °C in São Joaquim, while Paraná recorded 0.2 °C in General Carneiro. In the interior of São Paulo, temperatures ranged between 10 °C and 13 °C, and in the city of São Paulo, 6.9 °C was recorded in the southern part of the capital.

New cold incursions forecast for the weekend

Although the current mass is losing strength, MetSul forecasts a new incursion of cold air, with a continental trajectory, expected to arrive at the end of the week. The new mass will not be as intense, but it should cause a sharp temperature drop and more cold days in the first half of the following week.

Antarctic oscillations favor polar advance

The atmospheric dynamics in the Southern Hemisphere are conducive to successive cold air masses entering South America. The pattern is linked to the Antarctic Oscillation (AAO), which describes changes in atmospheric pressure and wind circulation between mid‑latitudes and high latitudes.

When the AAO enters a negative phase, the winds that are usually concentrated around Antarctica weaken and shift northward, facilitating the advance of polar air toward South America. This scenario helps explain the opening of the polar corridor, allowing successive polar‑origin air masses to move forward.

Impact on the climate of the South and Central‑West

The greatest impact of this series of cold incursions is on the temperature of the southern part of the country, especially Rio Grande do Sul and Santa Catarina. In the Southeast and parts of the Central‑West, a large amount of rain compared to the September normal is projected, with many mild days and some even cold.

In the Central‑West, where weather is usually warm, negative temperature anomalies do not necessarily translate into cold, but rather into fewer extremely hot days. In Mato Grosso do Sul, due to its closer proximity to the South, cold days occur.

What to expect in the coming weeks

The winter‑like thermal scenario, with conditions typically expected in June, July and August, remains in place. The high frequency of cold air masses observed in May, June and the first week of July may continue to affect southern Brazil in the coming weeks, with days of sub‑zero minimum temperatures and a high frequency of frost formation.

With information from MetSul Meteorology.