Meteorological forecasters have warned of a risk of thunderstorms across the Southeast and Central-West regions of Brazil on Thursday, May 21, 2026 [1].

These weather patterns are significant because the resulting atmospheric instability can lead to flash flooding and dangerous wind conditions in densely populated urban centers. The sudden shift in weather puts local infrastructure and transportation networks at risk during the peak of the work week.

Weather services, including Climatempo, said the instability is caused by a polar air mass moving across the country. This cold front is lowering temperatures and creating the conditions necessary for strong rain and thunderstorms [1, 3].

Reports on the specific areas of impact vary across agencies. Some forecasts focus on the risk of storms in the Southeast and North regions [1]. Other reports specify that the risk is concentrated in Paraná, Mato Grosso do Sul in the Central-West, and southern São Paulo [2].

In addition to the rain, some reports indicate a risk of frost in the South, specifically affecting Rio Grande do Sul, Santa Catarina, Paraná, and Foz do Iguaçu [4]. However, other primary forecasts focus exclusively on the thunderstorms without mentioning frost [1].

Residents in the affected areas are advised to monitor local alerts as the cold front continues its trajectory. The combination of low temperatures and heavy precipitation is expected to persist throughout the day on May 21, 2026 [1].

“A polar air mass moving across the country is lowering temperatures and creating conditions for strong rain.”

The convergence of a polar air mass with existing tropical moisture creates high atmospheric instability. When this occurs in the Southeast and Central-West regions, it often results in 'temporais'—severe storms characterized by high wind speeds and intense rainfall over short periods. The discrepancy in regional forecasts suggests a fast-moving system, making real-time monitoring essential for emergency services and commuters.