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Change in weather pattern in May brings combination of more intense cold in the South, advance of El Niño, and high volumes of rain in some regions, while areas of Central Brazil enter a drier period with temperatures above the historical average.
Brazil’s climate behavior is expected to undergo a significant change in May, with the advance of cold in the South, increased rainfall in part of the region, and consistent signs of El Niño formation in the Equatorial Pacific, according to MetSul Meteorologia projections.
This scenario is part of a climatic autumn phase where the atmospheric pattern is already gradually approaching winter, especially in the second half of the month, favoring colder nights, mild afternoons, and a higher frequency of frost episodes.
In the capital of Rio Grande do Sul, the climatological normals from 1991 to 2020 indicate an average minimum of 13.6°C in May, a value lower than the 16.8°C recorded in April, reflecting the increase in cold nights and the gradual reduction of temperature throughout the month.
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Brazil’s ‘best-planned’ city has 368,000 inhabitants, a 99% schooling rate, an ecological park with 111 hectares, avenues up to 70 meters wide, and a routine with commutes under 30 minutes.
Furthermore, the average monthly maximum drops from 26.4°C to 22.6°C, reinforcing the transition to a more typical winter pattern, with less hot afternoons and a greater predominance of moderate thermal conditions.
In São Paulo, data from the Mirante de Santana station indicate an average minimum of 14.7°C in May and an average maximum of 23.4°C, showing similar behavior to that observed in the South Region, albeit with less intensity.
At the same time, areas of the Central-West, Southeast, and even parts of the North may experience incursions of cold air of continental origin, capable of causing sharp temperature drops and characteristic cold spells.
Cold gains strength and frost risk increases in the South
As the month progresses, the probability of lower minimum temperatures increases, especially in the southern states, where atmospheric conditions favor frost formation on a greater number of days.
In more intense situations, the phenomenon can reach lower altitude areas and advance into regions of the Central-West and Southeast, bringing potential impact to sensitive crops, such as second-crop corn.
In Rio Grande do Sul, the expectation is an increase in the frequency of cold air incursions starting from the second week, driven by atmospheric systems that favor the entry of polar air masses.
In this context, the action of extratropical cyclones in the South Atlantic tends to intensify winds, reduce temperatures, and increase the number of cold nights throughout the period.
Despite this scenario, the occurrence of snow is still considered rare in May, as it depends on a specific combination of atmospheric factors that cannot be predicted far in advance.
Even so, there are relevant historical records, such as the 1979 episode in Southern Brazil and events associated with subtropical storm Yakecan, which in 2022 brought favorable conditions for the phenomenon to occur.
More recently, at the end of May 2025, an extratropical cyclone favored snow accumulation in São José dos Ausentes and the Southern Plateau of Santa Catarina, reinforcing the climatic variability of this period.
For 2026, however, this type of event cannot be anticipated in long-term forecasts, as it depends on specific conditions observed only in the short term.
El Niño formation gains strength with Pacific warming
Parallel to regional changes, the scenario in the Equatorial Pacific indicates the possibility of El Niño formation between May and June, in a process considered earlier than the historical pattern.
This movement is associated with the significant warming of the ocean’s subsurface waters, which have begun to show above-average values at different depths.
After the La Niña period, an intense Kelvin Wave advanced through the ocean carrying a large volume of warmed water between 100 and 250 meters deep.
In this system, thermal anomalies reached about 8°C above average, indicating a significant reservoir of heat ready to emerge to the surface.
With the tendency for this heat to rise in the coming weeks, the ocean-atmosphere coupling process should intensify, favoring the consolidation of the climatic phenomenon.
Even so, the most relevant impacts of El Niño on the Brazilian climate usually occur months later, when the atmospheric response becomes more evident and consistent.
Indicators such as the Southern Oscillation Index already show signs compatible with this process, suggesting that the transition should strengthen over the coming months.
Above-average rain in the South contrasts with drought in Central Brazil
While the South is expected to face frequent episodes of instability, rainfall behavior in the rest of the country tends to follow a distinct pattern, with a gradual reduction in volumes in areas of the Midwest and Southeast.
In the capital of Rio Grande do Sul, for example, the average monthly precipitation increased from 94.6 mm in the 1961-1990 series to 112.8 mm in the 1991-2020 series, indicating an upward trend over the last decades.
This scenario gained prominence in May 2024, when Porto Alegre recorded its wettest month in its historical series, which began in 1910, according to Inmet data.
At the time, the accumulated total reached 539.9 mm, surpassing even September 2023, which until then held the lead in the precipitation ranking.
For May 2026, climate models indicate volumes near or above average in most of the South, with the possibility of high accumulated totals at different times of the month.
In some areas, rain may occur recurrently, with weekly episodes capable of generating significant volumes already in the first half of the period.
Rio Grande do Sul and Santa Catarina are expected to concentrate the highest accumulated totals, while northern Paraná tends to show behavior closer to the historical average.
On the other hand, the Midwest and Southeast are entering a phase of reduced rainfall, accompanying the approach of winter and the advance of the dry season in Central Brazil.
In this context, capitals such as Brasília, Belo Horizonte, and São Paulo begin to record significantly lower volumes compared to the summer months.
In São Paulo, the May average is 66.3 mm, a value much lower than the 292.1 mm observed in January, the wettest period of the year.
Similarly, Belo Horizonte shows an average of 28.1 mm in May, against 330.9 mm in January, while Brasília records about 26.9 mm in the same period.
Above-average temperatures in part of the country and the advance of cold in the South
In addition to the contrast in rainfall, thermal behavior should also vary between regions, with a tendency for above-average temperatures in much of the Midwest and Southeast.
In these areas, several hot days are expected throughout the month, interspersed with occasional cold air incursions that may cause temporary temperature drops.
One such incursion may occur in the second week of May, reaching states like São Paulo and southern Minas Gerais regions with characteristics closer to winter.
Even with isolated cooling episodes, the predominant trend still indicates positive temperature deviations throughout the period.
In the South, the behavior should be more balanced, with Paraná and Santa Catarina recording values close to the historical average, although with areas of slight warming in northern Paraná.
In Rio Grande do Sul, however, a colder month is expected, especially in the Southern Half, where the frequency of polar air masses tends to be higher.
Another typical element of May is the increased occurrence of fog and mist between dawn and morning, phenomena that can reduce visibility in various regions.
In certain situations, the fog can persist until late morning or even extend into the afternoon, depending on local atmospheric conditions.
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