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May 2026 expected to have heat dome in Central Brazil, cold in the South, low humidity, and prolonged atmospheric blocking.
According to Climatempo, temperatures during May 2026 are expected to be much higher than normal in the region covering central, western, and northern São Paulo, Triângulo Mineiro and west-central Minas Gerais, Goiás, Distrito Federal, most of Mato Grosso do Sul, southern and east-central Mato Grosso, southern Tocantins, and western Bahia. The atmospheric blocking caused by the high-pressure system will hinder the passage of cold fronts from the South to the Southeast and Central-West.
As a result, a large part of the cold air of polar origin is expected to remain trapped over Southern Brazil, while the Central-West remains under the dominance of heat. The European ECMWF model, one of the main global references for extended forecasts, indicates that this pattern should persist throughout May, with the center of the heat dome reaching anomalies between 6°C and 10°C above average in certain weeks.
What makes the phenomenon exceptional is not just the magnitude of the anomalies, but their duration. The ECMWF projects the heat dome acting for at least five consecutive weeks, which, by Brazilian meteorological criteria, constitutes a prolonged heatwave. It’s an autumn that doesn’t feel like autumn.
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What is a heat dome and why is this atmospheric blocking expected to persist over Central Brazil in May 2026
The popular term “heat dome” corresponds to a precise technical phenomenon: the presence of a stationary high-pressure system over a region. This system suppresses cloud formation, isolates the air mass from external influences, and allows the ground and atmosphere to progressively accumulate heat.
In meteorological terms, it is an upper-level high-pressure block. It is a configuration where the Rossby wave, responsible for moving weather systems across the globe, becomes stationary or very slow, trapping warm air in the same region.
The blocking dominating Central Brazil in May 2026 is related to the warming of the Equatorial Pacific Ocean. A reservoir of superheated waters between 100 and 250 meters deep advances towards the coast of South America in the form of a Kelvin Wave, with anomalies of up to 8°C above average in the subsurface.
The Equatorial Pacific, the Kelvin Wave, and the possible onset of El Niño help explain the persistence of the heat
When the warmed water associated with the Kelvin Wave surfaces, the resulting atmospheric heating can reorganize winds and further favor the already established blocking pattern. This type of ocean-atmosphere interaction is decisive for large-scale circulation.
The Southern Oscillation Index, SOI, an atmospheric indicator of the Pacific, is already beginning to show values compatible with a transition to El Niño. The ECMWF forecast indicates that the phenomenon may establish itself throughout May or, more likely, between May and June.
This means that the May heat dome should not be treated merely as an isolated event. It could be the first symptom of an atmospheric reorganization capable of influencing the second half of 2026.
Week by week, what ECMWF projects for extreme heat, rain in the South, and thermal contrast in Brazil
The ECMWF forecast for May 2026 has sufficient weekly resolution to identify the moments of greatest and least intensity of the dome. In the first week, from May 1st to 4th, the dome will be expanding, with anomalies between 6°C and 8°C above average over the Central-West and parts of the Southeast.
On the 1st, maximums could approach or exceed 38°C in Mato Grosso, Mato Grosso do Sul, Goiás, Tocantins, and Piauí. In terms of precipitation, this week is expected to show a strong contrast: up to 60 mm above average in Rio Grande do Sul and 33 mm on the country’s northern coast, while Central Brazil remains slightly below average.
In the week of May 4th to 11th, the dome reaches its peak. The center of the anomalies could reach 10°C above average, a value described by meteorologist Alexandre Nascimento, from Nottus, as a “summer heatwave occurring in the middle of autumn.”
The greatest thermal contrast is expected to occur between May 11th and 18th, with cold in the South and persistent heat in the Central-West
In the week of May 11th to 18th, the thermal gradient reaches its maximum value. Southern Brazil is expected to face its coldest week of the month, with anomalies of up to 6°C below average in parts of Rio Grande do Sul and Santa Catarina.
Meanwhile, the Midwest still maintains temperatures between 3°C and 6°C above average. The simultaneous difference between the two regions can reach 12°C between areas separated by less than 1,500 kilometers.
In the second half of the month, the heat bubble loses intensity but does not disappear. The main highlight becomes above-average rain in the South, while the Midwest maintains less intense heat until at least May 25.
Why a prolonged autumn heatwave can persist for weeks over the Midwest and parts of the Southeast
The persistence of five weeks of positive temperature anomalies in the same region is classified by meteorology as a prolonged heatwave. According to the Brazilian criterion, which requires at least five consecutive days with a maximum of 5°C above the monthly climatological average, May 2026 meets the requirements in exceptional temporal extent for autumn.
The persistence mechanism lies in large-scale atmospheric circulation. When a high-pressure system stabilizes at altitude, it creates an atmospheric “hat” that compresses subsiding air, heating it by adiabatic compression and reinforcing the block itself.
In the case of May 2026, two external factors help sustain this pattern. The first is the oceanic Kelvin Wave in the Pacific; the second is the Antarctic Oscillation in its negative phase, which weakens circumpolar winds and allows greater advancement of high-pressure systems to lower latitudes of South America.
What Porto Alegre in May 2024 helps us understand about the risk of above-average rain in the South in 2026
The forecast of above-average rain in the South in May 2026 has an important historical context. In May 2024, Porto Alegre recorded the wettest month in its historical series since 1910, with 539.9 millimeters in a single month, exceeding the 450 millimeters observed in September 2023.
In May 2026, El Niño is still forming, not fully established. Models project a condition between average and above-average in the South, with more regular episodes and some high volumes, but without the same extreme configuration that trapped rain systems over Rio Grande do Sul for weeks in 2024.
What is repeated is the general mechanism: more active cold fronts, above-average oceanic humidity, and the Antarctic Oscillation favoring the passage of precipitating systems. The difference lies in the intensity, as the projected scenario does not equate to the pluviometric disaster of 2024.
Humidity below 30% in the Midwest and Southeast expected to increase respiratory risk during the heat bubble
While public debate tends to focus on the storms in the South and the heat in the Midwest, an equally impactful technical datum is being underestimated: the relative humidity in the Southeast and Midwest. The high-pressure block not only prevents the advance of cold fronts but also the transport of humidity from the Amazon to the interior of the country.
The result is the dominance of dry air. In Brasília, the average rainfall in May is only 26.9 millimeters, compared to over 200 millimeters in December and January. With the high pressure being more intense than climatology, the few rains become concentrated, and dry days multiply.
Relative humidity below 30% raises a public health alert, especially when it occurs for several consecutive days. With maximums above 33°C in parts of inland São Paulo and persistent dry air, the risk is concentrated in children, the elderly, and people with rhinitis, asthma, or chronic obstructive pulmonary disease.
How the heat bubble transforms May 2026 into an autumn with a summer pattern in Central Brazil
Brazilian meteorology has a direct expression for what May 2026 is configuring: “autumn with a summer face.” But the phenomenon has implications beyond thermal discomfort, especially for agriculture, public health, and energy consumption.
For agriculture, above-average temperatures during autumn interfere with winter crops planted in May and June, such as wheat, barley, canola, and oats. With maximums above 33°C in parts of inland São Paulo and Goiás, thermal stress can affect germination and initial development.
For electricity, prolonged heat increases the consumption of air conditioning and industrial ventilation during a post-summer recovery period for hydroelectric plants. If consumption in the Center-South remains high and the reservoirs of the Southern System do not recover, the second half of the year could start with a smaller energy margin.
May 2026 is expected to combine cold fronts in the South, extreme heat in the Center-West, and an autumn that has lost its natural transition
Climatempo summarizes the May 2026 pattern as a second half of autumn marked by temperature extremes. The Center-South will experience significant drops in temperature, while the Center-West will remain under persistent heat.
The negative phase of the Antarctic Oscillation favors cold fronts in the South. The oceanic Kelvin Wave accelerates Pacific warming. The ECMWF projects five consecutive weeks of extreme heat in a region that, at this time of year, should already be feeling the onset of autumn.
The result is a climatic contrast difficult to ignore: the South receives cold air and recurrent rain, while Central Brazil faces heat, atmospheric blocking, and low humidity. It’s the summer that didn’t leave in the middle of autumn.
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