Brazil's national weather service, the Inmet, has issued a red alert, the highest level on a scale that runs from yellow to orange to red, for the first heat wave of spring, in force from Saturday (26) through Wednesday (30). The warning covers all of Mato Grosso do Sul, Paraná and São Paulo, plus parts of Mato Grosso, Rio de Janeiro and Minas Gerais. Temperatures could reach 41°C in Cuiabá on Sunday (27) and Monday (28), 40°C in Campo Grande on Monday and 32°C in the city of São Paulo on Sunday, the agency said.
How the heat reaches the Center-South
One driver of the warming is a low pressure system set to intensify between northern Argentina and western Paraguay, pushing hot air toward southern Brazil, Paraguay and Argentina, Inmet meteorologists said. High above, a zone of high pressure over parts of the Center-West, Southeast and South pushes air down, where it compresses and heats further. The result is what forecasters call a heat dome: an atmospheric lid that traps warm air over the same region and blocks cooler air masses. As long as the pattern holds, temperatures should stay high.
In São Paulo, where the alert was activated on Thursday (24), Saturday starts cloudy with a low of 16°C in the metropolitan area, according to the city's climate emergency management center (CGE). Sun breaks through during the day and lifts the high to 32°C, with relative humidity near 45% and no rain expected. The coast stays milder thanks to the sea breeze: Santos ranges from 17°C to 26°C and Guarujá from 20°C to 22°C, weather service Climatempo forecasts. The interior of the state should be the hottest area.
Spring under El Niño
Spring, which began on September 22 and runs to December 21, marks the transition from the dry to the rainy season in central Brazil. Inmet said the season will be influenced by El Niño, the periodic warming of the tropical Pacific that shifts rain patterns worldwide. The institute expects the moisture flow from the Amazon to strengthen and feed rains over the Center-West and Southeast, and warns of heavy rain, wind gusts, lightning and possible hail in the South. At the other end of the country, northern Piauí and northwestern Ceará face a drier than normal stretch. Average temperatures should top 28°C in the North and Northeast and in parts of the interior, a setup that favors more heat waves, thermal discomfort, changes in soil moisture and fast-forming severe storms, according to the agency.
Forecasts released in early August by the CPC/NOAA, the US climate prediction center, put the odds of a very strong El Niño between September and November above 90%. The event earns the label "super El Niño" when Pacific sea surface temperatures run more than 2°C above the historical average, a threshold crossed only four times in about 150 years: 1877-78, 1982-83, 1997-98 and 2015-16. NOAA researchers caution that stronger episodes raise the probability of severe impacts, though they do not guarantee them. A joint bulletin by Inmet, CPTEC/Inpe and Funceme, the Ceará state meteorology foundation, forecasts above-average rain in the South, especially Rio Grande do Sul, and in parts of São Paulo and Mato Grosso do Sul, and drier than normal conditions in the North, the Northeast and parts of the Center-West and Southeast.
Heat waves tend to push up electricity use as air conditioning runs harder. For now the power sector has room to spare: the regulator Aneel set the green flag, the no-surcharge level of Brazil's tariff flag system on power bills, for October, ending five months of extra charges, Agência Brasil reported. The Inmet red alert runs through Wednesday (30).