Why São Paulo is Facing Its Wettest September in History

São Paulo is experiencing an extraordinary weather event. The capital city has officially registered 253.8 millimeters of rainfall in September alone, making it the wettest September in the city's history since official measurements began in 1943 by the National Institute of Meteorology (Inmet).

## Breaking an 83-Year Record

The accumulated volume in just the first two weeks of September has already surpassed the previous historic record set in 1983, when 243.1 millimeters of rain were recorded.

Meteorologists note that the pattern of consecutive rainy days is also well above historical averages. While the historical average for consecutive wet days in September is just 1.8 days, 2026 has brought extended sequences of wet weather, keeping skies overcast and preventing the usual transition to spring-like conditions.

## What's Causing the Heavy Rains?

According to weather experts, the unusual combination of persistent rain and low temperatures is driven by strong wind currents known as cavados, fueled by ocean moisture and intensified atmospheric patterns. Unlike summer storms that are typically followed by quick periods of sunshine, current weather systems are locking in continuous cloud cover.

Temperatures in the capital have remained unseasonably low, hovering around a maximum of 17°C (62°F) with sharp drops down to 14°C (57°F) at night, restricting the natural warming cycle that typically marks the approach of the Brazilian spring.

## Why It Matters: Stakeholder Impact

The relentless downpours present immediate operational and public health challenges for South America's largest metropolis. With soils already heavily saturated from persistent downpours, civil defense authorities have issued high-priority alerts for potential urban flooding, flash floods, and landslides across the Greater São Paulo metropolitan region and the coastal baixada Santista.

Infrastructure networks face strain as drainage systems struggle to cope with the sheer volume of water. Meanwhile, everyday life for millions of residents has been disrupted by severe traffic congestion, transit delays, high indoor humidity, damp living spaces, and a complete lack of sunshine.

## Future Outlook

Looking ahead, meteorological models indicate that while the intensity of the atmospheric blocks may begin to shift by late September, the immediate horizon offers little relief. Interior regions of São Paulo state remain under threat of isolated severe storms accompanied by lightning and potential hail, while the eastern coastal strip will continue to absorb residual moisture. Authorities advise monitoring local real-time alerts as the region navigates this historic climatic anomaly.