Meteorological Piracicaba/September 27, 2023 Storm

The storm that hit the city of Piracicaba, in the brazilian state of São Paulo, on September 27, 2023, was a severe, highly destructive weather event. It lasted less than 10 minutes but caused significant damage. The storm was a wet microburst, with wind gusts estimated at up to 108 km/h (67 mph), toppling over 130 trees, utility poles, walls, and roof tiles, and even causing a warehouse to collapse on Comendador Luciano Guidotti Avenue. The heavy rain, which totaled almost 20 millimeters in a short time, also led to flooding in parts of the city. This was the most violent storm Piracicaba had seen since July 21, 2013. The study will delve into the storm’s causes, impacts, and unique characteristics.

Causes

Cloudy sky before the microburst.

The storm in Piracicaba on September 27, 2023, was primarily caused by the arrival of a cold and humid air mass on a particularly hot day, with temperature reaching 38 °C (100 °F). This combination of atmospheric conditions led to the formation of cumulonimbus clouds, known for their significant vertical development and potential to produce severe storms.

The microburst that occurred can be attributed to evaporative cooling, which happens when a substance loses heat as it transitions from liquid to gas. When a drier air mass encountered the cloud, the water droplets inside it, along with those from precipitation, evaporated, causing the cold air to become denser. This dense, cold air then descended rapidly, creating a powerful downdraft. Upon reaching the ground, the cold air spread out in all directions, generating winds exceeding 100 km/h (62 mph) and triggering the microburst.

Aftermath

Tree fall near a school in the Piracicamirim neighborhood.

The storm that hit Piracicaba on September 27, 2023, caused significant material and environmental damage, highlighting the city's (or at least a large portion of it) lack of preparedness for extreme weather events. The destruction was comparable to an F1 tornado on the Fujita Scale, concentrated in a narrow strip measuring less than 3 km wide and about 4 km long, stretching from the Caxambu neighborhood to Vila Monteiro and from Higienópolis to Pompéia.

The storm resulted in 132 fallen trees, which disrupted the power grid, traffic, and altered the urban landscape. Additionally, several utility poles between the Jardim Elite, Nova América, and Caxambu neighborhoods were knocked down, along with some water tanks due to the powerful winds. A warehouse undergoing renovations on Comendador Luciano Guidotti avenue in the Higienópolis neighborhood also collapsed. In Caxambu, part of a condominium wall was torn down, and its reconstruction wasn’t completed until the end of November that same year. In the Pauliceia and 1º de Maio neighborhoods, roofs were damaged. The storm also caused flooding in various areas of the city.

Epicenter and major affected areas

Map of the main affected area (in red) and the possible epicenter (marked by an X).

The epicenter of the storm in Piracicaba on September 27, 2023, appears to be located between Professor Alberto Vollet Sachs and Comendador Luciano Guidotti avenues, which were hit hardest by the microburst. This phenomenon, a type of downburst usually affecting areas smaller than 4 km, spanned less than 3 km in width and caused significant damage within a 2.5 km radius of the epicenter. The neighborhoods most impacted included Caxambu, Jardim Elite, Nova América, and Higienópolis, where residents faced falling trees, poles, walls, and water tanks, along with flooding and roofs being torn off. In this area, wind gusts exceeded 90 km/h. Paulicéia also experienced roof collapses. Other neighborhoods, such as Morumbi, Pompéia, Vila Monteiro, Alto, and Paulista, were affected as well, though with less intensity, with gusts ranging from 70 to 88 km/h according to the Beaufort Scale. The "Luiz de Queiroz" College of Agriculture (ESALQ), located in the Agronomia neighborhood, recorded a gust of 64.0 km/h.