Possible bomb cyclone puts southern Brazil on alert

The National Institute of Meteorology (Inmet) reported today (August 4th) that it is monitoring the evolution of a low-pressure system that could give rise to an intense extratropical cyclone between Argentina, Uruguay, southern Brazil, and the Atlantic Ocean in the coming days. Forecasts indicate favorable conditions for the occurrence of a process called explosive cyclogenesis, also known as bombogenesis or, popularly, a bomb cyclone. 

This phenomenon occurs when a low-pressure system rapidly intensifies over a short period of time. Confirmation of this process will depend on the observed evolution of atmospheric pressure during the cyclone's development.

According to the forecasts of the Inmet Cosmo model, the system begins to organize itself from 9 am (Brasilia time) on Thursday, August 6th, from a trough (elongated region of low pressure) over northern Argentina and Paraguay. This system evolves into a low-pressure center between Uruguay and the coast of Rio Grande do Sul, giving rise to the extratropical cyclone.

In the following hours, the cyclone is expected to intensify rapidly. The forecast indicates that the central pressure of the system may decrease to approximately 967 hPa on Friday, August 7, and continue to fall to about 941 hPa on August 8, representing a reduction of approximately 26 hPa in 24 hours, a characteristic consistent with a bombogenesis process.

The cyclone is expected to reach its lowest predicted pressure, around 940 hPa, on Saturday, August 8th, when it will be at its most intense. Following this, the system will move southeast over the South Atlantic, gradually moving away from the coast of South America.

After the passage of the cyclone and its associated cold front, a high-pressure area will move over Argentina and southern Brazil, favoring the entry of a cold air mass and a drop in temperatures in the following days.

The forecast from the main meteorological models analyzed by Inmet shows a similar evolution to that indicated by the Cosmo model, increasing confidence in the scenario of the formation of an intense extratropical cyclone.

Weather conditions forecast

The main impacts in Brazil are expected between August 6th and 8th, mainly in the Southern Region. The formation of the cyclone will favor the advance of a cold front, with the possibility of rain, storms, lightning, wind gusts, and hail.

Conditions are expected to be favorable for severe storms, especially in southern Brazil. The Cosmo model indicates areas with the potential for hail associated with the cold front and increased atmospheric instability.

The strongest winds are expected mainly over the Atlantic Ocean, where gusts could exceed 100 km/h in areas near the center of the cyclone. In coastal areas and parts of the Southern Region, especially in Rio Grande do Sul and Santa Catarina, gusts exceeding 60 km/h are also expected between August 6th and 8th.

Furthermore, the intensification of the cyclone may favor the formation of instability lines and gust fronts associated with the passage of the cold front, especially in areas between southern Brazil and Paraguay.

After the cyclone moves away into the ocean, the advance of high pressure will favor the entry of cold air over south-central Brazil, causing a sharp drop in temperatures, with the possibility of impacts also in areas of the Midwest and Southeast in the following days.

Although models indicate a scenario favorable to the formation of an intense extratropical cyclone, the definitive classification as a bomb cyclone will depend on confirmation of the intensity of the drop in atmospheric pressure during its evolution.

Current weather warnings

Inmet maintains weather warnings for areas in the Central-South region of Brazil due to the conditions forecast for the coming days.

For Wednesday, August 5th, a yellow warning of potential danger for storms is in effect for areas in south-central and western Mato Grosso do Sul, all of Paraná, Santa Catarina, Rio Grande do Sul, and the border between São Paulo and Paraná.

For Thursday, August 6th, the intensification of the system and the advance of the cold front raise the level of alert. An orange warning for storms is in effect for southern Mato Grosso do Sul, western and southern Paraná, Santa Catarina, and all of Rio Grande do Sul, areas where the most significant impacts associated with the system are expected.

The yellow warning for potential storm danger also remains in effect, covering other areas of southern Brazil, central and southern Mato Grosso do Sul, and the border between São Paulo and Paraná.

Inmet also recommends that the public follow updates to weather warnings and forecasts, as the intensity and trajectory of the system may be adjusted as new runs of numerical forecast models are incorporated.