Subtropical Storm Potira Forms Southeast Of Brazil

Tropical Weather Update Overview:
— A subtropical storm has formed southeast of Brazil in the South Atlantic Ocean, forecast to gradually strengthen as it remains offshore
— The system poses no threat to T&T or the Lesser Antilles
— There are no tropical threats nor tropical waves east of T&T in the North Atlantic Ocean. The official start of the 2021 Atlantic Hurricane Season is June 1st.

An uncommon subtropical storm has formed in the South Atlantic Ocean, southeast of Brazil on Tuesday 20th April 2021, according to the Brazil Navy Hydrography Center (CHM) and Brazil’s National Meteorological Institute (INMET).

The Subtropical Storm, named “Potira,” can be traced back to a non-tropical disturbance that moved off the coast of Brazil over the weekend before stalling over the ocean just off the coast. As it sat over the water, it gradually started to organize and take on some characteristics of a tropical storm, eventually being declared a subtropical storm by Brazil’s Navy.

At the time of the formation on Tuesday, the subtropical cyclone, which was at storm strength, had estimated winds between 35 knots (46 to 55 KM/H) located at 27° S and 43° W. As of 00Z April 20th, the system’s intensity remains unchanged with a central pressure of 1006 millibars, now at an estimated location of 26° S and 41° W, moving to the east-southeast at 10 KM/H.

Latest information on Subtropical Storm Potira, southeast of Brazil in the South Atlantic Ocean
Latest information on Subtropical Storm Potira, southeast of Brazil in the South Atlantic Ocean

According to INMET, atmospheric conditions are conducive for intensification, and this cyclone could become a subtropical storm in the coming hours. According to the Regulations of the Maritime Authority for Maritime Meteorology (NORMAM-19), for a cyclone to be classified as a subtropical storm, the winds must be equal to or greater than 34 knots (63 KM/H).

As the system strengthens, INMET is forecasting strong winds near the coast of the states of São Paulo and Rio de Janeiro, reaching 102 KM/H at sea, in the south and west sectors of the cyclone, and 75 KM/H along the coast, throughout the period of cyclone activity.

Brazil's National Meteorological Institute Surface Analysis as of 00Z April 22nd, 2021 showing the Subtropical Storm Potira southeast of Brazil. (INMET)
Brazil’s National Meteorological Institute Surface Analysis as of 00Z April 22nd, 2021 showing the Subtropical Storm Potira southeast of Brazil. (INMET)

INMET added that these strong winds would cause waves in open waters to be very rough, with wave heights between 3.0 and 7.0 meters. Near the coastline, waves from the southeast to the east, up to 3.5 meters high, are forecast in the coastal areas between São Paulo, Rio de Janeiro, and Espírito Santo, south of Vitória, until the morning of April 24th.

Potira is forecast to slowly drift to the south off the coast of Brazil over the next several days with small fluctuations in strength but maintaining subtropical status. The system will likely dissipate off the coast of Brazil and have no direct impact on land apart from rough surf along the nearby coast and local shipping routes.

What is a subtropical storm?

GOES-16 IR-13 Imagery of the Subtropical Storm Potira southeast of Brazil (Weathernerds)
GOES-16 IR-13 Imagery of the Subtropical Storm Potira southeast of Brazil (Weathernerds)

subtropical cyclone is a non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Subtropical cyclones originate over tropical or subtropical waters and have a closed circulation about a well-defined center. Across the North Atlantic, they require central convection fairly near the center and a warming core in the mid-levels of the troposphere.

Those with sustained winds below 62 KM/H (33 knots or 38 MPH) are called subtropical depressions, while those at or above this speed are referred to as subtropical storms.

Only a handful of tropical systems have ever been recorded over the Atlantic Ocean south of the equator. However, the frequency of this phenomenon has been trending upward in recent years.

Before Potira developed, there had only been 14 named tropical systems in the southern Atlantic Ocean, a majority of which were subtropical storms. This means that they have meteorological characteristics of both a tropical storm and a non-tropical storm.

Second Subtropical Cyclone In South Atlantic For 2021

Tropical Storm 01Q in the South Atlantic on February 6, 2021 (NASA, Suomi/NPP)
Tropical Storm 01Q in the South Atlantic on February 6, 2021 (NASA, Suomi/NPP)

On February 4, 2021, an extratropical storm off the coast of Rio Grande do Sul developed into a bomb cyclone.

On February 6, the storm began separating from its weather fronts and developed subtropical characteristics, before fully separating from the frontal zone and transitioning into a fully-tropical storm later that day.

As a result, the NOAA classified the system as a tropical storm at 17:30 UTC, with the system being designated as Tropical Storm 01Q. However, the storm was short-lived, as it lost its tropical characteristics several hours later, with the NOAA issuing their final bulletin on the storm at 23:30 UTC that day. The storm dissipated soon afterward. Although the NOAA issued bulletins on the storm, the Hydrographic Center of the Brazilian Navy did not monitor it.

Only one system has ever reached hurricane status in the South Atlantic. In 2004 near the end of March, Hurricane Catarina became a Category 2 hurricane on the Saffir-Simpson hurricane wind scale before making landfall in Brazil.

The Brazilian Navy has the forecasting duties for tropical systems in this region of the Atlantic Ocean and is forecasting Potira to remain a subtropical storm over the next few days.

South Atlantic Ocean Tropical Cyclone Names

The following names are published by the Brazilian Navy Hydrographic Center’s Marine Meteorological Service and used for tropical and subtropical storms that form west of 20ºW and south of the equator in the South Atlantic Ocean. Originally announced in 2011, the list has been extended from ten to fifteen names in 2018. The names are assigned in alphabetical order and used in rotating order without regard to year. The names of significant tropical or subtropical systems will be retired.

  • Arani
  • Bapo
  • Cari
  • Deni
  • Eçaí
  • Guará
  • Iba
  • Jaguar
  • Kurumí
  • Mani
  • Oquira
  • Potira (unused)
  • Raoni (unused)
  • Ubá (unused)
  • Yakecan (unused)

There have been over 84 recorded tropical and subtropical cyclones in the South Atlantic Ocean since 1957. Like most southern hemisphere cyclone seasons, most of the storms have formed between November and May.

Strong wind shear, which disrupts the formation of cyclones, as well as a lack of weather disturbances favorable for development in the South Atlantic Ocean, make any strong tropical system extremely rare, and Hurricane Catarina in 2004 is the only recorded South Atlantic hurricane in history.

Track map of all significant systems of the South Atlantic tropical cyclone since March 19th, 2004. The points show the location of the storm at 6-hour intervals.
Track map of all significant systems of the South Atlantic tropical cyclone since March 19th, 2004. The points show the location of the storm at 6-hour intervals.

Since 2011, the Brazilian Navy Hydrographic Center has assigned names to tropical and subtropical systems in the western side of the basin, near the eastern coast of Brazil, when they have sustained wind speeds of at least 65 KM/H (40 MPH), the generally accepted minimum sustained wind speed for a disturbance to be designated as a tropical storm in the North Atlantic basin.

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