Subtropical Storm Potira
© NASA/WorldView
Subtropical Storm Potira off the coast of southeastern Brazil on Tuesday, April 20, 2021.
The start of the North Atlantic hurricane season is still more than a month away, but a new and unusual tropical system has developed in a part of the Atlantic Ocean where tropical systems rarely occur.

Subtropical Storm Potira took shape on Tuesday morning with sustained winds of 40 mph outside of the traditional Atlantic Basin. Instead of spinning up in the Gulf of Mexico, the Caribbean Sea or the northern Atlantic Ocean, it developed south of the equator off the coast of Brazil.

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.

Only a handful of tropical systems have ever been recorded over the Atlantic Ocean south of the equator, although 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.

Most recently was Subtropical Storm Oquira which developed on Dec. 28, 2020, according to Brazil's Navy.

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.

"Catarina landed in its namesake state in southern Brasil and had enough strength to bring loss of life," AccuWeather International Meteorologist Jim Andrews said.

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.

It also uses a different set of names for tropical systems than the National Hurricane Center.

Why are tropical systems so rare in the southern Atlantic Ocean?

Every year, around a dozen named tropical systems form in the Atlantic hurricane basin, the area of the Atlantic Ocean, Caribbean Sea and the Gulf of Mexico where the conditions come together for a tropical storm or hurricane to take shape. However, the conditions south of the equator are different than they are north of the equator.

One of the biggest factors is wind shear. When wind shear is low, it allows disturbances to organize and potentially develop into a tropical storm or hurricane. This disruptive wind is typically higher in the southern Atlantic when compared to the northern Atlantic.

"Having strong wind shear, it becomes very difficult to nearly impossible to have genesis of tropical cyclones," the National Hurricane Center explained.

Another factor is that the water temperature is typically lower in this region of the southern Atlantic, limiting the potential for tropical development.

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.