What to expect for the Brazil climate in December 2021

RIO DE JANEIRO, BRAZIL – The month of December marks the beginning of the so-called climatic summer, which comprises the quarter from December to February and the beginning of the astronomical summer, which will start at 12:59 pm on December 21.

It is an essential month for commerce, which, due to the festive season has an increase in sales, as well as for tourism, due to the end of the year trips, and agriculture, which advances with the summer harvest. December begins for the second year in a row with the La Niña weather phenomenon in action.

In this century, there have only been two consecutive December’s with the Equatorial Pacific under La Niña in 2007 and 2008, 2010 and 2011, and 2016 and 2017. Thus, rural producers who maintain rainfall histories on their properties can check what occurred in 2008, 2011, and 2017 to reference what to expect from an analogy standpoint.

The month historically marks a significant increase in the number of hot days in Rio Grande do Sul and rainfall volumes in southeastern Brazil. Note what happens in two state capitals.

For example, in Porto Alegre, the historical average maximum temperature in December is 29.5ºC, second only to the average maximum temperatures in January (30.4ºC) and February (30.0ºC). In the city of São Paulo, December rainfall averages 224.4 mm, the third-highest monthly precipitation average, only surpassed by January (288.2 mm) and February (246.2 mm).

Rain is more abundant at this time of the year in the Southeast and Midwest of Brazil because of a condition called the South Atlantic Convergence Zone. This moisture corridor comes from the Amazon and goes to the Southeast that persists for several days with frequent and voluminous rain, usually triggered by a cold front.

Rain in December November will end with below-average rainfall in almost all Southern Brazil. Most Mato Grosso do Sul and São Paulo, while many Mato Grosso, Goiás, and Minas Gerais areas will end the month with rainfall above the historical average.

In December, in turn, the picture does not change much. Most of the South of the country tends to have below-average rainfall again due to the influence of the La Niña phenomenon. However, unlike November, there will be places where the rain should be above average. Areas of Mato Grosso do Sul and São Paulo should also have precipitation below historical standards.

In the Center-East of Minas Gerais, in the North of Rio de Janeiro, and Espirito Santo, rain should be above average with localized excesses.

The map above shows the rainfall anomaly projection of the European model for December. In the case of Rio Grande do Sul, another month with below-average rain (which is different from no rain) should have repercussions on agriculture. Corn production will be stressed and will suffer from the lack of water in many municipalities.

With warmer days and below-average rainfall, the loss of soil humidity will accelerate. Rainfall in the Center-South of Brazil varies a lot from one point to another at this time of year due to the nature of the precipitation, which is convective in origin due to the heat and humidity.

At the end of the month, we can expect “islands” of above-average rainfall within a region with below-average precipitation. For this reason, isolated storms can bring very high volumes of rain in a short period in isolated points that contribute significantly to this variability of accumulations. These storms are often intense with the occurrence of windstorms.

December Temperature MetSul’s forecast is for temperatures close to or slightly above average in most of the Center-South of Brazil with the possibility of some negative anomalies in areas where the month will have more rain in the Center-West and Southeast.

The highest probability of above to much above average temperatures is in Rio Grande do Sul, particularly in the West and South of the state, because of the low number of rainy days and hot air in central Argentina and Uruguay.

With dry periods, areas of the Rio Grande do Sul state may face sequences of very hot days with heatwaves in which the temperature can get close to or above 40ºC on some days, notably in the West. Mild or pleasant days, which have been more common than usual so far this spring, tend to become rarer.

Source: MetSul