Tropical moisture may move south-eastward over Botswana in the coming weeks, which could cause relatively good rainfall over the eastern and southeastern parts of the country.

By Alani Janeke, Senior Journalist at African Farming and Landbouweekblad

Rainfall of up to 20mm may occur in the last week of September over the eastern and southeastern parts of the country. This will follow tropical moisture from the intertropical convergence zone over Central Africa, moving south-eastward over Botswana.

This rain is predicted from 25 September to 2 October over the Eastern Free State, all of KwaZulu-Natal, most of Mpumalanga and Gauteng, as well as the southeastern half of the Eastern Cape. Less than 10mm of rainfall is predicted for almost all the rest of the country, including parts of the winter rainfall region. Little to no rain is expected over the southwestern parts of the Northern Cape during this period. “It’s important to note that the forecast covers a fairly wide area, although rainfall totals may still be small,” says Johan van den Berg, independent agricultural meteorologist.

Impact Of La Niña

“If this is the beginning of the summer rainfall season, it could have implications for rain conditions later in the season. The expected La Niña phenomenon for the season is also going to start developing earlier, which could mean it will peak earlier in the coming summer than in the 2024-’25 season.” Van den Berg says if one considers that wetter and drier periods occur in almost every season, it could mean that a more traditional midsummer drought could occur by the middle of the 2025-’26 summer season.

“During La Niña seasons, the rain season begins over the eastern parts of the country and later expands westward. If the La Niña system is going to peak early, it’s possible that the western parts, as well as Namibia, won’t get the normal La Niña effect of good rainfall from February to April.”

“It’s still very early in the season, though, and we need to watch in the coming weeks when the rainy season will begin,” explains Van den Berg. “Although both the 2024-’25 and 2025-’26 summer seasons are La Niña-inclined, the rain pattern could differ significantly due to when in the season it develops.”

How La Niña Develops

All four Niño regions’ sea surface temperatures are slightly cooler than usual (between 0.2°C and 0.4°C cooler), which is very close to the level where they transition to La Niña levels (which is 0.5°C cooler). “Projections are that from the beginning of October, it will reach the level that transitions from neutral conditions to where it becomes a weak La Niña system.”

“Predictions show that the expected La Niña system will be relatively weak with minimum sea surface temperatures not dropping lower than 0.8°C cooler than normal,” says Van den Berg.

Also read: Initial signs of La Niña climate patterns start to emerge

Temporary Cooler Temperatures

Meanwhile, a series of weak frontal systems may move across the country from the southwest from 16 September to about 22 September. These cold fronts could cause minimum temperatures to drop sharply to near or at freezing point from 17 to 19 September over the interior of the Eastern Cape, the central, southern and eastern parts of the Free State, as well as the southern parts of the Northern Cape. “Predictions don’t indicate that temperatures in the Lower Orange River will drop to levels where frost could occur, but it’s not impossible. Further northward, minimum temperatures will be 10°C or warmer.”

Until 22 September, minimum temperatures over parts of the Western Cape could drop to below 5°C, although frost is not expected over these areas.

From 16 September to 22 September, maximum temperatures could drop sharply for periods over the southern half of the country, in places by between 4°C and 10°C. In the northern half of the country, maximum temperatures will remain very high until 24 September, when predicted rain could bring cooling. Heatwave conditions with temperatures of up to 38°C are expected to persist in the northern and northwestern parts of Limpopo until 23 September.