Vicious weather: Why December has been so rainy

Widespread rainfall and extreme thunderstorms resulted in some places across the country recording their highest December rainfall within the first week of the month.

Associated professor in meteorology at the University of Pretoria, Liesl Dyson, said during the last few weeks of November and the first week of December, there was a series of cut-off low pressure systems (COLs), influencing the weather all over South Africa.

“The cut-off low-pressure systems are large low-pressure systems that develop in the upper atmosphere at around seven to 10km above sea level,” she said.

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Dyson said these lows are known to cause widespread rain on their eastern flanks and about one in five cause floods.

“COLs are not uncommon during this time of the year, however, what is noteworthy is that there have been so many of them in close succession. Even though this rainfall caused disruption, it is not unknown to happen during this period,” she said.

Dyson said the rainfall and severe storms with hail that occurred over the southwestern parts of the country were of much greater significance.

“The Western Cape province lies in a winter rainfall area and rainfall in summer is rare. The south coast has had unseasonable rainfall during the past few weeks,” she said.

Dyson said the South African Weather Service records showed several places along the south coast and adjacent interior which received their highest rainfall on record for a December during the first weekend of the month.

“It is also important to note that widespread rainfall occurred over the Eastern Cape province, which has been drought-stricken for several years. This rainfall also partially occurred in the catchment of Kouga Dam that was only 10% full at the start of December,” Dyson said.

South African Weather Service scientist Lucky Dlamini said the December average rainfall for Gauteng was 115mm calculated over the period 1960 to 2020.

Dlamini said the provincial rainfall for Gauteng for December last year was 114mm.

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In November, Johannesburg received 191mm of rainfall, while Irene recorded 130mm of rain and 115mm of rainfall was recorded in Pretoria.

Yesterday, Rand Water recorded the Vaal dam was 83,5% full, followed by Grootdraai Dam (88.2%), the Bloemhof Dam (94,9%) and Sterkfontein Dam was 100.5% full.

Director of the adventure tourism provider Adventure Standards Africa, Professor Graeme Addison, announced the rafting course on the Vaal River was cancelled due to its water levels and strong flow.

It’s likely to rise further, and there was a very high chance of a big flood coming this year, Addison added.

Even a flow of 800 cubic metres per second is risky, with 400 safer to raft on the Vaal River, an expert said.

Apart from the strong flow, fallen trees and islands now underwater posed a very serious risk to river rafters.