Experts warn of more flooding as Johannesburg reels from cloudburst

The vicious thunderstorm which hit northern Johannesburg on Wednesday afternoon, causing widespread flooding, infrastructure damage and power outages, is classified officially as a cloudburst.

But predicting one of these with accuracy is almost impossible with current technology, says a weather expert.

A cloudburst

Vox Weather meteorologist Michelle du Plessis said the storm that hit Johannesburg and surrounding areas was not a supercell, but a cloudburst.

“A cloudburst is defined as 10cm (100mm) or more of rainfall within one hour, leading to rapid water accumulation.

This explains why areas such as Johannesburg, including Roodepoort, Krugersdorp, Sandton, and Kempton Park, recently experienced extreme rainfall,” she said.

The Witwatersrand escarpment played a key role in intensifying the event, as southwesterly winds forced moist air upwards, triggering a cloudburst that delivered over 100mm of rain in just an hour, Du Plessis said.

“Cloudbursts are difficult to predict because they develop quickly and affect limited areas. Unlike widespread thunderstorms, they depend on local wind patterns and topography, which can cause some storm warnings to seem exaggerated or, in other cases, underestimate severe conditions.”

‘Unpredictable storm’

Du Plessis said a cloudburst on a small scale was impossible to predict.

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“Models are not that advanced yet. So, yes, we expected scattered storms in Mpumalanga and Limpopo yesterday, which were large-scale and the models showed, But this type of small-scale severe storms is unpredictable.”

A weather warning has been issued for today because, said Du Plessis, “we know the ground is saturated and rivers flood, so more rain today could lead to further flooding”.

Du Plessis couldn’t comment on the weather warnings issued by the SA Weather Service, but said weather warnings were not usually issued a week in advance.

“The weather changes so quickly from day to day. These types of storms are unpredictable and localised. Yes, Johannesburg washed away, but the weather just rumbled in Pretoria.”

A similar cloudburst is thought to have hit Pretoria last weekend, when rugby at Loftus Versfeld stadium had to be delayed because the torrential rain left the pitch looking more like a swimming pool.

At the same time, there was little rain over Johannesburg, just 60km away.

Highveld thunderstorms

SA Weather Service chief forecaster of disaster risk reduction Kevin Rae said the highveld was one of the most thunderstorm-prone areas in the world, particularly in summer.

“Any thunderstorm has the potential to become severe and can sometimes be accompanied by one or more extremely strong, damaging surface winds, large hail, an abundance of small hail and even tornadoes.

“Any of the aforementioned phenomena would classify the storm as being severe,” he said.

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Generally, on the highveld, large hail and strong damaging winds are the primary phenomena to be concerned about, Rae said.

“Large hail is particularly prevalent in October and November in Gauteng. However, historically, large damaging hail can occur at any time of the year, including in winter.”

Supercells and tornadoes

Historically, it was thought supercell storms were relatively rare in SA.

However, recent research by authors such as Liesker (2021) has proved conclusively that supercell storms on the highveld – and, by extension, Gauteng – are far more common than previously thought, Rae said.

Tornadoes were relatively uncommon in South Africa.

Annually, the country only experienced 1% or less of the events experienced annually in the Midwest of the United States.

“Strong or violent tornadoes – EF3 to EF5 on the Enhanced Fujita scale – as well as extremely large hail (golf ball-sized or larger) are almost exclusively associated with a special class of long-lived thunderstorm, called a supercell thunderstorm.”

At the heart of a supercell storm is a unique rotating updraft, termed a mesocyclone, which significantly assists such storms to be particularly long-lived.

It increased the capacity of such a storm to inflict damage during its lifetime, which can be a matter of hours.

ALSO READ: WATCH: Flood damage assessed after brutal storm

“Research by Liesker (2021) suggested supercells occur most frequently on the highveld in October and November,” Rae said.

It was important to note that any thunderstorm – whether it is a supercell or not – has a significant capacity to inflict some sort of weather-related damage, he added.

“Consequently, the public, as well as aviators, should always exercise due care and caution when thunderstorms are nearby.”

La Nina

Weather scientists predicted late last year that a La Nina phenomenon would be present – although in a possibly weakened form – in the first quarter of 2025.

In southern Africa, this weather phenomenon normally results in increased rainfall, as opposed to droughts as a result of the opposite El Nino patterns.

KwaZulu-Natal and other parts of the country have experienced heavy rain in the past few weeks, confirming the accuracy of the long-term forecasts.

Yesterday morning, the Vaal Dam was just under 99% full.

More inflow today and over the weekend could push it over the 100% mark, raising the possibility that some of its flood gates may be opened.

ALSO READ: Residents living downstream from dam told to evacuate amid flood gate test

How a cloudburst forms

• Moist air encounters a hill or mountain – When warm, humid air moves towards elevated terrain, it is forced to rise.
• Air cools and condenses – as the air moves higher, it cools down quickly, forming clouds.
• Continuous upward winds prevent raindrops from falling – strong upward air currents keep condensation suspended in the cloud.
• Moisture builds up with no escape – the lack of outward flow means the condensed water stays in the cloud.
• Sudden heavy rainfall – when the updraft weakens, all the stored moisture falls at once as an intense downpour.
• Flash floods may occur – if the rain falls over steep slopes, it can quickly lead to dangerous flooding. – Vox Weather

What is a ‘supercell’?

• A supercell is a thunderstorm characterised by the presence of a mesocyclone, a deep, persistently rotating updraft.
• Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (supercell, squall line, multi-cell, and single-cell), supercells are the overall least common and have the potential to be the most severe.
• Supercells are often isolated from other thunderstorms and can dominate the local weather up to 32km away.
• They tend to last 2-4 hours.
  – Wikipedia

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