Africa Wearther

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What is causing the unusual thunderstorm activity in western parts of South Africa.

Satellite and lighting 9pm last night (Monday 12/02/2018)

Satellite and lighting 9pm last night (Monday 12/02/2018)

Pressure and vorticity at 30000ft at 2pm today. (13/02/2018)

Pressure and vorticity at 30000ft at 2pm today. (13/02/2018)

Surface temperature at 2pm today (13/02/2018)

Surface temperature at 2pm today (13/02/2018)

Total forecast rainfall accumulation today (Tuesday) and tomorrow (Wednesday).

Total forecast rainfall accumulation today (Tuesday) and tomorrow (Wednesday).

Yesterday (Monday) a short lived thunderstorm developed over Cape Town and there was also thunderstorm activity over western parts of the Northern and Western Cape with Brandvlei recording 30mm of rain. AfricaWeather is forecasting further thunderstorm activity over these areas for the next couple of days and there is the chance that the storms could affect Cape Town and the SW Cape today.

So what is causing the thunderstorms to develop so far west and how is this impacting the rest of the weather over South Africa?

For thunderstorm activity to develop the atmosphere has to contain three key ingredients, moisture, instability and a trigger. This was discussed in detail in a previous blog – What causes large damaging hail to form.

In short, a mass of air gains moisture from sitting over a warm ocean for a long period of time, this moisture or humidity is therefore far more likely to occur near a warm ocean, in South Africa’s case over eastern areas near the Indian Ocean.  Humidity is usually in short supply in western parts of South Africa due to the influence of the cold Benguela current. The second ingredient is instability, for the current thunderstorm activity this is being caused by an upper low pressure system to the SW of Cape Town and a surface low off the coast of Saldanha – this instability is usually absent over the summer months over this part of the country due to the South Atlantic high pressure dominating the weather and leading to SE winds.

The final ingredient is a trigger and this is due to the hot temperatures that are developing in western areas. Hot temperatures causes air to rise and if the air is unstable, which it is, it will continue to rise high up into the atmosphere and cause a thunderstorm to develop. Temperatures are expected to reach 35C today in Cape Town, western parts of the Northern Cape and the interior of the Western Cape with up to 40C likely in places along the West Coast.

Cape Town and the West Coast are most likely to see a thunderstorm today (Tuesday) as tomorrow temperatures are expected to cool slightly as westerly winds develop along the coast. Scattered thunderstorms are likely today over the whole of the Northern Cape and these will persist tomorrow throughout the province except in the extreme W.  The central and eastern interior of the Western Cape can expect scattered thunderstorms today and tomorrow with northern areas most likely to be affected. Total rainfall amounts of 10-15mm are expected with 30-50mm possible in isolated locations – similar to what Brandvlei received yesterday.

In other parts of the country, Gauteng northwards can expect mostly fine and settled weather until Friday with only a small chance of an isolated thunderstorm or two. From Friday and into the weekend, thunderstorm activity will become far more likely over these northern areas of the country.

Over the Free State, eastern interior of the Eastern Cape, the southern and western interior of KZN widespread rain and thunderstorm activity is expected until at least Friday, widespread falls of 20-40mm are likely with up to 80mm in places.

AfricaWeather will continue to keep you posted on this weather event via our Twitter Feed and Facebook page.

AfricaWeather, Smarter Weather Insights.

 

One Response to “What is causing the unusual thunderstorm activity in western parts of South Africa.”

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  1. - Memeburn says:

    [...] explained that a low pressure system west of Saldanha, together with water sea temperatures around the [...]


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