3 September 2008
Cape Town — It was a clash of the titans, with the Western Cape caught in the middle, the unwilling victim of the fallout.
When the fierce storms thundering in from the deep southern ocean struck Cape Town at the weekend, it was pretty much business-as-usual for the vast systems that control the weather over this part of the world.
What was somewhat unusual was the energy it developed as it approached, and the effect it had on the seas lapping at the coastline of the sub-continent.
The titans involved were the usual suspects - the south Atlantic high pressure System and its low pressure opponent from even further south.
And weather watchers who know the business will point out that the stronger the two cells are, and the closer they are together, the greater the effect their clashes will have on anything within reach.
"Both the high pressure cell and the low pressure cell involved in this storm were particularly strong, and when they moved closer together, the wind gradient became very steep," said veteran forecaster Niek Koegelenberg of the South African Weather Services office at Cape Town International Airport.
"The low pressure cell moved in from the south-west and the high from the north-west as the system developed, giving rise to high winds."
But it was not only the wind from the storm that caused so much destruction. Huge swells that rolled in over an area of ocean about twice the size of the country slammed into the coast, damaging beachfront properties and even the railway line between Muizenberg and Simon's Town.
Koegelenberg and independent marine weather forecaster Jean-Pierre Arabonis of Ocean Satellite Imaging Systems both pointed out that the so-called fetch, that area of sea behind the system over which the wind had an effect, had become very large, which gave it a lot of opportunity to work up large swells on the ocean surface.
The size of the swells were off the scale.
On synoptic charts in which swell height is indicated by different colours of shading, the largest area of the fetch had to be left in white, because red indicates the highest normal average of nine metres.
"We just did not bother shading it in, because red usually indicates the area of highest average swells of nine metres," Arabonis said.
Koegelenberg agreed that the average swell heights recorded during the storm were unusual. His equally experienced colleague, Rian Smit, said he had never seen the average that high, at 10:5m.
At the same time as the wind beat down on the fetch, a strong cold front developed as warm, moist air from the north clashed with the leading edge of a cooler and drier mass of air from the south, and began cooling rapidly to form the heavy, rain, hail and snow-laden cumulonimbus clouds that crept ominously in over the sea and the city.
Arabonis explained that the system moved relatively slowly, despite the power of the forces at work within it.
This caused those forces to have a greater effect on the immediate environment, allowing it more time to cause havoc in one spot.
Evidence of this was how the sea developed a thick foam, with the wind and movement of the water acting much like a beater, beating cream into a lather.
The slow movement of the system also had to do with the blocking action of another high pressure cell off the eastern coast of the country, over the Indian Ocean.
Eventually, that cell would force the storm to turn away from the land mass and veer off to the southeast, Koegelenberg said.
The wind not only flows from the high to the low, but also circles anti-clockwise around the high, and then clockwise around the low, to determine the differing wind directions over the affected area.
Surf forecaster Steve "Spike" Pike said the fact that the fetch was so vast would allow the swells generated by further weather systems, such as the one that arrived last night and those expected tomorrow and on Sunday, to keep up the momentum of the swell.
As a result, the seas around the country would remain extremely rough.
"For surfers, conditions are best when the wind drops, but the swell remains large," he said.
"The guys are going to have plenty of time to surf this week."
Pike pointed out that it had become an annual occurrence for Cape Town to be struck by unusually powerful systems at this time of the year.
"All the really bad storms we have had in recent years have been between the end of August and the beginning of September," he said.
Arabonis said this week's systems would have been considered quite strong, had they not been preceded by the monster of the past weekend.
"It seems rather silly to have to issue warnings now, after what we have had," he laughed.
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