Rain and storm for Cpt

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Geoff.D said:
Ja sure windy.com shows a bit of rain but at really insignificant levels. Less than 2mm is not what I normally consider as rain. Sure it wets the roads and gardens but it does very little in terms of run off to fill dams.
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The rain on Fri 15th will be produced by a cutoff-low. These are notoriously difficult to predict in advance, hence the wildly differing scenarios shown by the various model forecasts.

Two things are very clear: Most of the rain will be offshore, and most of the rain will miss Cape Town (and the dam catchments).

This chart shows the surface isobars, with Black South Easter conditions, and 6 hourly rainfall up to 8pm on Fri 15th:

mslp_6h_totp_066[20170915_1800_crop].gif
 
The thing that I found quite interesting/worrying when I checked for rain with Windy during winter was that many cold fronts that approaches SA from the west where indicated as supplying lots of rain, but then when you fwd, the fronts seemed to change direction and pass us on the the south. As if the continent repelled the fronts. This prob has to do with the fact that we have warning of the lan masses due to global warming?
 
Spacerat said:
The thing that I found quite interesting/worrying when I checked for rain with Windy during winter was that many cold fronts that approaches SA from the west where indicated as supplying lots of rain, but then when you fwd, the fronts seemed to change direction and pass us on the the south. As if the continent repelled the fronts. This prob has to do with the fact that we have warning of the lan masses due to global warming?
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The South Atlamtic High repels the cold fronts. If its strong we get no rain, if its weak then plenty of cold fronts make landfall. Nothing to do with warm land temperatures. More to do with jet-stream winds and the https://en.wikipedia.org/wiki/Antarctic_oscillation

Edit: The Australian BOM has a nice article: http://www.bom.gov.au/climate/enso/history/ln-2010-12/SAM-what.shtml

The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere.

The changing position of the westerly wind belt influences the strength and position of cold fronts and mid-latitude storm systems, and is an important driver of rainfall variability in southern Australia.
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Does anyone know of a similar study regarding how the AAO affects the weather in SA? Or do all the experts in the field simply assume the Australian view also applies to the weather over southern Africa?

I started taking an interest in what other cause there could be to describe the rainfall variability we experience when SAWS published an article in which they attributed only 30% of rainfall variability over SA to el nino and la nina. Hence why I take note of what weather patterns are present in the southern and mid atlantic.

The current longer term forecasts by SAWS and others suggest that low pressure areas are going to extend further south than normal over the western parts of SA but not quite reaching CT. That means as I understand it better rainfall than normal for western SA leaving a rather dry "no mans land " in parts of the Karoo this spring and maybe summer.
None of this augers well for most of the Western Cape however for spring an summer and possibly even autumn next year.
 
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Spacerat said:
The thing that I found quite interesting/worrying when I checked for rain with Windy during winter was that many cold fronts that approaches SA from the west where indicated as supplying lots of rain, but then when you fwd, the fronts seemed to change direction and pass us on the the south. As if the continent repelled the fronts. This prob has to do with the fact that we have warning of the lan masses due to global warming?
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This pattern was predicted back in 2005 already. COCT even had tshirts printed up
At a water meeting . The greater worry is why such incompetence with the same people in COTC has been allowed to continue unchecked.
 
windy.com has just added pressure contours to their animations making it much easier to understand and interpret weather movements.
 
cheers to the informed inputs by a number of the regular posters, very much appreciated!
 
Predicting the weather is just a hobby for me. It is fascinating to try and see what drives the weather.

So as to the next week after this current system passes the Cape? Rather bleak prospects again till sometime late next week. The low pressure area extending down the West Coast of Africa is still there and is not changing much.
The high pressure area in the Atlantic intensifies and diminishes and very successfully pushes the fronts south of the country.

What we are seeing though is the start of the summer patterns developing. Moisture being pushed eastwards out of the Indian Ocean over Africa south of the equator and then drawn down the west into Angola, and the very far north of Namibia
 
Nice rainfall in Jonkershoek, Lourensford stretching all the way to Rooi Els. Sunshine and fine weather for me. Perfect!
ca77ce778839ecfe384073bc8c1fc4ae.jpg
 
Next chance for rain in the catchment areas of the main dams is next Monday, Tuesday.

Low rainfall but at least something.
 
Gordon_R said:
Not much rain predicted, but rather cold! Some forecasts are showing snow for the SW-Cape mountains on Tues 26th...
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Yup I notice the predictions of Snow. That is good news for water run off. I do not know if you agree, the predictions are based on the Mid/South Atlantic high ridging in behind a cold front and thus not blocking the cold front from reaching our shores?
 
its amazing how lush and green it is around voelvlei dam in riebeek/tulbach valley, water puddles and varkblomme all over. strange to think that the dam levels are so low.


more like underdeveloped water system than a "drought" it seems sometimes. did not keep up with development and now blame the weather?
 
access said:
its amazing how lush and green it is around voelvlei dam in riebeek valley, water puddles and varkblomme all over. strange to think that the dam levels are so low.


more like underdeveloped water system than a "drought" it seems sometimes. did not keep up with development and now blame the weather?
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There are many factors playing a role in the current drought. It is not about "blaming the weather" but rather to understand what causes rainfall variability, which means understanding the weather.
 
Geoff.D said:
There are many factors playing a role in the current drought. It is not about "blaming the weather" but rather to understand what causes rainfall variability, which means understanding the weather.
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yeah, I was just practicing some tinfoilhattery. so weird to see lush green all around a damn area but theres a drought. but then a bit further towards the karoo and you can see rivers and dams empty, reeds dead.

but as you say, many factors, such as planning...
 
access said:
yeah, I was just practicing some tinfoilhattery. so weird to see lush green all around a damn area but theres a drought. but then a bit further towards the karoo and you can see rivers and dams empty, reeds dead.

but as you say, many factors, such as planning...
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Planning only allows society to mitigate against the effects of an extended drought. No amount of planning will ever prevent droughts from happening. In other words a shortage of water in our taps is not a drought.
 
access said:
its amazing how lush and green it is around voelvlei dam in riebeek/tulbach valley, water puddles and varkblomme all over. strange to think that the dam levels are so low.
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We had some late season rain in August, so many areas are now superficially green. The rain did not soak into the soil, nor run off significantly into the rivers and dams. 6 weeks rain is not enough to last a whole summer.
 
Geoff.D said:
Planning only allows society to mitigate against the effects of an extended drought. No amount of planning will ever prevent droughts from happening. In other words a shortage of water in our taps is not a drought.
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planning should mitigate the reliance on precipitation and look towards other sources.

it is Africa after all, not like low rainfall is something thats creeped up on us
 
access said:
planning should mitigate the reliance on precipitation and look towards other sources.

it is Africa after all, not like low rainfall is something thats creeped up on us
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Historically, winter precipitation in the SW-Cape has been fairly reliable (as predictable as the summer South Easterly winds).

We may be part of Africa geographically, but our weather patterns are dependent on mid-lattitude extra-tropical cyclones (otherwise known as cold-fronts). Just saying...
 
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