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Understanding Weather – not predicting – 26 April 2013

What happened?
That the weather patterns prevail to their own set of rules is a key reason why weather observations are performed so frequently in the course of a day, yet the observations do not necessarily provide the answer why these patterns choose to come and go seemingly to their own whim. Yet again these last few days provided another example of this seemingly fickle behaviour.
A week ago the weather outlook noted the major vortex moving far to the south and its broad trough extending to our southern latitudes. A strong anticyclone was pushing this vortex and its trough into and across the sub-continent. This more or less matched the given outlooks until the trough stagnated as it moved inland to the central Karoo. Here the stagnation created its own circulation and a cut-off low formed extending its inflowing circulation high into the upper atmosphere. This development sees two weather patterns form: one ahead of the core and the other behind on its western side. Ahead the deep inflow tapped Congo air bringing widespread rain to the eastern sub-continent while it also strengthened the southerly inflow west of the core. Much cooler air spread across southern Namibia as far as the central regions before flowing east toward the vortex core. Further south, the anticyclonic thrust slid around the inland system, but weakened as it entered the Indian Ocean arena. The effect in our skies saw a return of the upper air anticyclonic patterns of the past weeks with southerly airflows dominating.
As the new week arrived, a lower level anticyclonic core formed replacing the departing closed, cut-off, vortex to bring easterly to northeasterly airflows across the country. Such an airflow, not really strong enough to be called “wind”, extends across all the country and descends into and across the Namib before turning southward to assist potential vortex development over the southern ocean. The Namib experiences this flow as “oosweer”.
For the week, largely cloudless skies prevailed amid mild to warm days.
What’s coming?
Southern ocean predominance of intense vortex cores persists throughout the new week. The complex core far south of the Cape advances and over the weekend a new trough passes Gough Island, arriving south of the Cape by Monday. Neither of these systems will push their frontal troughs into the sub-continent and our air-space. Hence the northerly airflow drive is retained.
The warm by day weather persists for another week. The easterly flow can bring some cloud to the northeast, while “oosweer” prevails coast-wise.
The Pacific is still at odds with itself.
April sees no prospect of its rainy potential being realised.

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