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Understanding Weather – not predicting – 19 April 2013

What happened?
This last week saw an unexpected switch from the generally dry weather patterns which prevailed these past months. Although high pressure cells remained very much in evidence, the heart of these anticyclonic cores moved rather more to our east, than their previously predominant situation to the west of Namibia. The immediate upshot was that when a promising weather pattern presented itself, the ability to tap a moist air source to our north was given a better chance of success.
That this switch coincided with the new moon also attracted comment. The moon’s ability to pull both water and air cannot be denied but the compendium of other influences show that its effect on tides is more pronounced than its effect on weather, especially so late in summer.
But the new moon did coincide with the arrival of a broad system stretching all the way from the Western Cape to the South of Namibia, and producing showers of limited intensity of less than 10mm at best.
The synoptics patterns of the passing anticyclones and vortex cores, proceeded at will. Their ability to provoke activity further north, at least over central and southern Namibia, has brought an ability for showers to return to the countryside. But the expectations, based on April’s historical records, were still left wanting.
The Pacific is still described as “neutral”, but this should not be confused with an inability for local systems to develop to their full intensity
As we see reported events elsewhere, a conflict of interests does appear likely. It presents a scene whereby the clash of the synoptics merits attention. This season has seen favourable patterns in the lowest layers being overridden by opposing air flows (often immediately) aloft, coupled with a briefly more favourable middle to upper air pattern yet a widespread invection of moist air was denied at every occurrence.
A major trough passed quickly south of the Cape, only to be succeeded even quicker by another system, while a northwesterly drift persisted aloft, as witnessed by several successive morning’s turbulent altocumulus sheets. Such clouds augur well for convective activity hence those few showers. Again, the required advection of a deep moist inflow was denied, despite the easterly flows into and across our northeast.
What’s coming?
The upper air trough with a link to the northern sector of an intense vortex (pressures below 940hPa) is present at some 65oS. This system departs eastward during the weekend. The driving force behind this major disturbance, another anticyclone along 40oS, arrives slowly as it weakens while still south of the sub-continent This offers generally mild condition for the week. Cooler days seem likely for the south until after the weekend when warm by day is the outlook into the new week. Slack wind-flows offer no major change in the overall synopsis, a return to weak anticyclonic control is expected to hold sway in the new week.

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