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Understanding Weather – Not Predicting – 13 Jan 2012

What happened?
For more than just a few weeks the main anticyclonic core in the southern hemisphere has extended from the western South Atlantic eastward to cover southern Africa. This belt of high pressure is well marked throughout the troposphere from sea level up to the high atmosphere. From the standard point of view, this is the likely pattern to be found within our climate zone: the sub-tropics.
For some years though the world has experienced a change where climate patterns are concerned. These past few weeks have provided a deja-vu of the historical angle. The practical result has been a dry control of our weather pattern. At the same time, an equally active region of low pressure has taken root from just south of this anticyclonic core down to its regular southern home adjacent to Antarctica. To compound matters further, around the western edge, moist warm air from South America has been tapped and fed into the western part of the vortex area, fueling vortex development and restricting any high pressure weather pattern from invading this vortex-ridden area and pushing it along to the east. On occasion, ridges of high pressure have budded off, squeezed around the Cape and formed a weak anticyclone south of Madagascar. These weak cores have assisted in maintaining a vortex development area in the Mozambique Channel which has absorbed much of the cooler, moister flow which the continental heat will attract.
Across the rest of the southern hemisphere though, the weather patterns have persisted with the La Nina-style pattern to be found representing the recent departure from the climate patterns of old.
A change does seem to be trying to manifest itself. Upper air anticyclonic control does mean clear skies with unremitting sunshine and resultant unremitting daytime temperatures for much of the western sub-continent. The heat low is to be found daily on our weather maps. The widespread convection peppers the underside of the upper anticyclone with volatile warm to hot ascending air currents. The upper anticyclone does seem to be wobbling, while the tropical cyclone which evolved in the Mozambique Channel has weakened and been caught up in the west wind push across the mid-latitudes. Increasing cloud has formed across the northern mid-subcontinent, enabling the widespread but still isolated thunderstorms over the northern half of the country.
What’s coming?
A cold front approaches the Cape with a broad trough extending northward. Most outlooks see this development doing much to provide increasing thunderstorm activity in our airspace, even more hopefully, the new anticyclone on the far west Atlantic has an identifiable ridge extending well to the south of its core. Though as the weekend arrives, this is smoothed out while the core ridge advances eastward and around the Cape. Generally, favourable heat low conditions prevail across Namibia. The ability to advent any of the abundant moisture from mid-continent seems limited to the north and northeast most of the days. Occasional incursions are shown by the various models, but not quite at the same time; in other words, as the heat persists so there will be almost an hourly response to its local and overall effects

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