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Understanding Weather Not Predicting – 27 April 2012

What happened?
Weather headlines feature the northern hemisphere’s abnormal events while our southern world escapes  such headlines and their stormy facts. This is as much due to the geography as it is to any other factor.
Not for the first time can the changing climate be cited for all this drama. This change sees the out-flowing anticyclonic cores that much further south at the 40oS latitudes. Their relatively warmer outflow incites more intense vortex development around 50 to 55oS latitudes, drawing in polar air on their western sides, pushing it all the way north to reach Africa. This results in enhanced intensity.
Last week’s activity began to crowd somewhat overhead but, perhaps more explicitly, was concentrated more southward. This, too, excites the climate change watchers as it extends autumns’ rainy patterns just that much further south into areas where the expectation is that only wintry rain patterns bring     moisture. This saw the speeding-up of the developing systems, gone by Sunday but returning by midweek with clearance by Thursday.
The practical synoptics were provided by the southerly anticyclone pressing along, so tending to isolate and cut-off areas where vortex development can occur. These areas extend aloft into the middle layers, at least, creating a considerable depth of activity. It does seem that the various computer models have caught up with this type of development also, which says much for the intellects driving this broad range of work.
The two rainy patterns which crossed central and southern Namibia were part and parcel of this whole concept. The divide between the totally dry north and the showery south provided yet another aspect where lower level activity could prevail, even further away from this core. The dry upper pattern held sway blocking the advection of any moisture from the Congo.
What’s coming?
The latest vortex activity survives with its middle layer circulation clearly marked just north of the Cape, keeping our airspace at a distance but within the sector where northerly airflows predominate.
A brief colder intrusion appears over the south somewhere between Friday night and Saturday midday, but is carried away rapidly as the prevailing patterns regain a grip.
Differing from the hemispheric patterns, the South Atlantic’s anticyclone is divided into a rather weak, more northerly cell lying west to east at 25 to 30oS while quite intense activity off Patagonia enables a more powerful, low level, core to appear by mid-week. This core perpetuates an intense vortex around 45oS, spawning two troughs but too far to our south. The result is it will remain generally dry across Namibia.
From May, what of winter? There seems no due reason for the anticyclonic cores to leave their present track. North-south-north flows with some moisture too,  usually dictate quite rapid movement so, while a deep cold air flow can be advected inland, but given the high mobility of systems, long cold spells are not expected.

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