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Understanding Weather – not predicting – 18 May 2012

What happened?
That complexity is a part of weathers’ pattern is endemic; not always apparent but is shown on today’s charts with complex vortex areas in both mid-Atlantic and, even more so, in mid-Pacific. Both of these lie between two anticyclone cores supporting the vortex with warm overriding inflows on the east and colder air undercutting behind. The complexity extends as secondary, cold air vortices form with narrow intervening ridges supporting and maintaining these developments. Ridges as they push around, are capable of creating a cut-off low.
Recently, expected anticyclonic controls were in place but with some variation. For some days there  were lower pressures in the Atlantic, but still in control, beneath an extensive upper air feed with regular vortex activity pursuing its course unabated away to the south. While this yielded somewhat, providing our current pattern. the Pacific now shows a remarkable copy of this.
All this activity proceeds while the pundits debate the mid-Pacific: the equatorial belt in particular. That by given standards La Nina has devolved to the normal ranges is evident, its characterizations persist.
The north-south-north flow with its effects still evident world-wide and the readiness for anticyclonic cores to ride the higher latitudes (our 40oS) are most obvious. The now-weaker Pacific pressures can reflect on lesser Trade Wind strength, this would not match La Nina’s hallmark; equally perceived departures from recent equatorial rainy activity also spell out another story.
Where did this place our weather? Still among the back-seats! The upper air divide of the anticyclonic cores coupled with an evident, generally lower level, west coast trough advected a thin, but evident, moister air mass: midday clouds formed. This spells convection, the cloud levels (height above ground) and their topmost form matched the alto-cumulus range (moist advection). Being on the west side of the gentle upper flow, with a match in the surface layers, daytime temperatures kept to a warm autumnal range; night, a sunless 12 hours, becomes cooler at least.
Returning to the hemispheric patterns, this does not quite match the north-south-north effects. But with quite rapid movement of the vortex cores and their cold fronts, longtitudinal effects barely get a look in where sub-continental weather is concerned, although rainy occasions along the eastern escarpments and partly cloudy condition above northern and northeastern Namibia have indicated some input.
What’s coming?
On Thursday a cold front touched the Cape on its eastward journey, while a secondary cold front appears by Saturday deepening as it too departs ahead-of an intense, but also a cut-off, mid-Atlantic development. As it moves eastward, it weakens while it too slips past, south of the Cape, around mid-week.
Upper air control will maintain the current airflow pattern being light variable from a northerly orientation hence providing consistently warmer days and colder nights due to radiation. Frost is not expected, not even in the south, as there is a marked adiabatic effect of the dominant high pressure cell in the middle layers.
The cold fronts passing the Cape do not get the opportunity to influence the local weather pattern for more than a few hours at a time, as they are moving simply too fast.

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