Understanding Weather – not predicting – 10 Nov 2011
At an unusual time of the year we have seen, one way or the other, a departure from synoptic patterns prevalent across the southern hemisphere for many months previous to the past few weeks so giving a return to “old fashioned” winter-style patterns.
The persistent zone of cyclonic vortex development, one after the other across the southern oceans, is coupled with an equally well marked anticyclonic core hovering above the sub-continent at some 19000 feet. It is seen on upper air charts at the 500hPa level.
On recent charts, this cell comes out as being the strongest of all the high pressure cores around the entire southern hemisphere.
The current synoptic pattern is typical of late spring but there are still some winter vestiges that present itself now and then. This is witnessed by the relatively cool to cold night temperatures across the interior, and the cold and windy conditions in the south western corner.
This pattern prevailed from the central parts southward, but the northern parts lay on the fringe of more normal spring weather. In the lower level above mid-continent some moist air associated with the rainy weather predominating over eastern Zambia and Zimbabwe was drawn that bit further west. Variously across northern Namibia showery weather developed. The rainfall table gives some idea of the spread of this more seasonable weather.
The current pattern persists. The Atlantic anticyclone has enriched itself to pressures between 1035 and 1040hPa but remains in mid-ocean but just that bit closer to the 40oS latitude (between Tristan da Cunha and Gough Island) but its ability to progress further east is blocked by the complex vortex pattern present south of the African continent.
This vortex area develops a trough extending north to about the Orange River, better marked on the 700hPa charts, in the course of this Friday and moves south-eastward during the weekend, gone by Sunday. This pattern keeps a shallow moist input between 10000 and 16000 feet. There is instability and there is surface heat and daytime convection will occur. The upper ridge, although tending eastward, is still dominates those heights into which cumulus cloud should develop. Also this upper ridge leans north-westward so even in the lower levels where a northeasterly drift could occur (moister) the air above remains unfavourable.
But the northern borders lie on the moister fringe and some good development can occur.
Further into the new week a ridge across the Cape brings more southeasterly air into southern and central Namibia.
The latest La Nina bulletin remains favourable, bearing in mind that the northern hemisphere season began late. La Nina is weak but all global conditions are still in favour of a stronger Nina later in the season.