Windhoek — What happened?

Weather activity subsided across our side of the subcontinent as an intense anticyclone ensured warmer conditions by day and a slight relaxation of the cold, overnight, grip on temperatures. The surface air around the lower level anticyclonic cores brought moisture into the Zambezi valley and cloud formations appeared over the northeast with the expectation of light showers.

The cold front procession continued unabated across the southern oceans but just as unabated was the inability to penetrate even to the Orange River. The much-needed prospect of winter rain for the South did not materialise.

On the other side of the globe, the vast southern Pacific Ocean is steadily shifting to a La Nina with the so-called Southern Oscillation, the relative difference in barometric pressure between Australia and Tahiti, strengthening. Weather researchers are in consensus that La Nina will remain until deep into 2011.

So what is happening?

For very many months now, the anticyclonic tracks have readily followed a route some 5o of latitude, or more, south of their regular track. This is seen as a response to the identifiable influence of global warming driving these cells towards colder, more amenable, surface conditions. The surface values of the core pressures readily ascend to the upper 1030hPa values and quite frequently into the 1040+ hPa range. This higher pressure range does much to ensure increased Trade Wind flow.

Recently, the synoptic charts showing these increased pressures and their outflowing winds also show an ability for vortex development in the lower latitudes just like last year, but as opposed to last year, these trough lines lie more north to south whereas last year their lie was generally west to east. The north to south flow does nothing to weaken the Trade Wind flow, in fact more likely, it tends to promote it. Hence the La Nina effect is enhanced.

Our interest in such development is that La Nina events have, and more pronoucedly in this decade, taken on a rain-prone stance. The probability of a north-south-north flow (as is there to be seen in the South Pacific) offers the opportunity for increased moist air input from the east.

What's coming?

Antartica still hosts an intense cold vortex above its western parts. Vortices and interspersing anticyclones develop and break away from this turbulent zone. While, now some 5 days away from its parent, an upper air trough approaches the Cape, its surface cold front strength is given little prospect of intervening in our weather. The arrival, by this Friday evening, can see a westerly wind blowing into the South but just as quickly, departing. The intrusion is given less than a 24hour duration. The succeeding, but weak, anticyclone moves southeast of southern Africa, intensifying in the close presence of the upper anticyclone so swirling moisture inland. Middle layer cloud development recurs above our northeast. Such an inflow also ensures daytime warmth for the interior.