Understanding Weather – Not Predicting – 23 March 2012
Autumn’s equinox has gone, but the remaining vestiges of La Nina’s presence is still the dominant feature of global weather. Anticyclonic high pressure cores persist along 40oS around the hemisphere with core pressures readily reading 1035hPa or stronger, so powering Trade Wind flows toward the equator. In the opposite direction, ridges extend to 60oS pushing warm air into the Polar Front zone so exciting vortex development with equally deep low pressure cores often going as low as 940hPa.
Every polar front vortex develops a warm front and its warm sector brings warm, moist inflow swirling aloft with strong cloud and, of course, stormy weather. These systems that originate in the south pole are marked by intense northerly flows just ahead of the front, and higher pressure immediately following the front. As these cold fronts pass the Cape of Good Hope, they create enormous storms in the south Atlantic and southern oceans. We feel their impact as cool winds from the south, especially early mornings. This active picture has been present on the southern African doorstep for much of this past week.
The sequence of the ridge pushing and extending round the Cape was limited to some extent until mid-week, by which time new developing cores got into the 1020hPa ranges, so providing some inflow on the northern side into the mid-continent. But while the lower level pattern held sway the upper layers were dominated by the almost static anticyclonic cores feeding their surface counterparts.
But the high pressure area over the south Atlantic collapsed as it crossed the Cape barely maintaining a sufficient relative strength to identify a low pressure trough that was rolling down the coastline from Angola in the north.
With an area of somewhat higher pressure situated over southern Africa with its pivot somewhere over Zambia, the convection that did take place was killed in the upper air by the relative absence of moisture in the higher altitudes. The result was widespread rain by the showers were isolated and of very limited intensity.
Still the Inter-Tropical Convergence Zone maintained its position just north of the Owambo Angola border with some influx into our territory. The defined ITCZ trough line lay from Angola, across Zambia, across the ocean to Australia, thence past New Zealand, only fading to allow Trade Wind flows to be maintained across the broad Pacific, where the north-south lie of the anticyclonic core saw La Nina still in control.
This synopsis weakens in our skies as this Friday sees a brief lower level trough shape but weaken into Saturday. A deeper curve appears during Sunday into Monday. More favourable wind flows, deeper in their stretch across Namibia, set in, retaining their more northerly orientated flow to mid-week. The prospect of increased shower activity improves as the new week continues. The area of higher pressure over the sub-continent stays in place until about Tuesday whereafter it is expected to drift east. As this happens the advection of moisture on its western periphery, moves more over central Namibia and further south, bringing in air from as far north as the Congo. This dramatically improves the conditions for an extension of the rain season. However, it will only be by about Wednesday that a clearer indication of this development will be observable. There is a chance that this trough can extend all the way to the Orange River.