Understanding Weather – not predicting – 13 Septmber 2012
Anticyclonic control is to be expected within our airspace: generally clear, sunny skies are the hallmark and this persisted for yet another week. Although a procession of troughs and associated fronts on the surface and in the upper air, marched from their mid-Atlantic source eastward, once they reached land, they were deflected around the Cape hence their impact on land was limited. Colder mornings were the only local sign of these fronts passing the Cape.
Not quite unusual, but its consistent recurrence has dominated a range of past seasons. There has been brief wobbling but the pattern settles within hours rather than days. On the other side, our northeast, an exposure to moist advection from the east, gave an early indication of approaching Spring. For the time of year, this resembles the expected ranges.
During the winter there was a notable decline regarding winter rain. The west to east stance has given way to a northwest to southeast range above southern Namibia.
A further aspect has seen the ability for shallow anticyclonic cores to form and then ridge their way around these mid-Atlantic vortex patterns. Shallow they may well be and the various charts show limited vertical extent, but the pressure regularly exceeds 1030hPa.
The persistence of this level of activity augurs well for the forthcoming season as its natural extension sees the integration of the mobile core with that of the more or less stationary intense Indian Ocean anticyclone holding sway between some 25 and 35o South beyond Madagascar. This geographic positioning serves two purposes: maintaining practical support to the active but declining Indian Monsoon while also boosting the Trade Wind advection toward the low pressure area that develops daily over land due to surface heat.
All this smacks of the more regular, historic synoptic record. With this prevalence, limited variation compared with the previous few days was registered. Passing cold for the distant south, occasional cooling northward with mild to warm days further north, bordering on hot, with maxima daily above 30oC for the northern regions.
With anticyclonic control showing scant ability to weaken, the wind-flow is given a northerly orientation in the lower to middle levels throughout this next week. Occasional increase in both wind-flow strength and depth are inviting some cloud to appear. There is a slight chance of light rain in a belt more or less from Windhoek to the Kalahari but potential precipitation is not indicated above 2mm. This is expected for Tuesday and Wednesday. The moisture source seems limited, but some advection from the 15 to 20oS latitudes coupled with a limited convergence could see cloud condense into rain.
Cold air intrusion into the south remains transient with limited north or even eastward thrust: typical for this time of year.