Coen Welsh | Nov 14, 2017 | 0
Understanding Weather – Not Predicting – 02 Feb 2012
Within the range of the sub-Tropical High Pressure Belt, good weather has much to do with rainfall.
In the season when rain us expected (summer to autumn) and with an overall weather pattern leaning towards La Nina, there is much optimism for the rainfall expectations of the next three months.
January 2012, in the realm of southern Africa, was beset with unusual activity in the way of Tropical Cyclones. These vortex patterns also have an effect of disturbing upper air flows and, hence, disturbing expected weather patterns. The consequence was a January with rainfall generally much less than expected.
As the disturbing influence has waned so also has the upper air gradually returned to a more regular pattern. Our southern African geography is conducive to weather patterns and resultant airflows which are fed on the lower levels with maritime air being advected towards the heated interior and also tapping the fringes, at least, of the Congo airmass. So when an alien element is dissipated, the return to normal is on the cards. This last week has seen the departure of the old and the return of the normal. This is not only our own position, it is in keeping with weather around the hemisphere.
The Inter-Tropical Convergence Zone continues identifiably across our northern border area, away and across the Indian Ocean, Australia and far into the Pacific, yet with an anticyclonic core on its northern side feeding the Trade Winds keeping the La Nina’s hemispheric control in place and, at the same time, maintain the north-south-north flow equally active.
For us, this has culminated in the return of Tropical air in depth to much of our skies and with commensurate rain (and prospects of rain) to match. As one may have noted, in the last few days, the type of rain indicates the links to Tropical air influence: large drops (plenty of them) and a broad spread to match. The disappointing influence of Tropical Cyclone input into the upper air has departed our skies.
This departure from the previous pattern evolved in the course of Wednesday when an identifiable trough line was evident north-south across central Namibia. Moving along this convergence area, a core of lower pressure brought about a thundery area coupled with nimbo-stratus and a broad area of rains.
The prospects of, at least, a vaguely similar pattern holding sway into the new week are building rather than weakening. A succession of cold fronts and upper air troughs (two at least) move along south of the Cape but with sufficient curve to attract a flow from the northwester in the middle layers to maintain the moist advection into our airspace.
Between the frontal troughs, high pressure ridges thrust in and around the Cape and develop into fully fledged anticyclones with respectable pressure values to enable worthy lower level maritime input into the sub-continent, so supporting the reasonably moist air presence.
The second trough attracts attention. By Tuesday, a cut-off vortex forms west of the Cape: it is “locked in” by a shallow ridge. The vortex intensifies so drawing moist air southwards into its circulation while it drifts slowly closer to the Cape. The rainy outlook is generally promising, much improved from a fortnight ago.