Understanding Weather – not predicting – 17 May 2013
Major disparities are prevalent across the three dimensional atmospheric range as revealed by the synoptic charts for surface levels: the ranges north to south, east to west as well as through the upper air within the troposphere.
Because there is considerable mobility throughout this system, changing patterns develop remarkably quickly only to fade or transform showing even more divergence.
For the past few weeks, the upper air anticyclonic control has maintained a semblance restricting the activities of other quite potent synoptic patterns which have appeared close to or even emanating from a developing pattern associated with this upper air control.
What we have seen is a gradual weakening of this upper control as the core has split with a remnant away to the west. A new core lies eastward and somewhat in touch with the surface anticyclonic cores which recur, pushing frontal troughs, close to sea level, barely extending to 3000 feet: 925hPa.
This weakening enabled a major trough to form and intensify up to the upper troposphere, 200hPa level and, by model prognostics, shape into a neat cut-off vortex pattern. This weeks’ weather saw this build-up take shape but during the week, the pattern wilted to become a mere trough while another promising, intense trough with cut-off tendencies formed to the west.
This range of poor cohesion does seem to be more defined across our part of the hemisphere, rather than elsewhere.
But as the week closed, tangible evidence of this pattern brought middle level altocumulus across the central and southerly parts of the country, ahead of and in conjunction with the initial trough. Although this first system shallowed, its moisture content was sufficient to provide virga patches to appear here and there. The moisture source appeared no more remote than fringe air from west of Angola. Satellite images show little of this admittedly thin cloud band. Yet Thursday’s cloud patches were, by late autumn standards, promisingly thick. As the whole flow advances further south, cooling and hence thickening, the rainfall probability increases considerably. This is an historic stance from which winter rain develops.
This second intense vortex is expected to produce more cloud during the weekend with some showers expected in and around the Kalahari. As the pattern departs, the lower level wind flows seem set to return to an easterly to northerly orientation as the pressure patterns return to the more normal situation so keeping Namibia within a warmer flow overall. The disparity between surface and mid-level patterns persists with surface cores having little match above 5000 feet, 850hPa, across the sub-continent. Temperatures will drop considerably during Sunday but by Tuesday, milder weather will return as the wind turns north east again. The wind at the coast will blow from the north.