Understanding Weather – not predicting – 28 March 2013

What happened?
These days when weather analysis is to be found at the fingertip through a keyboard, the hard-earned expertise based on observation put together in the ‘dark ages’. comes into its own should the computer models go out of sync. Directly observing changing weather patterns can be very rewarding.
The reluctant skies of the past few months inched toward more purposeful patterns as the new week unfolded. Occasional clouds developed, pushing into the unrelenting dry air aloft.
Something had to give.
Observation showed crisp cumulus heads slowly becoming more numerous and resultant showers that bit more prolific but still short of the required intensity. As the week progressed, other factors, long absent from our skies, loomed.
A return to the fingertip activity confirmed the shift away from the dominant patterns of the past umpteen weeks. The high pressure column now lay eastward and its the controlling grip through the middle layers was fading as two major vortex cores appeared over the southern mid-Atlantic. These carried two extensive troughs northward and, at last, with considerable vertical extent. The effect was to advect some moisture from the maritime source west of the continent (not a rich source really) while the core to the east could attract an inflow round its north-side toward Namibia in the lower layers.
Middle level clouds still lacked the visible turbulence expected as the indicator of more favourable patterns, but the improving synoptics were not to be denied. Thunder-heads were thrusting deep into the upper troposphere, pushing moisture into these generally drier layers, while more favourable wind patterns were displacing the dry layers. Occasional falls above the 10mm value were now being reported. Tuesday saw extension of this moist advection inland of the escarpment:spreading to the southeastern border.
Typically though, such moist air arrivals gain prominence above the northwestern parts (little over the Kavango region) as they spread across Namibia. By Tuesday, scattered outbreaks of rain occurred with occasionally better intensities.
What’s coming?
The second of the intense troughs arrives south of the Cape by Friday. Its cold front and trough quickly depart while the cold-air secondary vortex deepens, advancing east and separates to form an intense cut-off core west of our southwestern coast. Moist air advection is identified up to and above 30,000 feet as the flow increases from northwest with some input from the Zambezi valley.
Even the least optimistic outlook offers rainy prospects from the escarpment inland as the trough approaches and the cut-off core deepens. Intensities in the early weekend are expected to top the 10mm mark, more widespread than scattered in a band from the Kunene to the southern interior.
This sysem advances east and across the whole interior during the weekend, clearing all but the Caprivi by Sunday. But weather patterns are notorious for the ability to change swiftly so the next few days are to be viewed with caution, but the overall, more rainy, outlook retains its optimism: the moister input is more closely maintained. Showers should recur inland, from the escarpment eastward to the Botswana border.