Understanding Weather – not predicting – 21 September 2012
The rumble of thunder across September skies offers an augury of contrasts for the now and the future. Historically disturbed weather so early in the new rainfall year presage a season disparate from the regular. In a climate zone where variability is predominant, “the regular” may seems to be closer to science fiction than the day-to-day reality. Mix all this in with changing climate patterns and the term abstruse looms larger.
We have incongruous mixes of weather patterns in the atmospheres’ three-dimensions.
This last week brought this jumble before our very eyes. Anticyclonic control placed most of Namibia beneath a drifting northerly to northwesterly air flow. This gentle advection was best seen in the middle cloud at altocumulus level. This was limited to mere hundreds of feet thickness with little rain potential.
During last weekend local instability gave rise to some cumulus cloud but the vertical extent did not exceed 2000 feet. Unseasonably, this instability turned into thunder weather with very light showers recorded over a broad swathe of the central to south eastern areas.
September matches May as being a month of limited rainfall potential. Such records presage a disjointed season. In an arid climate, perplexity is in the mental forefront. Will historic disappointment loom or is this an unexpected positive revealed by a changing climate?
Synoptic patterns indicated a col aloft at some levels while a weak inflow was identified in between. While major systems came and went away to our south, these were peripheral and the effect was minimal. After the weekend irruption, a replay appeared by mid-week. What is unclear is the source latitude(s) of this unstable advection. Is this tapping the broadened range of the tropical air-mass or is there a shallow synoptic pattern brought in moisture from the Gulf of Guinea.
Friday sees a departing pattern so the thin cloud-band disappears eastward, while a deepening upper air trough approaches the Cape. The direct relationship between surface and upper air, especially in the middle layers, is tenuous. Rain is not expected except, by Tuesday, when a weak trough forms from the north, advecting some moisture from western Zambia but its influence will remain contained over the Caprivi and Botswana.
As the one pattern departs, a weak anticyclonic core forms, strengthening somewhat above the mid-continent. Daytime cloud appears over the Kavango and Caprive but clearer skies return elsewhere. Another intensifying upper trough approaches the Cape by mid-week, but slides away although some inflow offers a return of cloud patches. Lack of deep convergence limits rain potential to light isolated showers over the north, east and central parts.