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Understanding Weather – not predicting – 11 January 2013

What happened?
The full weather stage has been brought into focus these last 3 weeks; emphasizing the clash between local individuality and the entwined complexity of a climate scene wrestling with progressive change and resistant elements of the past regime.
The final days of 2012 saw an active Tropical Convergence belt close to our northern borders coupled with an ability to tap some of this airmass and advecting into and across northern and central Namibia. This was in spite of an equally persistent upper air anticyclonic belt with a home base above the mid to lower latitude in the south Atlantic.
The appearance of an equatorial wave, the Inter Tropical Convergence Zone in its most active form, from late December through to this week, situated between 15 and 20oS, ensured a consistent moist air presence close to our northern regions. The ability to advect a broad band of this moisture was restricted by an equally persistent upper air anticyclonic core extending from our mid sub-continent westward across the Atlantic to South America, holding sway from some 16000 to 27000 feet and its core between 25 and 30oS. This high pressure presence was limited to only this quadrant of the hemisphere; across both Indian and Pacific oceans the anticyclonic patterns have changed little from the pre-Christmas stance.
The Pacific scene gave the mid-winter outlines of a weak El Nino Southern Oscillation only to fade after a matter of weeks. Such transience should have scant affect on the overall weather patterns yet for quite some time thereafter the weather offered outlines more accustomed to the zonal (west to east) alignment associated with an ENSO event.
This has limited anticyclonic activity southward and round the Cape, hence surface moisture from the east have been similarly restricted (another ENSO footprint). Nevertheless despite this, a rainfall amenable situation could not be completely discounted. Both patterns made their footprints.
Across the years’ end; frequent outbreaks of rain have been recorded across northern Namibia. Intermittent westward extension brought heavier falls to the escarpment and even to the coast. Similar advection brought tropical storms much further south. 70mm at Opuwo, 95mm at Gochas, and 30mm at Sesfontein were the result of this intrusion.
What’s coming?
There is the semblance of a westerly drift of the equatorial core, with advection to the south to be expected during the next few days. Outlook charts offer conflicting light to variable flows aloft, although the rain expectations persist for much of the northeastern areas with some southward spread.
The whole synopsis maintains a stance similar to, but rather lopsided by comparison, with the more favourable patterns we have become accustomed to in this new millenium. At this juncture, there is a match favouring the copy of the delayed pattern of this past northern sub-tropical summer season.
As the high pressure system rounds the Cape, local conditions start clearing up at least until Monday. The effect of the high pressure is to clear the western half of the country of moisture. More favourable conditions only return next Tuesday, also, at this point, marking the start of heavier precipitation from Mariental northwards.
In the East, conditions remain positive with rainfall indicated all the way down to the Gemsbok Park for most of the week.

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