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Weather 16 January 2015

What Happened
The excessive heat of the holidays has abated somewhat creating space for the zonal airflow across the sub-continent from east to west, advecting moisture onto our doorstep. This is in no small measure assisted by the strong low pressure vortex present in the Mozambican channel, acting as a scoop that transfers moisture from the ocean up to the middle levels. From there, it is only a matter of two days to reach southern Angola and western Zambia, and from there enter Namibian airspace. The very large swath of warmer than normal water remains between South America and Africa with a noticeable disruption to both the position and the intensity of the South Atlantic high pressure cell. In the Pacific Ocean, the equatorial zone provides all the signals that the weakish El Nino that has been building for six months, may have crossed its highest point. It is not clear how quickly, or whether, conditions will return to neutral, or even enter the very first phase of La Nina. What is empirical is that Sea Surface Temperatures across most of the equatorial Pacific have receded to within half a degree of normal. In the Indian Ocean, what was only a cyclonic depression last week, has developed, as expected into a full-blown tropical cyclone. Named Bansi.

The South Atlantic high pressure cell that was stuck offshore for about a month and a half, has finally become dislodged, and started on its regular path swinging around the southern tip of the continent. This movement has brought significant changes to the local synoptic pattern compared to a week ago. As the stagnant high pressure cell gained momentum and drifted east, the core of the South Atlantic high basically collapsed followed by a fairly substantial shrinking of the outer fringes of this cell. The 1016mB line still reaches far to the north but the bulk of this cell now lies directly opposite Namibia covering a distance of some 5000 km between Africa and South America.High pressure control of the upper levels over the Atlantic is still evident out in the ocean but closer to the continent, the 500 mB level has developed to its expected altitude. This definitely signals better, or less negative prospects for enhanced moisture and condensation. Tropical Cyclone Bansi is swirling eastward at a sedate 4 knots, and is expected to collapse early next week.

Rainfall reported on 09 Jan: Eros Airport 8.8; Whk 4.8; Keetmans 2.4; Eenhana 1.6; Oshikango 1.4; Rehoboth 1.2; Ondangwa 0.6; Oshakat 0.6; Katima 0.3; Otjiwarongo 0.2; Rundu 0.2;

10 Jan: Omaruru 6.8; Whk HQ 2; Hosea Kutako 0.3; Okaukuejo 0.3;
11 Jan: Naudubib 6; Eenhana 2.9; Okaukuejo 2.8; Oshikango 2.4; Omaruru 1.7; Okahao 1.2; Otapi 1; Rehoboth 1; Keetmans 1; Whk 0.2; Gobabis 0.2;
12 Jan: Rehoboth 6.2; Otjiwarongo 5; Oshikango 4; Oshakati 2.6; Outapi 2.6; Maltahöhe 2.6; Okahao 1.4; Whk HQ 1.2; Okaukuejo 0.6; Mariental 0.4;
13 Jan: Karasburg 21.1; Oshikango 17; Outapi 12.4; Aroab 5.3; Whk 3.8; Otjiwarongo 3.2; Keetmans 2.6; Maltahöhe 1; Mariental 0.8; Okahao 0.6; Omaruru 0.3; Rehoboth 0.2; Lüderitz 0.2;
14 Jan: Okahao 31; Whk 18.5; Oshakati 16.2; Eunda 15; Naudubib 13; Gfn 11; Hosea Kutako 6; Gobabis 3.6; Oshikango 1.2; Omaruru 0.1; Karasburg 0.1;
15 Jan: Rundu 5.8; Mpacha 3.2; Omaruru 1.6; Opuwo 0.9; Gobabis 0.6; Gfn 0.2; Oshakati 0.2;

What’s Coming
Another week of intermittent rainfall features on most forecasts but again with clear skies over the southern Namib and western Karas. Unstable air at surface level up to the 500mB level (18,000 feet) is indicated over most of the north and north-east with a southward extension across the north-western and central parts. The next approaching South Atlantic high remains weak but it still clears two thirds of Namibia’s surface area over the weekend. Conditions in the Kavangos and the Zambezi, however, remain very favourable for enhanced precipitation. Early next week, the high pressure cell slips around the southern Cape, the windflow across the southern half backs from south-west to south-east, then due east by Tuesday, resuming its normal north-eastern orientation by Wednesday. This goes hand in hand with the heat-induced anti-cyclonic circulation over southern Africa meaning good rainfall prospects for the entire country resumes on Wednesday and carries on for the rest of the week.

 

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