Overview for the week and 5-day outlook to Wednesday 24 January 2018
Visual: Forecast map of the 500 mB surface on Sunday 21 January
Source: GrADS/Cola – George Mason University
The most notable and obvious local feature of this week was the persistent mid-level trough from the Kunene through Namibia, into southern Botswana and further into the South African interior.
The rainfall results were however very poor with only unconfirmed records of substantial falls in the Bushmanland area. Over the interior there were several reports of very light rain from Tsumeb further south over large areas of the interior above the escarpment but these all failed to produce 1mm.
In the bigger picture, the second most important feature was the gradual southward migration of the moderate tropical storm Bergguitta which hit Mauritius on Wednesday and Thursday. Although not officially classified as a cyclone, it has all the markings of a cyclone. Unfortunately for Namibia, it was on the wrong side of Madagascar, and by the end of the week, its only local impact was to reduced the strength of the high pressure cell south of Madagascar.
The southern boundary of the so-called Inter-tropical Convergence Zone (ITCZ), remained more or less on the Namibia Angola border. It was from this source of moisture that the mid-level trough was fed, and which produced the daily cloudiness over the interior.
The way the clouds developed this week, shows the negative influence of high pressure control in the upper air. Every day the cloud base was between 12,000 and 14,000 feet with limited convection. This means the clouds did not develop into proper cumulonimbus (CB) clouds. This is because of suppressed convection in the atmosphere above 22,000 feet. This illustrated just how important the relative strength and position of the South Atlantic high pressure cell is for Namibian rainfall.
The paradox of our local weather is that the same system that dispels the intrusion of moisture from the north, the South Atlantic high, becomes the driver of the so-called Indian Ocean transport, once the core has slipped past the eastern Cape. At the beginning of this week, the South Atlantic high lay a few hundred kilometres offshore Lüderitz. This is an anomalous position for this time of the season. As long as the South Atlantic high extends so far north, Namibia will not get substantial rain.
During the week, the high followed its customary trajectory around the southern Cape and into the southern opening of the Mozambican Channel from where it was supposed to drive the anti-cyclonic airflow over the southern African subcontinent. This did not happen due to the strength of Bergguitta on the other side of Madagascar. The further south Bergguitta moved, the more it weakened the high south of Madagascar. This in turn reduced the strength of the conveyor system that brings moisture from the Indian Ocean across central Africa into Angola. It was amply witnessed by humidity levels in Namibia which obstinately refused to go above 45%, even at the cloud base level.
The result was that only limited moisture entered Namibian airspace and only along the weak mid-level trough which covered only the convergence zone from the Kunene to Ariamsvlei.
Another feature which has been present for the better part of three months, is the persistent high pressure control in the upper air originating from the continental high over eastern and northern South Africa. Called ridging, it is a very strong weather factor and its influence stretches over thousands of kilometres. So, despite the intrusion of some moisture from Angola, upper air ridging prevented the clouds from forming tops up to 45,000 feet, suppressing convection and consequently, inhibiting precipitation.
Although local conditions are still not conducive for good rains, the mid-level trough stays over Namibia for the weekend but it slowly migrates to the east, taking light showers to the Kavango, Bwabwata and the Caprivi section of the Zambezi region.
The map above shows expected conditions for Sunday at the 500 mB surface which is around 18,000 feet aloft. The extensive pink areas show that ridging will still be a dominant feature over the next five days, but there is a strong cut-off low forming just offshore from Cape Town.
This leads to some pockets of unstable air, shown by the blue spots which indicate columns of rising air or convection.
The cut-off low draws in much moisture from Botswana but its effect will be limited to the Western Cape and the Northern Cape.
The map also shows a fragmented blue line from the north of Madagascar across the Mozambican Channel into Mozambique, Zambia and Angola. This is the mid-level signature of the ITCZ and it has become progressively more defined over the past two weeks. This is a good sign.
Namibian rainfall prospects for the interior on Monday, Tuesday and Wednesday remain restricted while the Western Cape seems to have a good chance of getting a summer shower or two.
In Namibia, only the north-eastern quadrant has a more than 50% chance of rain up to Wednesday.