The Week’s Weather up to Friday 28 April Five-day outlook to Wednesday 03 May
Temperature and Precipitation forecast from Friday 28 April to Saturday 06 May 2017
Source: wxmaps.org, GrADS/COLA
Persistent cloudiness was present over most of the country for the duration of the week except over the coastal plain in the Kunene region in the north, and the western districts of the Karas region in the south. Yet, very little precipitation was recorded on the ground.
No rainfall has been recorded on the official records after Sunday 23 April although it is possible that light showers occurred in Bwbwata and the Zambezi this Thursday and Friday.
In the Pacific Ocean, the brief La Nina phase is now officially over, usually a sign that rainfall in the western half of the southern African sub-continent will also subside. Similarly, the ENSO index maintained by the Australian Bureau of Meteorology has been negative for the past two weeks, another indication that La Nina is history.
Looking at conditions over the sub-continent, it is however not so simple to say what will happen during May. Although not typically a rainfall month for Namibia, especially with the onset of rainy weather in the Western Cape on Wednesday, the current conditions still hold potential for a late season surprise.
As the week progressed, the high pressure cell south of the continent continued to grow stronger. By Thursday, it had covered much of the eastern half of South Africa with the main core south of Madagascar reading an incredible 1034 mB, the strongest it has been so far this season.
However, on the western side of the sub-continent, an equally prominent low-pressure system developed ahead of a frontal system driven by the approaching South Atlantic high. By Friday, the low-pressure system has progressed to about 500km offshore from Port Nolloth, with a very strong circulation, but not a vortex yet. The barometric reading indicated a fairly low 1008 mB.
This produced a rather unique weather pattern. Offshore and onshore over the eastern half of the sub-continent, the strong high was in control while offshore the west coast, a moderately strong low, controlled local conditions. One being anti-cyclonic and the other markedly cyclonic, it created a pronounced trough that slowly crept down the Namibian coastal plane, eventually covering the entire west coast from Saldanha Bay in the south to Namibe in Angola.
Across Namibia, from west to east, the reigning airflow was north-east earlier in the week, then backing to due north by Thursday and Friday. This brought in warmer air from Angola, as could be witnessed by the slightly higher daytime temperatures during the second part of the week.
It was this strong north to south airflow that continued to advect moisture into Namibian airspace that produced the daily partially cloudy conditions. But these conditions were mostly restricted to the surface level up to an elevation of about 12,000 feet. At the 500 mB surface around 18,000 feet much so-called ridging was present. These are quiet conditions in the upper air with limited or very slow lateral movement. This stable upper air conditions restrict convection thus despite moisture being present in the lower levels, the conditions for Cumulus Nimbus cloud development, are restricted. The result was little or zero rainfall.
During the weekend, the cut-off low is situated a few hundred kilometres offshore Cape Town. It continues to enhance the north to south airflow over Namibia.
The high in the east, although somewhat weaker, remains strong at 1028 mB. This also helps to drive the airflow from Angola into Namibia.
No rainfall is expected although clouds will form daily over the interior east of the escarpment. But similar to this week, ridging in the upper air will restrict convection so not a single forecast indicates any precipitation.
By Monday, the cut-off low has spent most of its energy and moved far south of Cape Agulhas. For Monday, Tuesday and Wednesday, the entire southern Africa is under high pressure control except the north-western corner of Namibia, where another surface low pressure system will start developing.
With a high of 1016 mB over the eastern half of Namibia and 1012 mB at the coast, it portends another round of (mild) Oosweer from Wednesday but then only over the Namib north of the Kuiseb.
Current conditions during the transition from summer to winter are extremely difficult to forecast accurately more than 48 hours out and even then the models are not very robust. It is therefore quite possible that the coastal low, combined with the north to south airflow, may still produce unexpected isolated showers over the interior during next week.