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Weekly overview and short-term outlook to Wednesday 01 May 2019

Weekly overview and short-term outlook to Wednesday 01 May 2019

Visual: Cloud cover forecast for Sunday 28 April.

Source: GrADS/COLA, George Mason University, http://www.wxmaps.org/fcst.php

Recent Developments

For the duration of the week, Namibian weather has been in a classical holding pattern marking the inter-seasonal transition.

The remnants of the trough that brought widespread but light rain over the Easter weekend, lingered in the Kavango region for about two days but was finally dislodged by ridging in the upper atmosphere from the east.

For the rest of the country, surface conditions earlier in the week were controlled by the northern rim of the South Atlantic high pressure cell as it migrated around the southern Cape. It was quickly followed by the next approaching South Atlantic high sitting only a few hundred kilometres offshore Saldanha Bay by Friday, but with a relatively weak core at only 1020 mB.

This gives Namibia a very distinguishable local weather pattern which is typical of the season’s change. On the surface is high pressure control (from the south-west) and in the upper levels above 25,000 feet is ridging from the east. This ridging is always a protrusion from the high pressure cell after it has crossed the southern Cape and started moving up the Mozambican Channel. It recurves over land moving across Mozambique, Zimbabwe and Botswana but in doing so, it acquires a certain buoyancy due to solar heating so it rises. It tends to “slip over” the warmer, lighter air from Zambia.

The visual for this week, although still two days out, clearly shows this dynamic. The three layers of moisture, ground, mid-level and upper level are shaded respectively in blue, green and red. Note that moisture will be present only in the middle layer over the weekend.

From an existential perspective, the high pressure control on the surface is easy to discern. It is marked by cool nights and cold early mornings. The mid-level intrusion from the north is visible from the occasional cloud bands, and the upper level ridging is felt in the afternoon when the high pressure control causes the entire air column over Namibia to sink albeit it ever so slightly. Due to the enormous size of weather systems, a downward movement of only one hundred feet already shows a rise in temperature.

On the surface this effect is amplified by the height of the atmosphere leading to the warm to hot late afternoons. It is best described as a see-saw weather pattern, influenced on a daily basis by elements from both west and east, and felt in the large swings in temperature.

On the Radar

This week’s pattern continues into the weekend with only one significant change. There is a substantial intrusion of moist air from Angola hugging the escarpment but leading to cloudiness on both the coastal plain and the interior adjacent to the escarpment (within one to two hundred km).

High pressure control continues on the surface as well as in the upper levels. The moisture intrusion is very similar to a so-called inversion layer, it is only much thicker in vertical extent, typically around 10,000 feet deep.

It stays in place for Saturday and Sunday and there is again a chance that it may link up with the trough to its south, which will bring more cloudiness than what is currently expected but it is restricted to Namibia’s western half, although in this case it is expected to traverse the entire length of the country from the Kunene to the Orange.

The mid-level lower pressure intrusion slowly morphs into a more defined trough by Monday, shifting slightly to the east, situated more or less over the centre of the country. Prospects for rain are very limited due to the high pressure control in the alto levels.

By Tuesday, the South Atlantic high makes landfall between Oranjemund and Lüdertiz bringing quiet but cold conditions to the Karas region which will last for two nights. It will also bring high level clouds to the Orange River valley however no form of precipitation is expected.


 

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