Overview for the week and 5-day outlook to Wednesday 18 July 2018
Visual: Wind vorticity and height of the 500 mB surface on Sunday 15 July
Source: GrADS/COLA, George Mason University, http://www.wxmaps.org/fcst.php
An understanding of the interaction between the southern Indian high pressure cell and its South Atlantic counterpart helps to complete the bigger weather picture at it developed and emerged during the course of the week.
The week started with the southern Indian high due south of Madagascar and a core pressure of 1036 mB. It linked back over the continent, linking up with the continental high, which in turn was the remnant of the previous South Atlantic high as it migrated across the sub-continent from west to east.
The next proper South Atlantic high was still offshore with the core, also at 1036 mB, some 600 kilometres out to sea. There were no major cold fronts and in fact, over the Namibia the wind direction for most of the week backed from north-east to north to north-west. All in all, each day’s synoptic map showed a very regular mid-winter progression with the only exception the weak trough that curved in a wide arch across Namibia, from western Zambia in the north, along the Namibian escarpment and across the Orange River into South Africa. This trough was accompanied by a fairly expanded conversion zone on its east running through Botswana and the South African interior where it resulted in some rainfall, both over the interior and along the eastern littoral.
Night temperatures were not extreme, generally staying close to 10°C over most of the country. The weather was in a typical see-saw pattern with warmer air from the north at around a 13,000 feet elevation and colder air from the south on ground level.
This mechanism is typical Namibia and results from the daily-increasing sunshine hours. When the son is up, energy enters the system heating the surface air which encounters the mid-level airflow from the north as it rises. When the sun sets, the source of energy is removed and the colder (denser) air from the south quickly takes control.
As the week progressed, the South Atlantic high crept closer, making landfall south of Oranjemund during Thursday night with a significant impact over the Western Cape and adjacent interior but a very limited effect in Namibia. The north to south airflow ahead of the front remained strong and any cooling during the night was offset early in the mornings after sunrise.
The frontal system passes the subcontinent during Friday night so it will have an obvious impact on the Karas region, and in Namibia’s eastern areas along the Botswana border and the Gemsbok Park on Saturday evening. But the weak trough stays in place with only a slight deflection closer to the Orange River.
The high rapidly departs, driven to the east by exceptionally strong jetstreams in the alto levels. This opens up most of Namibia’s airspace for an equally rapid resumption of airflow from the north-east and the north.
In the wake of the departing high, lower pressure develop along the coastline with a weak low pressure system developing offshore Walvis Bay on Sunday. This is the irregular large patch of blue on the visual. In this area, the cloud base will be considerably lower and it will be windy although less so on the surface than at the 500mB level.
The area immediately east of this system, shown on the map as a collage of blue and mauve patches, is where the activity will be. The cloudbase over the southern half should sink to about 9000 feet creating some room for convection meaning that rain is a strong possibility but any showers will be light, brief and scattered wide apart.
The instability continues on Monday and Tuesday but the bias remains in the south, and on Tuesday in the Karasburg district. Despite the cooler winter conditions, frost is unlikely.
Over the northern half, it will be warm with quiet days and much cloudiness over Bwabwata and Zambezi but no rainfall is forecast in these areas.