Weather 16 September 2016

What Happened
During the course of this week, the South Atlantic high pressure cell slowly made its way across the Atlantic while the continental interior was mostly under low pressure control. The results was a relatively quiet week with not much happening weatherwise other than the gradual transitioning to an early summer synoptic pattern.
In Namibia, general conditions were much in line with the expected seasonal stance. The southern half was warm to hot with windy conditions close to the escarpment, the northern half was hot to very hot, and the general airflow was from the north-east, backing to north and then later in the week, north-west. Not much happened on the coastal plain either – no excessive wind and no excessive fog.
Towards the end of the week, this scenario changed substantially, especially over the southern Namib and the Karas region.
Whereas the South Atlantic high was far offshore and not well-defined earlier in the week, it became fairly strong by Thursday with a clearly demarcated core reading 1032 mB, not insubstantial for spring. The low pressure conditions over the interior eventually covered most of the subcontinent with a medium strength mid-level trough running from southern Angola into the South African interior. This brought more than a bit of clouds to Namibia and there were even expectations of light precipitation in the Karasburg, Aroab, Ariamsvlei triangle.
Just offshore from Cape Town a strong cyclonic (low pressure) circulation developed but it never blossomed into a proper cut-off low. But what it did, was to act as an engine to drive the outer rim of the approaching South Atlantic high towards the north and over Namibia. By Thursday evening the core of the South Atlantic high lay some 600km offshore from Oranjemund. The activity of the low just off Cape Town however, brought the high’s into the southern Namib and from there further north to the central Namib. Very windy, wet foggy conditions spread north to beyond Swakopmund.
The first evidence of the effect of this high/low combination was felt at Aus on Thursday morning when the early dawn temperature went below 5°C.
By Friday, this system has spread almost to Hentiesbaai in the north, across the southern coastal plain, and to Aus and Rosh Pinah just above the escarpment.
Over the interior, the surface impact of the high was felt as far north as Otjiwarongo with night temperatures dropping below 10°C.
What’s Coming
The southern and south-western quadrant face a cooler weekend. While Friday night will be chilly, the effect is limited to the southern half of the country. Some moisture lingers in the Karas region and cloudiness must be expected to continue through the weekend, but the chances for rainfall is very limited.
As the high migrates to the east, it has a marked surface impact, pushing up the cloudbase to around 19,000 feet, which by Monday may lead to a typical September phenomenon – virga. This is the Latin for Old Man’s Beard and is an apt description when it rains in the alto-levels but the moisture never reaches the ground. This may be witnessed during the early part of next week.
As from Monday, a more typical summer pattern sets in with the first substantial low pressure system developing along the northern Namib, beginning in south-western Angola and spreading down the Namibian coastal plain. Meanwhile, the high shifts east leading to higher surface pressures over Botswana and the eastern half of Namibia. This will create windy conditions over the northern Namib, but it will also advect substantial moisture from the north. How deep into Namibia that will penetrate remains to be seen.