Weather 20 November 2015
Two distinct meteorological features dominated the week’s synoptic progression. The first was the persistent high pressure control on the surface, and the second is the continuing, very well developed trough (low pressure band) in the mid levels between 15,000 and 30,000 feet.
The high pressure control on the surface is the direct result of the outer rim of the South Atlantic high pressure cell as it crosses the bottom half of southern Africa. It roughly divides the sub-continent into two halves, northern and southern. The divisionary line runs more or less from Walvis Bay horizontally across the sub-continent to Beira in Mozambique. North of this line temperatures are substantially higher, barometric pressures much lower, humidity is higher, and the atmosphere reaches considerably further aloft, to as high as 56,000 feet. Airflow on the surface circulates slowly in an anti-cyclonic rotation with the bias mostly north-east to due north.
South of this line, the opposite is true. Temperatures are lower, especially during the night, surface level humidity is much reduced, barometric pressures are higher, and the vertical extent of the atmosphere is restricted to about 45,000 feet, limiting the scope for enhanced convection.
This division cuts Namibia into two halves, the same with Botswana, it effects the southern part of Zimbabwe, and southern Mozambique south of the Zambezi River. In the mid levels, the synoptic picture looks somewhat different. The continental anti-cyclonic circulation covers a much larger area. In fact, about the only territories excluded are the south-western half of Namibia, the Kalahari and the Karoo region of South Africa, typically the arid areas.
At this level, humidity is enhanced with relative humidity above 70%, the airflow is from the north-west, and as the trough moves towards the south-east it builds, becoming stronger over southern Botswana and the South African interior. The apex development happens over the eastern Cape Province and the adjacent offshore area. The eastward progression of the system as a whole follows a fairly well demarcated convergence line stretching from around Mocamides in Angola, across Owamboland, through the Omaheke Region, through central Botswana and into the Northern Cape Province. East of this line, cloud formation happens daily, while west of this line, the skies remain clear.
This development and progression of the system has played itself out this week. Local temperatures across the Namibian north, the north-east, and east along the Botswana border, remained elevated. Humidity penetrated Namibian airspace over Ohangwena, Kavango West and East, and the Zambezi. The southward extension reached to around Buitepos. But local convection was suppressed and only very light, isolated precipitation resulted. The south-western quadrant was hot by day, but noticeably cooler at night. The southern Namib had its signature fresh winds from the south, up to the escarpment.
The South Atlantic high pressure cell is much stronger than a week ago. By Friday, its core lies about 2000 km offshore from Oranjemund at its usual latitude at 32o .It is fairly strong with a core pressure around 1032 mB. The southern Indian high pressure cell is weak and very far east of Madagascar. Only its outer rim at 1016 mB comes close to the island.
In between runs a prominent trough from southern Angola, across north-eastern Namibia, through Botswana into the South African interior. This trough remains the key synoptic feature throughout the weekend.
By Friday evening, the outer rim of the approaching South Atlantic high makes landfall brining colder, windy conditions to Oranjemund, Lüderitz and the southern Namib. By Saturday, the entire southern half of Namibia is under high pressure control on the surface. It will lead to cooler conditions during Saturday night and Sunday night.
By Sunday, the high has shifted to the east, allowing airflow from Angola to enter Namibia across the Kunene river and move down the coast to the Kuiseb Valley.
By Monday evening the effect of the high has passed, airflow will be from the north, with very little wind above and below the escarpment. The days will be hot to very hot for the entire country.