The Week’s Weather up to Friday 22 September. Five-day outlook to Wednesday 27 September 2017
Map: Accummulated precipitation forecast for the nine days from Friday 22 September to Saturday 30 September. Source. GrADS/COLA, George Mason University.
Except for the southern-most strip of the Karas Region along the Orange River, the whole country continued to be under high pressure control in the upper air. The clouds that appeared from Lüderitz to Ariamsvlei on Thursday and Friday formed the northern limit of a very wide convergence zone running from the Western Cape to the Orange River Valley.
For the duration of the week, the weather followed a regular transition pattern except for the persistent and widespread ridging in the upper air.
The southern Indian high pressure cell is back in full strength with a very strong core that developed to 1036 mB towards the end of the week. Similarly, the northern Indian low pressure system whose core shifted south of the equator a fortnight ago, has now moved even further south. Where this low pressure system meets the southern Indian high, a powerful conveyor belt is formed advecting moisture from the Indian Ocean westwards to the African continent.
However, as can be seen from the map for this week, the Indian Ocean transport is blocked by the fragmented atmosphere over east Africa and it will be a few more weeks before substantial moisture starts penetrating central Africa.
In the Pacific, although the official ENSO stance is neatral, La Nina conditions are present over large areas of the eastern Pacific with the inflowing South Pacific current now some 2°C cooler than normal. Infact, the water near the Ecuadorian coast is only 20°C, indeed a rare occurrence even during prolonged La Nina phases. The Pacific subsurface temperatures up to a depth of 300 metres are also cooler than normal, in some place by between 1°C and 2°C.
On the continent, the southern boundary of the Intertropical Convergence Zone is touching the Angola Kongo border but is widely fragmented and displaced in the east. Typically, from the time the ITCZ moves into northern Angola, it is about six weeks before any significant rain conditions develop over northern Namibia.
This week’s cold front missed the continent staying offshore south of Cape Agulhas but the surprise is the next cold front, expected to move into the Western Cape during Monday night.
The weather at this stage is marked by the continuing struggle between northern and southern systems. The nights over most of the country remain cool while the afternoons are hot to very hot. The required conditions for summer are not there yet.
Throughout the weekend and into Monday, lower pressures are present over Botswana and over Namibia except for the Orange River Valley.
From Sunday afternoon a weak trough forms from Angola/Zambia in the north across the Kavango, Otjozondjupa, Omaheke, the eastern half of Hardap and the eastern half of Karas. This will bring in the first substantial clouds for this season but it will be limited to the north-eastern quadrant and the eastern border with Botswana and South Africa.
Monday evening, the South Atlantic high pressure cell is expected to move into the Western Cape and penetrate Namibia in the Karas Region.
Because of the growing daily dose of sunshine, this intrusion is very brief lasting less than 24 hours.
By Wednesday, the airflow will revert back to east and then to north-east with very hot conditions resuming over both the northern and the southern half of Namibia.
As the map indicates, the GrADS/COLA forecast sees limited precipitation in the north-eastern quadrant.