Weather 01 July 2016
The very strong vortices south of the high pressure belt continue to modify local weather with fairly significant changes over short timeframes. The synoptic map this week started with a fairly conventional mid-winter pattern with the South Atlantic high pressure cell just offshore the Western Cape and the core of the southern Indian high south of Madagascar. But by Wednesday, a very complex picture evolved, setting the stage for very cold conditions to arrive in Namibia during Friday night.
Between the two high pressure cells, the vortices in the southern ocean created a well-developed trough. This band of lower pressure was tentative at first but by Thursday, it stretched from Angola, across the entire length of the Namib from the Kunene to the Orange and into the Northern Cape. While the nights were still cold over the Namibian interior, it prevented frost and brought pleasant, mild days to most of the country. The exception was the north-eastern quadrant from Kavango East through Babwatwa up to the Zambezi where it was unseasonally warm, and even hot, reaching almost 30°C on Wednesday and Thursday. This was the spin-off from the strong north to south circulation that formed on the western rim of the high prevalent over South Africa and Botswana.
The lower pressure lead to windy conditions along the northern Namib but without any significant Oosweer. The flow towards the south was simply too strong for the pressure differential between the Botswana border (high) and the northern Namib (low) to create a east to west airflow. The driver of this system was the vortex some 2000 km south of Cape Town, behind which follows a strong frontal system. Ahead of these frontal systems the airflow is typically from north to south, advecting warmer, moister air from central Africa into southern Africa. However, the progression from west to east of the whole system depends on the strength of the high pressure cell over South Africa, the distance away of the next approaching South Atlantic high, and the strength of the trough that develops between them.
By Friday, the approaching South Atlantic high was impressively strong with a core reading of 1036 mB. The southern Indian high has subsided and moved east of Madagascar.
Ahead of the very strong South Atlantic high, a major frontal system is pushed along, making landfall in the Western Cape during Friday bringing very cold and wet conditions.
The frontal system over the Western Cape starts impacting local weather during Friday and the night will be very cold in the Karas Region and in the Kalahari along the Botswana border.
Due to the strong anti-cyclonic circulation of the high, the airflow will be from south to north. This brings in cold antarctic air and the effect will be felt up to Grootfontein.
For the duration of the weekend, the Namibian interior will be cold. Frost is very likely along the Orange River valley and into the Karasburg district.
These conditions spread further north during Saturday taking the cold into the Omaheke Region, and even further north. Saturday night, the high’s core lies over the Drakensberg region in South Africa, creating a strong zonal flow over the subcontinent from east to west.
Of course, for Namibia it means the cold then comes from the east, possibly bringing frost to the Gobabis district. These cold conditions will spread into Otjozondjupa during Saturday night.
It is only by Sunday evening that milder conditions will return to the interior. Next week Monday, the airflow will have backed from east to north-east, and by Tuesday to due north.
This pattern splits Namibia again into a western and an eastern half with cold nights in the east, and warmer, more windy conditions over the coastal plane.