Weather 29 April 2016
The typical late-summer hamburger weather pattern is back, amply demonstrated this week by the opposing surface, mid level and upper level airflows.
On the surface the impact of the South Atlantic high pressure cell was felt as night temperatures went below 15oC. Colder nights were experienced as far north as Otjiwarongo. Over the south-eastern quadrant, the nighttime airflow remained south.
In the mid level (15,000 to 30,000 feet), a weak anti-cyclonic circulation intruded from the north and the north-east. This circulation, as it encountered the high pressure system from the south, split into a western and an eastern trajectory, flowing around and over the high pressure cell on the surface bringing hot conditions to the Kunene Region, Owaboland and Etosha, the Kavango and down the Botswana Border to the Otjozondjupa Omaheke border. This created anomalous conditions visible as wind from the south at surface level, while at cloud height, the movement was from the north and the north-east.
The South Atlantic high pressure cell has regained some strength, reading 1024mB at its core, with this core only a few hundred kilometres offshore on the Oranjemund latitude. Over the Namibian interior, early morning barometric pressure typically ranged from 1020mB to 1018mB, indicative of the surface high pressure control and the approaching winter.
In the upper levels (30,000 to 45,000 feet), the strong South Atlantic high caused some impressive jetstreams flowing from south to north. Pilots on the Windhoek Cape Town leg experienced this push when they flew from south to north (Cape Town to Windhoek), and the reciprocal drag when they returned.
Immediately south of the Cape a strong low-pressure system developed. By Wednesday this has amplified into a proper vortex, with strong winds at the Cape and much moisture over the Western and the Southern Cape. For Namibia this means the mid level trough is enhanced, gaining strength as it moves south-east but it is only beyond our territory that it develops into its full strength, hence very little local precipitation.
The mid level trough is further restricted by the surface level high pressure control, and it is moved rapidly from west to east by the upper level high pressure control. So whatever rainbearing potential it holds, slips away to South Africa very quickly.
The weather is at the seasonal tipping point. Within the next week or so, it will turn to early winter conditions. The South Atlantic high which had such a big impact this week on local weather, slips past the Cape on Friday, bringing a frontal system in its wings.
By Friday evening, temperatures in the Karas region drop dramatically, maybe as low as 5oC. By Saturday evening, the whole country right up to the Angolan border is experiencing strong airflow from the south, a result of the frontal system driven by the migrating high. Daytime temperatures will recede to just above 20oC with only the coastal plain in the Kunene region remaining warm to hot. It should also lead to very windy conditions over the northern Namib.
At the end of the weekend, the high has migrated further east, and the night cold now comes in from the east.
With the passing of the high however, a low pressure system develops from the north (Angola) but the bias remains along the coastal plane. Most of southern Africa is engulfed by the high.
It is only by Tuesday next week that airflow reverts to north, north-east, with warm to hot days returning. Not a single forecast sees any rain on the horizon for the next five days.