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Weather 17 June 2016

Weather 17 June 2016

What Happened
As typical a mid-winter pattern as one can get persisted for the duration of the week. Two high pressure cells lay on either side of the sub-continent, both in their customary locations. The southern Indian high was of moderate strength, at the 35° latitude and some distance south-east of Madagascar. The South Atlantic high was slightly displaced to the north, the core about 1000 km out to sea, while the effect of the outer rim was felt in the Karas region on Wednesday and Thursday, and the Omaheke region on Thursday and Friday. As the leading edge of the South Atlantic high made landfall early in the week, it brought cold, wet conditions to the Cape. This is usually a precursor for very cold conditions over the South African interior a day later, which then spreads northward to cover Botswana, and another day later, ridges back over Namibia, bringing generally colder conditions from the east covering the eastern half of the country with the most severe effect in the Karasburg district.
When the high pressure system from Botswana ridges over Namibia, it typically reaches from the Botswana border to the Ugab river valley. It happened again this week just as expected. On the ground, the experience of these conditions are characterised by very cold nights over the south-eastern quadrant with the Ugab as the boundary. The simple weather rule for winter is thus, south-east of the Ugab, cold at night, while north-west of this divide, milder nights and warm days. The Ugab river valley is also more or less the geographical limit of the lower pressure conditions that penetrate the Kunene Region from the north-west, originating from Angola. This is also a signature element of the local weather.
Between the South Atlantic high and the southern Indian high, an area of intense low pressure activity formed. In this zone, conditions are volatile and the air is very unstable. The airflow is generally from north to south, coming from Angola, then crossing Namibia and Botswana into the South African interior and on to the south-eastern Cape. These low pressure vortices are usually the pre-cursor for a frontal system which is the leading edge of the approaching South Atlantic high. On synoptic maps it is indicated as a so-called cold front, and it is the isotherm where the temperature behind it is at zero degrees Celsius or less, on the surface. The frontal system reverses the airflow, switching to a south north pattern, advecting cold polar air across the subcontinent. However,the low pressure system ahead of the cold front tend to siphon away some of the intensity of the approaching high, thus limiting the cold impact over southern Africa. The further north one goes, the less the cold intrusion. It happened exactly like that this week. Were it not for the presence of the vortex ahead of the frontal system, it would have been much colder. The absence of these strong vortices last winter (2015), was what brought the first frost already at the beginning of June.
What’s Coming
Saturday brings mild conditions across the entire Namibia with windy conditions across the Karas Regions. The wind will be north westerly. The vortex is still present south of Cape Agulhas and it contributes significantly to reduce the strength of the outer rim of the approaching South Atlantic High.
A strong frontal system lies some 1500 km out to sea, but the continent is shielded from its direct impact by the intervening high.
By Sunday low pressure conditions are present over the eastern subcontinent, creating space for the rapid advance of the South Atlantic high. The high arrives on Sunday evening when it will bring windy conditions to the coastline between Oranjemund and Lüdritz. The cold front moves rapidly across the sub-continent, from the Cape up to the central Namib. By Monday, it has spread across the South African interior and over Botswana and starts ridging back into Namibia. Monday evening and Tuesday will be very cold, bringing cold air from both the south-west and the south-east.
By Wednesday next week, Namibia is again split neatly into a western and an eastern half with stable cold conditions in the east, and windier, milder conditions in the west.
Frost is not indicated.

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