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Weekly weather overview and short-term outlook to Wednesday 06 February 2019

Weekly weather overview and short-term outlook to Wednesday 06 February 2019

Visual: Southern Hemisphere surface pressure synoptic on Friday 01 February 2019

Source: GrADS/Cola, George Mason Universtiy, http://www.wxmaps.org/fcst.php – select Forecast Maps – select Southern Hemisphere

Recent Developments

Earlier in the week, the daily focus was still on the excessive afternoon heat across the entire country. By Tuesday, a shift started appearing and by Thursday it was clear that the high pressure control in the upper atmosphere has been broken, for now at least. The only exception was the southern Namib and parts of the Karas region where blistering conditions continued. But even these abated somewhat by the end of the week.

It is the first time this season that a noticeable positive shift has occurred. During the week, the South Atlantic high slowly migrated around Cape Agulhas, moving slightly further offshore everyday. By Wednesday, the leading rim has reached the bottom (southern) end of the Mozambican Channel from where it ridged back across southern Mozambique, Zimbabwe, central Zambia, the northern half of Botswana, the Okavango and into southern Angola.

However, control was not very rigid and afternoon clouds appeared sporadically in these areas (on satellite images) indicating some mid-level moisture despite the low pressure conditions in the northern half of the Mozambican Channel.

Starting on Tuesday, the signatory mid-level trough from Angola through Namibia into Botswana and South Africa, first appeared. Contrary to previous similar developments in the first half of the season, this one had more substance and produced a wide area of cloudiness from the Kunene in the north-west, across the interior and into Botswana more or less up to Unie End in the Gemsbok Park.

With the core of the South Atlantic high far removed, the trough remained in place and produced sporadic light falls on Tuesday an Wednesday.

By Thursday, the combined effect of the ridging from the east, and the strong north-westerly airflow created a broad convergence zone (almost 600 km across) from the northern regions into the north-central area, with a weak extension across Windhoek and further to the south-east.

This system produced fairly widespread rains in the central parts south of Etosha with numerous reports of good falls between Windhoek and Tsumeb, and even some extraordinary (but very isolated) falls in the area south of Gobabis.

The importance of this week’s major shift in overall conditions must not be underestimated. The fact that the South Atlantic high has shifted its core in a single week by more than a thousand kilometres, is a very encouraging sign. This type of mobility is not usually associated with a high pressure cell, let alone the king of high pressure cells, the South Atlantic high.

The visual is only a computer-generated image but it clearly shows the short-term expectation that all the high pressure cores in the southern hemisphere (1024 mB) have shifted to positions straddling the 40°S latitude. Note also that most of the South Atlantic is covered by the 1016 mB isobar indicating that at least for a couple of days, a rainfall window has opened over Namibia and will stay there for three or four days.

On the Radar

A fairly strong low pressure system moves across the southern Cape during the weekend. The South Atlantic high regains some traction, slowly building again during Saturday and Sunday but the core is weak and stays far offshore.

A thicker atmosphere sets in over the southern Namib from Monday, remaining there on Tuesday and Wednesday. This means the south-western third from Walvis Bay through the southern interior and most of the Karas Region, will again be blisteringly hot on these three days.

The north-eastern two thirds of the country see some unstable air entering from Angola but this is dispelled by a return of high pressure control and the expected formation of a strong tropical depression east of Madagascar. This depression will leach the lower and middle layers of the atmosphere, consequently redirecting the northern airflow over Namibia.

The expected result will be a much-reduced chance for rain over the interior with only the northern areas close to the Angolan border indicated for light rain.


 

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