Weekly weather overview and short-term outlook to Wednesday 21 August 2019
Visual: 30-day Moving Southern Oscillation Index
Source: Australian Bureau of Meteorology, www.bom.gov.au/climate/enso/monitoring/soi30.png
Very little action over the interior for the duration of the week. In the south, nighttime temperatures showed the impact of the South Atlantic High pressure cell as it crossed the Western, central and southern cape, while a cooler early morning presented on Wednesday along the South African border south of the Gemsbok Park.
Along the coast, fairly strong Oosweer reigned in places north of Hentiesbaai with moderate fog at Swakopmund. Along the northern Namib coastline, conditions were very windy while only 50 km inland, daytime temperatures were very hot. On Thursday and Friday, temperatures exceeded 30°C. These hot conditions were present across the entire northern border with Angola.
Compared to a week ago, almost nothing has changed. Overall, the entire sub-continent is under alto level high pressure control. The only exception was that the epicentre of the upper level high has shifted from southern Mozambique to the area between Buitepos and Gantzi.
By the end of this week, a relatively normal mid-winter synoptic stance was in place with the standard features all where they are expected. The continental high has split off from the South Atlantic high which subsequently pulled back over the ocean but remained more or less in the same place as last week with the core about 1000 km west of Oranjemund.
The continental high brought cold conditions to South Africa, Zimbabwe and Botswana but the local impact was minimal. The high’s northern rim drove the easterly to north-easterly airflow which contributed to the exceptionally high temperatures (for winter) in Namibia’s northern regions and the hot, windy conditions over the northern Namib.
The visual for this week is an old favourite, the 30-day moving Southern Oscillation Index compiled by the Australian Bureau of Meteorology. While there is not a proven, scientifically researched link between the index and the weather in southern Africa, there is a definite observed correlation.
When the index is negative, El Nino is present in the equatorial Pacific and southern Africa generally experiences drier conditions. This is shown in the purple ellipse. When the index moves rapidly down, as shown in the dark grey ellipse, rainfall conditions commensurately deteriorate over the western half of southern Africa. Namibia’s current drought attests to the validity of this observation. However, when the index rapidly rises as in the blue ellipse, rainfall expectations for Namibia improve roughly ten to fourteen days later.
This does not mean that it will rain in two weeks from now, only that overall conditions will be more conducive for rain. It does not mean that it will rain, only that the probability has shifted.
On the Radar
The weekend sees a continuation of Friday’s conditions with very quiet, warm to hot days over the interior. A weak trough is present over the northern Namib by Sunday afternoon but the pressure differential is too slight to produce windy conditions.
Next week Monday, Tuesday and Wednesday are almost an exact repeat of this week’s first three days. The trough along the coastline migrates to the south and by Tuesday covers the entire coastline from the Kunene to the Orange River indicating that for at least two days, it will be windy and warm along the coastline.
On Tuesday and Wednesday it will be particularly windy along the escarpment, the windier the closer to the escarpment one gets. Wednesday morning should again be cool to cold in the Omaheke region.
A final remark is based on a very perfunctory observation. For the duration of this winter, there has not been a single swallow or swift in the skies above Windhoek despite a relatively mild winter. This was also confirmed by other observers in different parts of the country south of Etosha / Grootfontein. Last winter (2018), they never left although last year’s winter was noticeably colder than this year so far.