Guest Contributor | Sep 15, 2020 | 0
Weekly weather overview and short-term outlook to Wednesday 12 February 2020
Visual: Dynamical model of rainfall probability for central and southern Africa for the January, February, March season based on actual rainfall from 01 January 2020 to 05 February 2020.
Source: Climate Prediction Centre in the National Oceanic and Atmospheric Agency of the US Government.
The rather distressing visual for this week again comes from NOAA’s Climate Prediction Centre. The difference between this forecast and others is that it follows a dynamic methodology based on actual rainfall for the first month of this year.
Although seemingly, this prediction flies in the face of observed reality, we need to bear in mind that the rainfall season to date has not been spectacular. It is true that some spectacular 80 mm or more falls were recorded but these were very isolated, caused some local flash flooding, but within hours local conditions reverted to normal.
There are also some farmers in the Drimiopsis district who claim around 300 mm to date but this is not corroborated by rainfall measurements in other parts of the district, confirming the isolated incidence of whatever thunder showers have occurred.
Only the northern areas north and north-east of Etosha can be regarded as having had a normal season so far.
In short, the drought is far from over, and one can only pray that the variables incorporated in the predictive model, are based on too short a timeline to provide a reliable picture of expected rainfall in February and March.
What is very interesting to note are the similarities between Madagascar and Namibia / Angola. In both areas, the south-western quadrants are indicated as below normal, the central areas (straddling Namibia and Angola) as normal, and the north-western quadrant as above normal.
Finally, this is a probalistic model meaning that it only shows the probability, expressed in percentages, for the occurrence of one outcome against another. These predictions are typically volatile and were there to be good rains in Namibia over the next week or two, the predicted outcome will look remarkably different.
This week presented a fairly standard mid-summer weather development. Cloud cover over a very large area of the country was good while the South Atlantic high pressure cell was far offshore on Monday, Tuesday and Wednesday. It was only by Thursday that the high’s outer rim at 1016 mB approached the mainland, but locally the impact was seen only over the northern Namib.
The high’s approach did however restrict cloud development to above the escarpment but it went surprisingly far southwards, covering large areas of the Swartrand up to the confluence of the Koinkiep and the Fish rivers.
The results were typical Namibian – good but isolated showers in certain areas scattered over a large territory but less than 1 mm only five kilometres away.
On the Radar
The ever-present South Atlantic high clears the skies from the west during Saturday and Sunday. This happens at the 500 mB surface and there is no sign of the mid-level trough that usually advects moisture from Angola through Namibia into South Africa.
For the weekend, only the north-eastern quadrant is indicated as positive for rainfall. The pivot of the sub-continent’s anti-cyclonic circulation is situated over Western Zambia, and grows stronger during the weekend. This should bring lots of rain to the Caprivi, continuing into next.
As the South Atlantic high migrates around the continent, it eliminates any rainfall prospects over the weekend for Namibia’s southern, central and central-north regions.
But the vertical extension of the South Atlantic high is expected to drift further offshore as the surface cell moves south and east. This opens up some space at those levels of the atmosphere where precipitation is formed and the mid-level trough is back on Monday.
The trough stays in situ during Monday, Tuesday and Wednesday, growing in strength while the high’s impact wanes, again leading to positive rainfall expectations for most of the country above the escarpment.
There is an outside chance that the Kalahari and Karasburg districts may see some rain starting on Tuesday night continuing into Wednesday.