Rikus Grobler | Oct 18, 2017 | 0
The Week’s Weather up to Friday 17 March Five-day outlook to Wednesday 22 March
Precipitation and temperature forecast from Friday 17 March to Saturday 25 March
Source: wxmaps.org, GrADS/COLA
The rain season continues unabated albeit at a much lower intensity.
For the week, reports of rain came from the Rehoboth, Windhoek, Otjiwarongo, Otavi, Usakos, Ondangwa, Rundu and Bukalo districts with some impressive but localised showers north of Kamajab and throughout the Kunene region. In general, the falls measured between 25 and 55 mm with Windhoek the exception where only a series of brief showers fell late Tuesday afternoon.
The week started with the tail of the frontal system that upset Cape Town last weekend, passing around the southern Cape and moving up the Mozambican Channel. A weak high pressure cell formed over the eastern half of South Africa leading to an anti-cyclonic circulation on the surface which helped transporting moisture from the Mozambican Channel into Zimbabwe and Botswana.
This high over the sub-continent eventually linked up with the Southern Indian high pressure cell and by Friday, a very extensive area of the ocean south of Madagascar was under high pressure control.
The South Atlantic high remained more or less in place, its core straddling the 33°S latitude. In the first half of the week, the core briefly split in two but by Thursday, the core closest to the continent had dissipated into the strong trough that ran southward from Port Elizabeth. By the end of the week, the South Atlantic high has assumed its usual late-summer strength at 1020mB, the core some 1500 km west of Port Nolloth.
A well-developed low pressure system stretched from southern Angola across the Kunene river into the Kunene Region. It developed southward as the week progressed. Along the coast, conditions were variable with considerable differences in a relatively short distance. At Lüderitz and Walvis Bay, the wind was due south, becoming very strong and cold every afternoon. Only 100 km furhter north at Hentiesbaai, the wind was east causing warm afternoons, reminiscent of early Oosweer spells.
From a local perspective, the weather scene was complex. The northern Namib was under low pressure control while the Omaheke Region, adjacent to the Botswana border, had a weak high pressure influence. The difference in barometric pressure between east and west created the strong airflow from the east (from high to low) gradually shifting to north-east during the week.
By the end of the week, the synoptic chart showed the South Atlantic high pressure cell far out to sea, the southern Indian high and the high over the continent merged into a single cell and a strong divergence in the direction of the airflow – in the Mozambican Channel from south to north and over Namibia from north to south.
The outer rim of the South Atlantic high approaches the Western Cape. By Saturday the core has reached the continent and lingers for a day over the southern Cape.
Over Namibia, the weather continues along a western half / eastern half split. Low pressure conditionis persist over the coastal plain, covering both northern and southern Namib. Inland, however, the low pressure control extends only up to the escarpment. Over the interior, higher pressure is in control, again creating a fair difference in pressure between the high ground above the escarpment, i.e. the central plateau, and the coastal plain.
The Inter-Tropical Convergence Zone is still very active but it has shifted to northern Zambia and central Angola. In the west, it reaches into the Kunene Region.
The weather stance is a typical late-summer pattern. High pressure is in control over South Africa with low pressure over Angola and Zambia. This creates very fluid and unstable conditions over Namibia. According to the forecasts, it can rain anywhere across the entire country during the weekend, but the forecast reliability is low.
By Monday, most of Namibia is under high pressure control on the surface with only a very small chance of rain over the southern Namib, and near the Angola border but intensities will be low.
Conditions only change significantlyl by Tuesday when the low pressure system from Angola starts moving in over the northern half of the country. No rain is indicated for the south.
The overall rainfall bias still favours the central north, the Kunene Region and the northern half of the Erongo Region. Due to the eastern windflow dominance, the Namib will be very hot.