Weather overview and short-term outlook to Wednesday 11 November 2020
Visual: Decadal shift in position of the northern boundary of the Inter-Tropical Convergence Zone.
Source: National Oceanic and Atmospheric Administration (NOAA) of the US Government. www.cpc.ncep.noaa.gov/products/international/itf/itcz.shtml.
The Inter-Tropical Convergence Zone is in the news again, not because its southern boundary is moving through central Angola but because its northern boundary has unexpectedly shifted by some 4 degrees latitude in the ten days before the end of October.
In its most recent posting, NOAA said the Africa Inter-Tropical Front, which is the northern boundary of the Inter-Tropical Convergence Zone, was south of the mean position at the end of October by 2.6 degrees. Before that, it was about one degree north of the mean position. This is roughly 290 km which is not a lot in the bigger weather picture but which is significant because it happened in only ten days. This shift led to drier than normal conditions in the western Sahel but persistent above average rainfall in South Sudan.
In the visual, the current position of the front is indicated by the red line. The black line shows where it was expected to be (mean) and the yellow line where it was on 20 October.
All of this is happening north of the equator so the immediate relevance to weather in southern Africa is not obvious. But the ITCZ shifts seasonally from north to south as the austral summer progresses and then back again at the onset of winter. If it is retreating rapidly in the north, there is a reasonable expectation that it may also shift farther in the south, but this is not axiomatic.
The ITCZ is the broad swathe around the globe with the equator at its approximate middle. It is the zone where the northern and southern trade winds converge and the dominant airflow is from east to west. It is enormously important as a conveyor of tropical moisture around the globe, and from within the limits of the zone, both northward and southward to the adjacent land areas.
At this time of the year, the ITCZ’s southern boundary is often fragmented and not well defined. In the west of the southern African subregion, it tends to move into Angola first and then later into Zambia and Mozambique. This has been happening consistently over the past month and the relative proximity (central Angola) of the ITCZ to the Namibia Angola border has already produced two rather unseasonal wet intrusions over the past two months.
The implication is that if the ITCZ continues moving south and moving so rapidly, it may reach the Namibia Angola border before Christmas. This may corroborate other indicators like the 30-day Southern Oscillation index, which at this stage, suggests a very strong La Nina event. This will be good for the Namibian main rainfall season.
On the local front, the past two weeks again showed why the influence of the South Atlantic high pressure cell must never be underestimated. It has moved around the sub-continent from west to east in a regular 6-day pattern, bringing cooler nighttime temperatures to the southern, eastern and central regions, and have even helped dispel heatwave conditions which were again present in the north and the north-east.
The high’s migration again brought cloudy conditions to the southern Namib and the Orange River valley, this time however, without any significant precipitation.
On the Radar
For the duration of the weekend conditions are expected to remain much the same as during this week. There is no expected intrusion from the south and in the mid-level atmosphere, (500 mB or about 18,000 feet) there is mild high pressure control. This will translate to quiet, hot to very hot days, with clear skies an zero rainfall.
Visible moisture will only be present in the upper limits of the troposphere, above 25,000 feet.
During Sunday, the signature mid-level trough starts showing up which will produce a strong northerly to north-westerly airflow over the interior above the escarpment.
By Monday, the trough is much stronger and it runs from Ruacana across the country to more or less Buitepos where it enters Botswana airspace. West of this trough the skies will be mostly clear and the days very hot but east of it a convergence zone forms and much cloudiness is expected over the north-eastern quadrant.
It is uncertain how far west the convergence zone will develop as it depends on the position of the trough but there is an outside chance that conditions for rainfall will improve over the northern and central areas by about Wednesday next week.
Better prospects for precipitation are forecast only for the end of next week.