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Overview for the week and 5-day outlook to Wednesday 15 August 2018

Overview for the week and 5-day outlook to Wednesday 15 August 2018

Visual: Mean surface temperatures (top) and anomalies (bottom) for the next week.

Source: GrADS/COLA, George Mason University, www.wxmaps.org/outlooks.php

What Happened

This week again demonstrated the awesome power of the South Atlantic high pressure cell and why it is and will remain the dominant actor on the Namibian weather stage. At the beginning of the week, a 1040 mB beast of a South Atlantic high lay some 1500 km west of Cape Town. This system blew into the interior by Thursday.

Upper atmospheric activity was enhanced so the jetstreams drove the system towards the continent but with an expectation that it would slip around Cape Agulhas with little impact north of the Orange River.

Enters the second important southern African weather feature, the persistent anti-cyclonic surface and middle layer circulation that is as much an influencer of local weather, as is the South Atlantic high. In a sense, the anti-cyclonic circulation is also an aspect determined by the (former) South Atlantic high since it results from the continental high which is the remnant of the South Atlantic high once it has completed the track over land from west to east.

Both highs have discernible cores with the South Atlantic usually the strongest but not always. Much depends on the relative position of the southern Indian high and its north or south displacement. Between the two highs there is always a type of corridor marked by lower pressures but not necessarily a full-blown low pressure system. This corridor is often the conveyor area for a weak mid-level trough from southern Angola through Namibia and Botswana into South Africa.

The important aspect to observe is the distance between the cores of the two highs. If the distance is larger, there is much room for warm tropical air to be advected from the north, bringing warmer, milder conditions to the sub-continent including Namibia. If the distance is small, the interaction between the two highs is more pronounced, producing all sorts of unexpected, untimely weather phenomena.

This was such a week. The trailing (northern) rim of the continental high advects air from north to south while the leading rim of the South Atlantic high does the opposite. When these two systems are in close proximity, it generates a zone of compression but the air must still go somewhere, and this is typically up. So a strong cyclonic circulation is established as a result of the (forced) rising air creating a low pressure system that can even turn into a vortex.

Earlier in the week, this low pressure system developed a hundred kilometres or so offshore but south of Oranjemund. During the week, it moved over the sub-continent and by Thursday its pivot was located offshore the eastern Cape. Its rotation amplified the normal south to north airflow ahead of the South Atlantic high’s core, and the result was the first proper, widespread frost for Namibia’s southern, central and eastern regions.

The migrating low pressure system was most visible on 500mB maps, showing just what impact conditions at 18,000 feet can also have on the surface. In essence, Namibia had a severe cold intrusion despite the absence of a cold front at the Cape.

What’s Coming

The South Atlantic high rapidly moves across the sub-continent from west to east, at the same time reducing to a core strength of a more typical winter value of 1024 mB. This is still sufficient to cause sub-zero temperatures but it will be restricted to Namibia’s eastern half, and it will only be Friday night, possibly as far north as Grootfontein.

Saturday will be warmer but with a significant pressure differential in the north-western corner so expect windy, dusty conditions in the Kunene region and along the escarpment for the full length of the country. The closer to the escarpment, the windier it will be. By Sunday, conditions will revert to normal with a warm to hot day for the interior depending on latitude.

The next South Atlantic high makes landfall during Monday but the point of contact is far south, just north of Cape Town. Already this high is much weaker with an expected core reading of 1032 mB. The part that will penetrate Namibia, however, is not expected to go above 1020 mB.

As is typical for the South Atlantic high, it will cause cold conditions south of the Orange River, with only a limited impact in Namibia. By Tuesday and Wednesday the cold will come from the east as by then, the high will have moved into the continental position.

The top visual shows mean surface temperatures for the next 8 days indicating that Namibia will be split into three zones, southern, central and northern, and that cool conditions will prevail at night. The bottom map shows the anomalies or deviation, indicating cooler than normal conditions in the south, normal winter conditions in the central areas and warmer than usual conditions in the north, especially the Kunene Region.

Frost is not on the forecast radar for next week.


 

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