Typesetter | Jul 20, 2017 | 0
The Week’s Weather up to Friday 16 June. Five-day outlook to Wednesday 21 June
500 mB Height and Vorticity for Tuesday 27 June 2017
Source: wxmaps.org, GrADS/COLA
After several days of generally lower pressure over most of the sub-continent, the South Atlantic high pressure cell made landfall during Wednesday. This saw a minor drop in night temperatures across Namibia south of Grootfontein. A bigger drop and much colder nights were experienced in the Karasburg and Keetmanshoop districts with the cold coming from the east.
The week’s local weather was marked by several intrusions of moisture in the middle layers of the atmosphere roughly between 13,000 and 18,000 feet. Surface level humidity mirrored this activity running between 30% and 40% which is very high for Namibia during winter.
At the beginning of the week, the outer rim of the South Atlantic high was still some 200 km offshore. The core had a reading of around 1028 mB. On the eastern side of the continent, the southern Indian high pressure cell lay due south of Madagascar but at a slightly weaker 1024 mb. A cold front crossed the Cape early in the week but had little impact on Namibia other than a strong north to south airflow ahead of the front. This helped advecting moisture which was observed as the very unseasonal two days of scattered clouds.
On the surface, the local weather displayed a typical winter split with warmer condtions in the northern regions, and much colder weather in the Karasburg and Hardap regions. As the South Atlantic high slowly engulfed the sub-continent, a typical winter stance was visible on the synoptic map. By Friday, the South Altantic covered most of southern Africa but did not split off from the parent cell. The result was an extended, relatively strong high pressure cell that reached for almost 5000 km from east to west.
For the first time this winter, the so-called 540 dam line crossed the southern-most strip of the southern Cape. South of this line, the surface temperature is zero degrees or less and the extent to which it moves over land, usually indicates that very cold conditions will be experienced over the South African interior, with a spillover, a day later, to southern Botswana and southern Namibia.
The South Atlantic high morphs into the continental high over the weekend.
By Sunday evening the high covers most of South Africa and most of Botswana. Again it spills over to Namibia with the coldest conditions expected in the Karasburg district and along the Botswana border in the Omaheke region.
By Monday the continental high has split off from the South Atlantic high and lower pressure are present over the Namibian coastal plain. It will be particularly windy in the southern Namib.
By Tuesday a very strong frontal system is about 1000 km offshore. This system seems to be the strongest cold front so far this winter and it will have a marked impact over the Namibian interior. It is expected to arrive on Wednesday. The map that accompanies this week’s discussion shows that the forecast expects it to reach aloft to at least the 20,000 feet level.
The front hits the Karas region during Wednesday. A strong south-westerly wind will blow at the coast and the interior will be icy cold. It may bring the first frost to the Namibian interior but the spell is brief, only two days, due to the rapid advance of the system from west to east.
This system is a northern extension of the Western Cape’s winter rainfall and is expected to bring light rain to the Sperrgebiet as far north as Lüderitz. It may even brings some snow to Aus.