The Namibia Economist

Understanding Weather - not predicting -7 June 2013

What happened?
A quite rapid set of mobile synoptics was to be seen just about world-wide, this was not just unique to the Namibian scene. Yet there was a level of consistency in that the mobility revolved around a pattern associated with our geography.
An anticyclonic core floating above the eastern sub-continent was the controlling feature: more-or-less consistent with normality. As such, it ensured a northerly airflow throughout the lower and middle levels. This set of synoptics links and contributes to the active patterns created by warmer airflows being guided southward, so ensuring vortex development when meeting to override the colder air that originates over the Atlantic.
This turbulent mass with cold air surging northward undercuts the warm air. This is normal, but it has its own ranges of variability which were manifest during these past few days.
A week ago,the outlook saw a well-developed trough approaching from the west, extending into our south. Behind, cold air would drive the trough eastward by midweek when a new vortex would appear. In the cold, unstable air one new trough and vortex appeared as the old front crossed our far south, but this new core weakened the first one. There was no high pressure advance, while yet another secondary trough built behind, another frontal collapse, while in the upper air west of Namibia a major trough formed, evolving into a clearly marked cut-off vortex. This wavered, its prognoses varied across the  week.
The result was that the brief winter intrusion of last Sunday was quickly expelled when the airflow reverted to north, bringing warmer air from Angola to cover the entire Namibia. Except for the brief early morning cold, the days were unseasonally warm even reaching the upper twenties north of Otjiwarongo.
What’s coming?
While the current vortex area moves away, an anticyclonic ridge (high pressure) squeezes across the southern Cape eventually building into a substantial core to the east by Monday. This keeps the airflow north throughout and the complementary west coast trough persisting. Northern Namibia remains hot by day (30oC), central parts remain warm by day but clear skies provide for maximum radiation, local inversion sees temperatures only recovering once early morning sunshine breaks this down.
Southern Namibia returns to daytime warmth. The next major trough appears (once again) clearly in the middle layers by mid-week west of the Cape.
Whether or not this is complementary remains to be seen, but the Indian Monsoon and the cooling surface waters of the eastern Equatorial Pacific, both with a favourable ring for future developments, continue their evolution. Another semi-link sees recurrent surface anticyclonic cores making their way in southerly latitudes (+-40oS) pushing both vortex cores and their troughs to the north (Cape coast) with cut-off potential. The wind at the coast will be mostly from the north.

Understanding Weather - not predicting - 24 May 2013

What happened?
The departure from one pattern to a more favourable one is now an established feature on our synoptic charts for both the surface and throughout the upper air. The reversion to more normal patterns coincided with the approach of a very promising front which was expected to develop into a cut-off vortex and stagnate for a day or two. This system collapsed but gave rise to an equally well-marked pattern which also brought about the collapse of the extensive, dominant upper air anticyclonic presence which had held sway for the past few months.
As this new vortex pattern evolved it began the advection of moist air from the fringes of the tropical air persisting west of the Congo estuary. Because this pattern persisted, this advection also continued, intensifying as its moisture depth increased from a few hundred feet to some 2 or 3 thousand feet. This persistence was rewarded as local convergence lines appeared with comparable thundery cloud patterns identified both by satellite imagery and personal observation across much of Namibia.

Read more: Understanding Weather - not predicting - 24 May 2013

Understanding Weather - not predicting - 31May 2013

What happened?
After a prolonged season of an abnormal weather pattern, over the past few weeks, that stubborn deviation has given way and reverted to a more conventional, expected pattern. During this week, the synoptic charts presented an early winter pattern that is in line with expected conditions.
These consisted of cold front extensions coming close to southern Namibia so bringing closer the prospect of winter showers to the south. These have been scarce in recent seasons.
Historically our south could rely on winter rain recurrence across the May to September period with a level of certainty better than our summer-autumn expectation which is dogged by the Arid Zone identity of variability.
Present synoptics present these frontal extensions driven by anticyclonic cells which, and here is the mix, form far to the south of their normal (about 30oS) range. This complexity sees these cells tending to slip around (south of) the vortex, pushing it somewhat north while guiding the vortex and its higher extension southward, mixing with the warmer air on its western side. North of these events that occur in the southern seas, an anticyclonic upper extension is located east of Namibia.
Airflow across Namibia retained a northerly orientation with the potential of tapping the moister fringes still present over northern Angola. This has placed the weather pattern into the position which both resembles the newer and the older so that conditions warm for this time of year prevail (newer) while pressure patterns display typical early winter movement.
Although it is too early for the current pressure patterns to give any indication of developments later in the year, at least for now, they seem to have settled back into a recognisable pattern. The weak cold front of the previous weekend collapsed as it rounded the Cape, leading to a mild, sunny week with little wind, for the entire Namibia. The relatively warm conditions were supported by a constant northerly airflow.
What’s coming?
South Atlantic weather is dominated by an intense trough progressing east and, by weekend, bringing about cloudier skies to the south. At this stage some drizzle is expected below the escarpment across Saturday into Sunday. The trough slips away southeast by Sunday evening. This trough and its almost upper cut-off core are gone by Monday, so restoring the northerly flow across the interior plateau. Such a pattern restricts easterly flows hence no “oosweer” conditions. Clear skies and overnight radiation cooling are to be expected but the chill of winter is limited.
An upper air trough appears by mid-week.
These patterns fit well with both adjacent and distant southern hemisphere patterns, while intermittent ridges do form, extending to Antarctica, strengthening cold front activity and a potential for “cut-off” vortex development: again a mix of foundation weather patterns, while ensuring winter rainfall potential to the area south of the Keetmanshoop Lüderitz line.

Understanding Weather - not predicting - 17 May 2013

What happened?
Major disparities are prevalent across the three dimensional atmospheric range as revealed by the synoptic charts for surface levels: the ranges north to south, east to west as well as through the upper air within the troposphere.
Because there is considerable mobility throughout this system, changing patterns develop remarkably quickly only to fade or transform showing even more divergence.
For the past few weeks, the upper air anticyclonic control has maintained a semblance restricting the activities of other quite potent synoptic patterns which have appeared close to or even emanating from a developing pattern associated with this upper air control.

Read more: Understanding Weather - not predicting - 17 May 2013