Coen Welsh | Nov 14, 2017 | 0
Understanding Weather – not predicting – 09 August 2013
The week saw the departing anticyclone ensuring an easterly flow for most of the country, but a minor cold front developed to the far southwest, providing a brief incursion into the south which, because of the rapid movement of both front and a succeeding ridge, swept airflows within hours back to an easterly but chilly orientation. The warmth was revived quickly by sunshine and the winds backed even more. The north retained its overall warmth, the central parts had this brief intrusion, the south recorded day temperatures into the cold mid-teens, coupled with a moderate, wind-chill from the breeze.
This was only the start!
The new cold front with an extensive upper air trough, brought unstable air as its calling card. The instability saw the cold front form no less than three secondary vortex cores within a six hour spell, their centres lying from just off the south Cape coast away to the 50oS latitude. Such abrupt orientation easily taps polar air, sweeping it quickly into the sub-tropics. Such complex, multi-cored cold fronts have a history leading to snowy weather, no matter what season, into southern Africa. This extreme did not have time to make its mark this time around. Rain-bearing potential was also limited to Thursday with rain at Rosh Pinah Exxaro Mine (a valuable 13mm). Some drops were noted further east. This virile complexity saw cut-off cores form, but too far southward, moving quickly while fading and reforming within this icy but still unstable air mass.
Cold but unstable air reveals a temperatures lapse rate steeper than the 3oC normal. Undercutting and overriding of airmasses and their intertwining flows will ensure vortex development identifiable for each successive 3-hourly synoptic chart. The weather close to the various cores will be stormy, wind flow and wave development will be impressive. Namibia’s overall experience was limited to a return of cold with maxima below 15oC, to the south. A marked wind-chill was noticeable in the central interior while the north remaind mild to warm.
Nevertheless, some cloud persisted over the northeast, brief oosweer conditions occurred from mid-coast southward, as well as a reminder that August, the windy month, could provide some semblance of the historical norm, for both coast and inland.
An extreme example of a La Nina pattern, the north-south synoptic orientation, persists as the current anticyclonic presence drives past the subcontinent during the weekend as it departs eastward. But this is a distinct surface pattern which fades below the 850hPa level: this level and those above offer another steep trough arriving by Saturday, but passing by Sunday afternoon. The active weather prospects are limited to areas east and south of Namibia.
A colder inflow seems limited by both surface: moving too quickly, with upper flows maintaining a more westerly direction at all levels aloft.
After this Friday morning’s cold, milder temperature patterns are expected to prevail for the next few days.