13 December 2013
At this time of year the positive weather features may be present, but do not always receive support throughout the air to bring these positives to fruition.
Last weeks’ presence of moist air across Angola and Zambia and the favourable airflow in some depth which penetrated both our airspace and further southward was also assisted by an active convergence zone which produced the rainfall of the weekend.
This upper air vortex developed but moved only slowly during Friday through to Monday ensuring consistency to the airflow towards this convergence area. This steady influence enabled convective activity to mature to form rain clouds.
Across these few days, quite widespread rains were recorded. The tropical nature of the air ensured significant measures (25mm or more). The broad spread of this airmass, both in terms of geography and duration, saw medium to heavy falls measured over the three-day period.
Historically, such a widespread occurrence is either unusual or associated with a season marked by a disparate range of weather patterns. The influence of climate change has disturbed our previously accepted order of things.
The persistent presence of anticyclonic cores along the 40oS latitudes has provided unusual synopses to evolve within our airspace. These have formed in tandem with the surface heat, low pressure area and assistance of airflows on the western side of an equally persistent anticyclonic core away to the east. There is a semblance of the ideal pattern for rains across the country, but this is still early. This bodes well for the season and we may see this presence form and even expand during the first weeks of 2014.
The favourable synoptics ensured some 60 hours of enhanced convergence inland from the southern escarpment. Falls totalling 100mm occurred variously across the Karas region: more rain than even a season may provide: the upper vortex and the tropical inflow and its convergent ability flourished accordingly!
By December standards, a wet few days have brought relief for much of the environment.
Towards the weekend, the whole pattern has moved away, only leaving moist air presence close to our northern borders. An upper-air anticyclonic core, linked to the surface core close to 40oS, ensures dry conditions to prevail for the weekend and into the new week.
Daytime heat returns to support the daily development of the heat low pressure area, so attracting moist advection to increase by the new week.
Showers should return similarly to the northeastern areas by Monday, reaching further south every day reaching Windhoek by Wednesday night, and probably Hardap and Karas by Thursday. A favourable trough is expected to form by mid-week: its range of activity holds much promise.