Understanding weather – not predicting 27 september 2013
We had an unusual set of patterns intruding on our complex climate couplings this week.
A week ago saw anticyclonic controls developing to our south and west over the ocean. This is quite a regular pattern but within hours though, the high-pressure formed a southward extension. This ridge enabling the upper vortex to slide just ahead as the core moved to the east ensuring the airflow around the ridge to advect cold, moist air from far south of the continent. This brought colder than expected air to much of the country for two days.
Warmer temperatures were restored by Monday as the system slipped away to the east and warmer air from the north returned to central Namibia. The upper air still had a role. The slowed movement saw an upper trough form by Monday and quickly shape a core cut off from the vortex core. With warmer conditions returning the prospect of moist advection in these same middle layers sharpened on the developing vortex’ eastern side. The core deepened and by Tuesday, Congo air arrived en masse bringing in sufficient moisture to put advection to work. The first showers were recorded above our cnorthern coast as well as seaward. Overnight damp frost moved up the Orange River valley with cloudiness but no recorded rain.
East of the Okavango catchment, daytime maxima went into the mid-30oC range across the northern third, all in the same time span! The first showers fell in the Namib: Gobabeb recorded 3 consecutive days of 1mm or more totalling almost 30mm. Showers then spread countrywide from occasional in the Cuvelai Delta to scattered across the country to all but the far southwest, while still advancing southeastward. September rains leave an awkward feel as one recalls past occasions heralding disappointing seasons. With global and hemispheric patterns in some cohesion the expectation for the new season are slightly more positive but only marginally so.
Average rainfall for the week by region:
Cunene – 1mm; Owambo – 2mm; Okavango – 1mm; Caprivi – nil; Damaraland – 6mm; Grootfontein district – 5mm; Otjiwarongo district – 5mm; Erongo – 40mm; Omaheke – 10mm; Hardap West – 10mm; Hardap East – 10mm; Karas West – 1mm; Karas East – 1mm; Khomas – 10mm.
This cut-off vortex identifiable throughout the Troposphere drifts southeast. By Friday, its influence wanes as it weakens to a declining trough, while a new anticyclone advances across the southern continent. It too is expected to feed colder, moister air into the eastern sub-continent in the new week Air flows return to an easterly to northerly flow from Tuesday onwards meaning warmer days.
A mid-level high-pressure core is expected by mid-week to the west with an interior low pressure area forming during the day. Outlooks vary. These are not the regular synoptics but there is consensus that north-south-north flows reappear next week with strong potential for more advection. After the weekend, warmer conditions should prevail with some cloudiness expected (at least) over both our north and northeast. The north and north-east will remain hot while the central plateau will be warm.