Understanding Weather – Not Predicting – 18 Nov 2011

 What happened?
That the world’s weather patterns are at odds with themselves seems to be a common consensus. Primarily the cause is to be found in the jet stream levels and ensuing activity. The last few days saw changes across the southern Hemisphere.
The blocking systems have either become mobile or tended to melt away. The strong anticyclonic cores have weakened considerably. Earlier readings of some 1040hPa have fallen to the mid-1020’s, but the South Atlantic core is now 1030 while developing that rather important ridge that pushes around the Cape to establish a core over the south-east of the continent. This brings a broad inflow of moisture inland.
The various anticyclonic cores have also moved further north. Yet, while they brush the Cape to some extent they still maintain a “southeaster”. There is quite obviously considerable interplay going on which can only be observed by satellite observation. The developing synoptic pattern sees the associated upper air also depart from the grip of a winter-style pattern.
This is a continuation of La Nina and its north-south-north circulation remains in evidence.
But the current La Nina is much weaker than early in the year.
It seems at this point that the strength of La Nina has little effect on the current weather. How La Nina develops closer to the main rains season, will determine its outcome.
What’s coming?
With the synopses now mobile, cloudy weather is back in the picture and showery weather not far behind. This Friday and into the weekend, the prospect of thunder showers is to be expected from the coast inland. The broad inflow of moisture over southern Africa is more prominent over the western fringe. Heavier showers are expected over Kunene and Erongo regions with the system drifting to the east and south and clearing the interior by Monday.
As the pattern develops moving the cloud-band across the country and further eastward, the northern section drifts to the east, settling above Angola and the Caprivi for the rest of the week.
Meanwhile on the Atlantic a cold front approaches but ahead of this and well-marked in the upper air a trough, bending the jet stream flow through the 300 and 200hPa levels, advances southeastward with a smaller cut-off vortex just ahead of it. This activity does much to sharpen the northerly airflow across Namibia strengthening the shower prospect.
By next week, this complex pattern lies south of the continent, being pushed along by the new anticyclone strengthening in mid-Atlantic. The associated ridge thrusts, again, around the land so forming a new, active, cell to the southeast by mid-week. The moist inflow resurges and rain is indicated for the north from Tuesday onwards.