Understanding Weather – Not Predicting 30 March 2012
Officially, La Nina 2012 is dead, but its soul keeps marching on.
The latest update notes the return to the non-anomalous temperature ranges so placing the event into history, but at the same time, the Australian Bureau of Meteorology suggests that the evolving situation should be treated with a degree of caution.
The overall weather pattern persists in the pro-La Nina-type pattern. Warm water zones remain, Trade Wind flows are supported by strong anticyclonic cores which keep the north-south-north airflow in the cols between these high pressure cores, active.
While this synopsis is particularly visible across the Pacific arena, it has plenty of support across both the Indian and Atlantic Oceans. There are quirks defining local variation, but the overall has been and is little disturbed.
A week ago,. the prospect of an improving pattern for rainfall prospects loomed.
This prospect set the scene for typical La Nina-style synoptics to form. With anticyclonic cores ready to push east along higher than normal latitudes so did the possibility of a cut-off vortex remain in the offing. Such low pressure cores appear to maximum advantage where airflows become slower moving than in their more thicker counterparts:ie the upper air which is some hundreds of millibars thinner than at the surface.
Such an upper air development obviously draws middle, or even higher, level air flows towards its core, increasing the vortex action, and “weather” is about to develop. The synopsis last weekend favoured the presence of a west coast trough throughout the lower layers, so the succession of events was developing with increased compatibility.
A week ago, there was a defined, but weak, upper zone of lower pressure lying from west to east from between St Helena and Tristan, a rare event, but providing yet another feature of compatibility. The final contributing factor was the rapidly departing front moving away but with the vestige of a trough west of the Cape west coast. At this time of year also, the cut-off prospects are historically more probable.
This combination ensured that extensive rains would occur and, being fed by Congo air, intensities would readily cross the 10mm benchmark. There have been reports of various falls well in excess of 60mm from several places in southern Namibia.
As the trough pattern departs across the sub-continent, an invasion of cooler air with depth takes over until the weekend. The deep vortex away to the south also moves away. Only into new week will air flow patterns return to a more northerly direction, but generally with little cohesion on a scale similar to those of the past week. Generally a dry week lies ahead.
Windflow will be fresh from the South for most of the forecast period with the Atlantic High moving in from the West. This keeps the skies clear but the wind from the south will lead to cooler temperatures across most of the country. By the end of next week, however, there is an expectation for the low pressure area to slip south from Angola and start another wave of precipitation that slowly migrates from west to east.