Understanding weather – not predicting 04 October 2013
Last week’s rain was a seeming divergence from the static state of persistent clear skies associated with the Sub-Tropical High Pressure belt.
The rainy pattern departed as both its core pressure circulation pushed southeastward with some assistance from the anticyclonic thrust both feeding and pushing from the west. Both surface (up to some 5000 feet: 850hPa level) and into the middle upper air (from 10000 feet: 700hPa and aloft) were taken over by this controlling airflow to ensure a drier pattern arriving to dominate for the next few days.
Such a take-over would normally see an advection of cold air as its calling card. Contrary to such expectations, the advance was quick enough to ensure any such input would get no further than our southern parts before being carried away. The limited upper extension kept the hazy upper air mass more or less in place as we saw with the clear blue skies (like the previous week) just not in evidence.
This week the anticyclonic input brought some cloud to our northeast and light showers were recorded on Monday and Tuesday. The northeasterly flow produced midday maxima up to the mid-30oC range, virtually country-wide, simultaneously. A lower surface pressure core was the daily feature for the rest of the week beneath the middle air anticyclonic core, clearly identified at the 500hPs level. This is actually a very typical synoptic pattern for this time of year!
The complexity angle is enhanced by the fairly static anticyclonic high-pressure core persisting to the east of the sub-continent. Persistent upper airflow from the north, assists the increased low-pressure activity where troughs are formed. This complexity invites moist input into our middle air more readily than previously experienced. Such is the impact of the advancing spread of the Torrid climate belt and its associated moist air mass in response to warming and the consequent change of climate patterns.
The high-pressure area southeast of the continent is the driving engine that brings our signature northern airflow after a cold front has passed the Cape of Good Hope. The sharpening trough activity associated with the active synoptics of the Southern Ocean is due to the warmer thrust from the north which increases vortex development. A new trough advances as the weekend approaches.
During the weekend, mid-level flows also resume a northerly orientation. Some cloud may convectively occur. One trough moves past the Cape, departing by Monday, while a new trough appears above the Atlantic by late Tuesday with an upper air extension which (complexity at work again) is expected to create its own core, cut-off from the departing surface vortex. Mid-week can see clouds increasing in the middle levels with increased moister input also. As it becomes warmer by the day, with increasing convergence, the moister air-mass provides a shower potential for most of Namibia across a 24 to 36 hour range from Tuesday to Thursday. Falls can exceed the 10mm measure as both convergence and convection combine.