Understanding Weather Not Predicting – 20 April 2012
April still counts among the wet months so occasions of rain should be expected but the pattern has been modified over the past few years.
For weeks now, variance in airflow aloft Namibia has prevailed.. Pressure to create “weather” was building as a trough approached and deepend more sharply. The 2 lower levels: 700 and 850 hPa (our surface in effect) responded and the turbulence factor increased sufficiently to forge convective development across a broad band of the country. The mid to high upper air flow remained unmoved.
April’s convection was powerful enough to thrust up and into these levels with their reasonably moderate west-to-east flow.
No matter where you are, the risk of hail is endemic to each and any thunderstorm. The scene is the cumulonimbus cloud and its up and down draughts. These vary in vertical strength and extent. Usually, the collection of moisture, now as droplets, is sufficient to enable both turbulence and gravity to prevail so a shower of rain (at least) occurs. Occasionally, with clashing air currents inside the cumulus cloud, a serious downdraught is thrust by an even more serious updraught. Now the upper extent of the cloud is far above the freezing level, the duration of the water droplets at this height and its cold temperatures ensures rapid sublimation to ice. Within a few minutes this icy mass is just too much and precipitation starts. So descent to the ground happens and we see it on the ground as hail as was witnessed by residents of Brakwater north of Windhoek.
From the description, the hail tended to be mushy, already thawing, so the duration in the upper air above freeze level, was limited but the bulk could not be avoided.
Despite this very localised activity, a good range of showery weather was also reported from the northwestern interior to variously across the southern interior.
The lower levels quickly return to a favourable pattern, both 700 and 850 hPa responding to the developing trough. The 500 hPa level also bends somewhat accordingly. The outlook for showers returns during the weekend, persisting into the new week. A brief colder inflow from the south west fades quickly as the succeeding anticyclonic ridge thrusts round the Cape, establishing a new core by mid-week across South Africa. A favourable lower level circulation seems set to return, while the middle layers also return to their westerly flow.
Despite the collapse of the so-called Southern Oscillation, the La Nina influence on the anticyclonic cores persists. These high pressure systems are still some 15 degrees latitude further south than the norm, allowing the sporadic but prominent advection of airmasses from north to south. This creates rainfall opportunity for us, even late in the season as evidenced by the meagre but persistent positive expectation for rainfall over the weekend and early next week.