 DISCUSSION: When people think of tropical cyclones, most would quickly think of open ocean genesis situations and favorable environmental conditions which would keep an organized low-pressure system intact. After all, any tropical low is essentially a self-sustaining heat engine that transports heat from the tropics and poleward towards the mid-latitudes (i.e., regions located between 10 and 30 degrees North and between 10 and 30 degrees South respectively). Many people generally tend to think that once a tropical cyclone impacts land, weakening of the core circulation tends to happen rather quickly. That greater rate of weakening is presumed to be due to greater friction with terrain that effectively disrupts the center of circulation and negatively affects both spiral rain-bands and core convection alike. But a generous void exists in the meteorological literature about tropical lows that form over land (i.e., those which form primarily over central and southern Africa).
Most commonly found during the heavier precipitation months (i.e., between December through March), these tropical low-pressure systems form over land as a consequence of moisture that is displaced from the intertropical convergence zone (ITCZ). More specifically, the ITCZ is an area of persistent deep thunderstorm activity that oscillates between the Northern and Southern Hemispheres depending on the time of year. Around this time of the year, high moisture content and instability generated by the release of latent heat in the mid-troposphere (i.e., between roughly 2 and 5 kilometers up) combine with a favorable wind shear profile to generate a cyclonic structure similar to what is observed across tropical ocean basins. Although peak surface winds are not as strong as those found in association with tropical storms over open waters (e.g., the strong core wind speeds which were observed with both Hurricane Irma and Hurricane Maria), some of these land-based cyclones can still have high winds that can inflict substantial wind damage. However, these systems are mostly known for their potentially copious amounts of precipitation and thus are considered high-impact events as flooding is the principal threat in this otherwise semi-arid region of Africa. Fortunately, the development of such a tropical low-pressure system would likely weaken the grip of excessive heat impacting southwestern South Africa at a given point in time. In recent days, there have been confirmed observations for maximum temperatures over inland areas in excess of 38°C (100°F) and these same areas have been hit excessively hard by a long-lasting drought. This low-pressure system is expected to develop over Angola and slowly travel southward through Botswana and Zambia. Heavy (and very much needed) rainfall (i.e., on the order of 6 inches (or 0.16 meters) or more) is expected to fall over southwestern South Africa over the next 7 days. This heavy rainfall may also help to provide relief from the lengthy period of drought previously noted above. The photo attached above can be found at the following web address: https://www.eumetsat.int/website/home/Images /ImageLibrary /DAT_IL_10_01_24_A.html. To learn more about other high-impact weather events occurring across Africa, be sure to click here! © 2018 Meteorologist Brian Matilla and Meteorologist Jordan Rabinowitz
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