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UNDERSTANING WEATHER NOT PREDICTING |
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Written by JOHN OLSZEWSKI
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Friday, 13 November 2009 08:56 |
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What happened?
As last week closed, a complex vortex system was active, situated above an area of warmer than normal sea at the western end of the Agulhas current. The rather weak anticyclone lying west of this disturbance extended a ridge south of Bouvet Island linking with an Antarctic core. The intermix of cold and warm provided all the ingredients for aggressive vortex activity to take shape. A trough line developed and an upper cut-off vortex also formed. By Sunday, several observers noted the altocumulus patches streaking across the skies from the northwest pushed along by a strong wind. The rain area was confined to approximately the Lüderitz magisterial district. From the coastline inland to the escarpment, falls above 5mm were recorded. Along the escarpment and further inland, falls above 10mm occured. The duration, measured in hours, indicates the precipitation occurred ahead of the front, indicating warmer air lift as the core of the vortex area approached. As the active cores moved east, cold Antarctic air was carried north quickly dropping temperatures over the interior. Generally, the anti-cyclonic tracks remain further south than normal. An interesting thought concerns the extent to which these anticyclonic tracks so far south of their normal track can foment cut-off vortex development bordering the sub-tropical latitudes and what effect this will have on local weather.
What’s coming The final vortex of the week-long sequence lies well to the south of the Cape of Good Hope. Two anticyclonic cores, one between Tristan da Cunha and St. Helena and the other on the southern side of the vortex are squeezing it eastward. The final cold front of this series, is still developing (as the polar air flowing north around the southern anticyclone undercuts) to depart with its parent vortex. A ridge will move around the Cape coast, building a new anticyclone off the southeastern coast. Moist air is thus expected to push into the sub-continent during the weekend. The outlook expects moist air from the active Zambian rain-belt to be advected south and west into the northeastern quarter of Namibia by Saturday evening and showers can spread southwest and then south to the Gobabis area. Otherwise, dry, sunny weather seems likely to continue for the rest of the country. For southern Africa, the current pattern presents a normal picture for what should be expected at this time of year. Active weather systems are now situated over Zambia, Zimbabwe and eastern South Africa.
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