| Understanding Weather - not predicting - |
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| Written by John Olszewski | |
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On the face of it, this last week was a quiet week on the weather front. But, bearing in mind that local events are part of a greater whole, this does not mean that the weather world took leave. The southern oceans were (still are) the zone of ceaseless activity. The interplay between polar and semi-polar air and the southward thrust of temperate air creates the stage where this ceaseless activity performs.
Cold fronts have come and gone across the Cape regions of South Africa,
active rain-belts should be recurrent and very cold air arrives,
briefly at least. This is a likely winter-time scene. But with another
slant.
In between the vortices and their cold fronts are the anticyclones which push these systems along. The anticyclonic zone of movement is still to the south of the more regular track and the ability to extend ridges of higher pressure southward to the Antarctic is consistent. This pattern provides that north-south-north air flow which does much to set the scene for summertime rainy weather. Its presence at this time of year shows that while La Nina has faded, not all the playing positions have similarly collapsed. Still further afield, the Indian Monsoon is on track in time and rainfall values. The Sahel ITCZ is active, but according to rainfall estimates, not quite to expected levels. The Australian view notes with some concern that the Indian Ocean sea-surface temperatures are in oscillation from its normal pattern. While the Namibian far south has experienced cold (low daytime temperatures) the central parts remain mild to warm (pleasant daytime temperatures) and the north remains warm to hot for this time of year. It is winter, so cold nights are widely evident. The other common factor is the daytime wind. This brings us back to the north-south-north air flow pattern across much of the southern hemisphere. Across the northern areas, the soil (where flooded) is still very damp, but drying, the generally frost-free condition maintains some moisture in the dry grassland. Frost removes unwanted pests, but also damages vegetation. Farmers cannot have it both ways, but with outside pressures on the go, easier conditions in the veld do help. What’s coming There is scant reason to expect any major change. The weather patterns visible on the Australian ocean sea-surface maps, coupled with the South African regional charts, indicate a regular rhythm in the procession of polar/temperate vortices (storm areas at their best) and calming anticyclones that still produce noteworthy wind-speeds. Freezing levels are low, some 6000 to 7000 feet above sea level and the tropopause has descended to a winter level around 30000 to 35000 feet. Light variable upper winds are the expectation over the central plateau. An interesting anticyclone northeast of the Falkland Islands will move roughly eastwards. The cold front of the vortex it drives should reach the Western Cape longitudes by mid-week. The forward view is of very cold conditions (again) for the far south, extending perhaps to as far north as the Windhoek latitude, but (again) not much further north. So, until the solstice (21 June), little change. |
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