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Understanding Weather – Not Predicting – 01 Dec 2011

What happened?
Summer begins with December. From one aspect at least, a start is made to summer as pressure patterns move toward a summery stance. Rainfall patterns develop accordingly.
Daytime heat is part of the rainfall pattern ensuring convection is widespread.
Here the pressure patterns exert an influence: anticyclonic cores dominate the 35 to 40oS latitudes, rounding the land mass and advecting a good depth of moist air inland and toward the heat-low pressure belt developing daily across the mid-continent.
As the Atlantic anticyclone has moved those few points southward so did its ability to bring colder, drier air inland. This weakens another aspect of the typical summer pattern. The absence of moister air and hence cloud across the western parts now has a more restricted range than usual.
Meanwhile, across the northern half of Namibia, thunderstorm activity rumbled on with a marked preference for the northeastern parts in keeping with a broad flow from the tropical airmass, across Angola thence into the heartland of southern Africa.
The control of this flow lies with an anticyclonic core to be found above Zimbabwe and Mozambique. A core like this is more at home later in the season, but such a positioning has given rise to a more intermittent occurrence of rainy weather than is the usual set of expectations for the pre-Christmas weeks.
Whether or not this is part of the developing La Nina in the Pacific, flow across our parts of the world maybe open to varying opinions, but the overall stance still bodes well for above normal rain prospects across a very broad area.
What’s coming?
The promising pattern persists into the new week. A frontal trough is already departing, but is replaced by yet another one.
As has already been noted, anticyclonic cores bypass the Cape and in so doing tend to accelerate. The description “Race around the Cape” is frequently used. But by their very nature anticyclones are much more likely to be slow-moving or, as we have seen recently in the mid-Atlantic, just about stationary. But these mobile cores are really ridges breaking away from the parent core, they leave behind the vast, descending column of air feeding the main anticyclone. These ridges are shallow, hence their ability for rapid movement. Being “quick”, they push the trough along. But because part of the trough has to cross the southern part of the landmass, the trough’s northern section is slowed down, often separated from the maritime section. This remnant still keeps its vortex characteristics and develops its own core but it is now cut off from the parent trough, by definition a low pressure area.
A key here is the wind flow patterns well above the surface. Such newly developed cores can bring in an advection of moister air. Cut-off lows, their common name, have a history of rapid development and surprisingly heavy rains. But they also have an acknowledged level of unpredictability. In the current case, this new pattern has been on synoptic outlooks for the past 3 days. Despite their record, this persistence has survived and showery prospects are given for the southwest and south in the course of the weekend.
What is uncertain is just how much of the northern air can be drawn into this pattern and to what effect.
For the northern parts thundery weather persists.
Note: A trough is an extension of a low pressure area and a ridge is an extension of a high pressure area. Circulation in the low pressure area is cyclonic in the southern hemisphere while circulation in the high pressure area is anti-cyclonic.

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