15 February 2013

Namibia: Understanding Weather - Not Predicting - 15 February 2013

What happened?

A complex weather pattern has taken control of our skies for the past eight weeks. We experience it in the form of abnormally hot days, and the absence of any meaningful rain.

To our south, the range of troughs and their distant vortex cores would be much more at home in the winter months, interspersed by very shallow anticyclonic cores while maintaining a more zonal (west to east) flow pattern overall. The eastern parts of the sub-continent show a recurrent style representative of the Inter-Tropical Convergence Zone of summer. In between we find a zone identifying the eastern end of an extensive upper air, middle to high level, anticyclonic core hovering from southern Africa all the way to distant South America. This weather control maintains a generally dry air presence coupled with a southerly to southeasterly flow. This also smacks of a wintery range.

It is true to say that this weather mixture is unusual in our skies, it is also out of synchrony with the synopses away to the east extending to western South America.

There is an intermittent sliver of hope with a shallow but active lower middle air moisture range being advected across a good range of our skies: active enough to see healthy convective development but its upward extent restricted by the deep unfavourable airmass directly above.

The key question revolves around the synoptic set of patterns required to displace this unwelcome situation.

The precise sequence which saw the complexity evolve also demands attention.

No matter how we look at it, it does seem that the synopses of both hemispheres have a level of cohesion in the high upper air, the stratosphere aloft our usual weather range. Regardless of how conditions develop on a daily basis in the altitudes up to around 35,000 feet, the dry stratosphere above it is what is limiting our rainfall. Until the conditions at around 56,000 feet change, we do not have a positive projection for rainfall in this season.

This week saw effective rain limited to the eastern Kavango and Caprivi, but reminiscent of early summer ranges.

What's coming?

Although the active weather persists above the northeast, limited extension is promised in the next few days. A major vortex forms east of Mozambique by Monday, limiting air flow inland during the week. Two well-marked vortex cores pass the Cape during the weekend and early new week.

The unfavourable upper pattern is unlikely to either collapse or be displaced. While the warm water presence in the Indian Ocean continues, a distinct danger remain in the form of tropical cyclones which generally pulls moisture away from the western half of the sub-continent while leading to extensive flooding in Mozambique and northern KwaZulu Natal.

In Namibia, one more overall hot and dry week is the widespread outlook for all but the northeast.

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