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Understanding Weather – not predicting – 21 June 2013

What happened?

While weather around the world exhibited measures of extremes, we in Namibia were seated in the spectator seats. The considerable ranges of synoptic activity formed, developed and flowed eastward but remained south of the Orange River. A brief intrusion circulated northward providing a chill start to the working week for all but the extreme north. With patterns moving quickly, warmer temperatures returned from Tuesday. The north-wind orientation persisted for the remainder of the week.
A sample of the high upper turbulence associated with a cold front could be seen as cirrus cloud earlier in the week. The 30,000 foot level hosts the core of the jet stream which is where the band of cloud was visible.
These jet-stream influences are regular at latitudes much further south so northward excursions provide an opportunity to see this turbulence. Jet-stream typically blow at around 100kph and often much faster.
The wind-torn cloud shreds gave witness of this level where converging winds more-or-less collide, creating a turbulent range, some thousands of feet thick which airliners are very careful to avoid.
The visible cloud-band is part of the upward extension of the surface level vortex created by the cold undercutting airflow thrusting the warmer air aloft.
The front, the divide between the cold and warm, correspondingly extends through the middle layers into the cirrus level and further to the highest levels of the troposphere. Such undercutting ensures cloud formation and, ultimately, precipitation.
What’s coming?

Although frontal activity continues unabated across the southern oceans for the next few days, northward undercutting advection remains unlikely. By Monday a surface ridge forms south of a well-marked vortex between 40 and 50oS.
These feed the lower inflow and higher outflow while the anticyclonic ridge situated above the eastern sub-continent keeps our wind-flow northerly for the most part. Winter means chill overnights but warmth prevailing by day.
The eastern Equatorial Pacific sees little change from the near La Nina situation. Strong high pressures maintain consistent Trade Wind flow from the more southerly core yet the further outlooks maintain a neutral state of the so-called Southern Oscillation.
The wind flow will remain predominantly north, also for the northern half of the coastline, hence zero or very weak oosweer conditions.

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