| Understanding Weather Not Predicting It |
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| Written by John Olszewski |
| Friday, 12 November 2010 07:19 |
What happened?
The past few days saw an interesting mix of winter style and La Nina formation weather. Winter style saw a major vortex area move slowly across the southern oceans, while doing so, rainy weather was brought into the Cape area and northwards (but not as far as our winter rainfall area). As the whole complex system carried on eastwards, yet another secondary formed in the cold, semi-polar air circulating briskly behind the active front. The succeeding ridge of the South Atlantic anticyclone was picking up air about 55 to 60oS latitude and circulating this northward. This cold advection made its presence felt across much of Namibia after the weekend.
This south to north flow typifies La Nina influence as shown by the continual north-south alignment of the various anticyclones as they form, build, moving eastward all the time and with their cores persistently hugging the 40oS latitude range.
The influence of the vortex area maintained a trough across the mid-subcontinent and a persistent flow of moist air to be found to its east. Thunderstorms formed in this air-mass. Katima’s 34mm was a good example of the potential of such an airmass.
Meanwhile, the northward moving airmass ensured morning temperatures well below normal for the time of year. The length of sunlit hours (12+) ensured rapid daytime warming. But, as this airflow is carried eastward, when it moves in from the east, it undercuts the resident airmass. Undercutting means uplifting the resident airmass, which means cooling to condensation level so cloud (or more cloud) forms and the probability of precipitation of excess moisture becomes more and more likely. This sequence is another example of a La Nina influence. It is also a definite example of the north-south-north airflow and its influence at work.
Meanwhile the retreat of the Inter-Tropical Convergence Zone from its far northern reaches across west and central Africa is now at its regular departure rate, although by being so far north of its normal lie, this southward movement may take that bit longer before it reaches its equatorial home.
Due to the very limited input (there are very, very few weather stations reporting) from the central and southern African area, the agency responsible for the tracking of the ITCZ has issued its last bulletin for this calendar year and so any idea as to where this major feature will be for the next 6 months (across our rainfall year 2011) can only be tracked from satellite imagery.
However, another valuable NCEP NOAA input involves the eventual issue of upper air charts based on Tropical Zone analysis. These charts come on line some days afterwards so up-to-date analyses cannot be available, but the retrospective value has much to offer. It did, for instance, provide exhaustive detail regarding the whereabouts of the ITCZ during the lengthy 2008 and 2009 rainy spell and the ensuing efundja and subsequent flooding of Owamboland and Etosha.
What’s coming?
The arrival of the cold inflow circulating around the anticyclone SE of South Africa will see further extension westward. The upper air flow around the vertical extension of this anticyclone will pull in some tropical air. These flows will be advected into our airspace by the weekend.
A weak upper trough appears above the west coast. so this combination is expected to provide a band of showers across much of the country from Saturday onward. Widespread productive falls are expected but only for a four to five days. At this point, the season’s first substantial rainfall belt is expected to cover most of the country even reaching into the pre-Namib. |
| Weekly Rainfall - 11 November 2010 |
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| Written by John Olszewski |
| Friday, 12 November 2010 07:17 |
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| Understanding Weather Not Predicting It |
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| Written by John Olszewski |
| Friday, 05 November 2010 08:55 |
What happened?
With moister air persisting around the Kavango valley, the prospect of showers persisted equally. An anticyclonic core above Mozambique maintained input from this moist air-mass across the mid-sub-continent. Quite widespread falls occurred across the Kavango, Caprivi and southward across the far east extending so far south as the Ariamsvlei district. Being on the western fringe of this airflow pattern, the prospect of heavier falls was, at best, doubtful. In reality, this prospect held sway, although local falls totalled as much as 10mm in the Ariamsvlei area.
Thunder activity is also reflected in numerous reports of lightning setting off veld fires. This is always a risk when activity is present, but moist air and rain showers are at a premium. Again, this is a likely feature of the late spring and early summer, bearing in mind that the core of the rain season lies some weeks ahead. At present, generally very dry air, considerable daytime heat and windy conditions, are descriptive of the daily round.
While this was the pattern across one part of the country, the south lay on the fringe of an active winter-style pattern prevailing across a vast stretch from some 30oS and further south to the Polar regions.
Here, cold fronts with developing waves (new vortex cores) and secondary vortices held sway. Upper air troughs completed this range of activity.
The influence of this complex system is continuously felt south of the Orange river but we are too far north to experience any significant weather. At most, cloud patches form sporadically over Namibia’s south and southwestern parts.
For the Windhoek area on Tuesday, sufficient daytime heat resulted in higher level cumulus cloud. Combining with the flow to the east and the northernmost edge of an approaching upper trough, in the late afternoon, towering cumulus clouds east of Windhoek lead to several showers. This was promisingly swift, in view of the overall picture.
Much further afield, the ITCZ begins withdrawing across the Sahel, but is still maintaining a level of activity unusual this late in the declining season.
La Nina reigns in the equatorial Pacific, being predicted to last throughout the southern summer.
Anticyclones persist along the 40 to 50oS latitudes, ridging both north and south emphasizing their range of hemispheric control.
At this stage, an interesting season assembles.
What’s coming?
Airmass flow from northwest to southeast seems unchallenged across the mid-sub-continent. The ability to build shower patterns seems restricted to our northeastern regions and only for the weekend period.
An upper air trough will see cloud development over the far south, but the actual zone of development lies too far away to the south to have a real impact.
During this time, the trough-line lies to the east, so southerly airflows cool the nights across much of the west and central parts, at least.
Meanwhile, another extensive vortex system approaches from the west accompanied by a well-marked upper trough. This combination arrives early in the week and induces more moist air from and across our north. Rainfall estimates are moderate, a band of showers can spread from the Etosha eastwards, while extending southward to the Orange. |
| Weekly Rainfall - 04 November 2010 |
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| Written by John Olszewski |
| Friday, 05 November 2010 08:50 |
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Weather
