| Understanding Weather -not predicting- |
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| Written by John Olszewski | |
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That a different scale of weather pattern appears to be evolving seems to be an apt conclusion. The lingering daytime warmth, the ready ability for clouds to appear, the limited northward thrust of colder air masses is what we have experienced, so far. This weather-on-the-doorstep reality is well-supported by the weather patterns experienced daily and visible on the southern hemisphere weather charts.
The key factor is the regularity with which the anticyclonic cores
build and progress in the the range of latitudes between 40o and 50o
South.
This character is to be associated with summertime and would advect considerable moisture inland. But this is the threshold of winter. One can also state that this characteristic is synonymous with a La Nina event continuing in the South Pacific and with the Southern Oscillation index in the high position. Meaning that the barometric pressure at Tahiti in the south eastern Pacific is still above that of Darwin in northern Australia. This indicates that La Nina is still in control leading to short bursts of summertype weather in Namibia. Such patterns are not just drawings on a weather map, they present a range of effects which are recorded in the day-to-day weather. This last week provided yet another sample. Anticyclonic cells push cold fronts along. These fronts, if allowed by a slackened push, will develop an individual vortex, create its own weather developments and the anticyclone will push past this, leaving it on the northern side and limit its northward thrust of the colder air mass which it propels. On the far northern side, the limited advection of semi-tropical air enabled cloud formation and even local shower development. Smaller disturbances formed and heavier storms occured. Several reports of falls amounting to some 40mm have been made from places across Namibia’s northern parts west and northwest of the Waterberg. Sadly, these falls have not occurred where a rain-gauge measure has been made, so word-of-mouth has to be accepted. In these instances, this dubious method appears much more solid, based upon the eyewitness accounts from a handful of individuals. But, nevertheless, the controlling pattern varies considerably from the “normal”. In the south more normal conditions prevailed with light showers recorded south of Keetmanshoop and Aroab. What’s coming During the weekend, two cold fronts approach the Cape one after the other. An upper air trough intensifies in association and, with the succeeding anticyclone pushing around these developments another cut-off vortex is predicted to form off the Cape West coast by early next week The resultant weather does not offer much prospect of change for most of the country. Cloud is likely, the isobars suggest windy conditions, a light shower or two seems probable for the far south, at least. Temperatures continue warm by day, cold overnight, though there is a risk that, following the passage of the “cut-off” vortex, some very cold air can reach at least our south for about a day, before warmer air returns blowing in from the east. With the various pressure patterns mobile, there is every prospect that windy weather will happen across much of the country. The risk of veld-fires is already present with several fires reported in the Kavango region. |
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