Understanding Weather – not predicting – 15 March 2013
There is a considerable need for greater detail in daily weather charts. Computer models are faced with blank areas on both surface and upper air charts. These lacunae are filled in from satellite images which are based on specific programmed assumptions. The results often, are not accurate.
This last week gave a good example.
For most of this season, the weather was dominated by an out-of-place middle layer anticyclonic core extending across the South Atlantic, bringing in cold dry air from the south. Any lower moisture input could make precious little convective effect on this upper presence. Future outlooks presented a more promising situation regarding air flow and moister input but this did not materialise and rainfall across the country remained far below normal.
So what went astray?
Across the southern hemisphere, charts show surface anticyclonic cells ranging from some 35oS to the 45oS latitudes regularly, but not for the South Atlantic. The overall explanation appears complex. It varies from the rest of the hemispheric pattern, it is equally variant from the northern hemisphere, too.
Can we investigate the equatorial Pacific? Sea surface temperatures are now in a positive, warm, range which, from past experience, provides similarities to the current patterns in our upper middle skies.
Such an overall season has provided a similar range with the early, quite widespread, rains. There were some heavier falls across the far north: Okongo’s 88mm this last Sunday was the best. Some rain fell across the Cuvelai Delta, but an efundja appears limited. Some rain also fell in a broad band from north to south over the edge of the escarpment more or less midway between Windhoek and Walvis Bay.
Otherwise, in this last week, the perceived fulfilment of last week’s outlook did not materialise as the high pressure core renewed as much above Namibia as anywhere else.
The overall state of the season is matching the driest ranges on record so far. The patchy, spasmodic results have all the dry, ENSO-style, qualities as their hallmark.
The outlook across the southern ocean remains unconvincing. A passing anticyclonic core at surface and between 35 and 40oS passes quickly by the weekend, but is limited in its ability to advect moist air to the interior.
Its quick departure sees a new vortex system control west wind flows across the Cape coastal latitudes into the new week as the middle levels keep to a dry air core for the next few days. The rainfall prospect diminishes accordingly.
The adjacent prospect sees a retreat of the active Inter-Tropical Convergence Zone further northward. The ability to tap this and advect deep levels to displace the intense dry upper pattern does not appear from the current charts and their outlooks for our part of the hemisphere. Yet, it deserves noting, the overall southern hemisphere patterns remain in their more southerly course, La Nina-style, with moist maritime advection accompanied by a deeper inflow across the southern hemisphere generally (other than above us!): More observational detail is truly required.