Upper-trough and frontal-low SW-Cape 25 August 2012

Numerous model runs and forecasts (up to 06Z on Wed 22nd) have indicated rain over the SW-Cape on Sat 25th. It has been hard to characterise this event until now, since it has some elements of a weak cutoff-low, as well as a 'normal' cold-front. The long-range model runs had been very uncertain about the details (flip-flopping), but have now converged to a sort of 'hybrid' system.

The first element that has been common to all of the predictions is that it will again be rather cold over Cape Town, due to the arrival of an upper-trough. This trough will also bring some instability, although the lack of moisture means that convective rainfall will be limited.

Very strong winds are possible if the surface low develops close to the coast, and intensifies as predicted by some of the model runs. This aspect of the forecast is still the most uncertain, and will only become clearer 24-36 hours closer to the time of landfall.

The current SAWS ship synoptic chart and EUMetsat Antarctic sector Airmass animation already show some elements of the approaching system, but do not indicate the complexity and fine details at this stage (06Z on Wed 22nd).
metzone.weathersa.co.za/images/articles/ma_sy.gif
oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/ANTARCTIC/

This custom GFS chart shows the 500 hPa level temperature at 06Z on Sat 25th, being about 10C below the seasonal average, as well as the position of the closed (but not symmetrical) upper trough.

Compiled by Gordon Richardson - Stormchasing SA