After what can only be described as a dull summer
Numerous model runs have indicated the likely formation of a cutoff-low (COL) over the south-eastern parts of the country between Sun 24th and Tues 26th. All indications are that this system will produce heavy rainfall (> 100mm) in places, and extremely heavy snow (more than a meter) over the Drakensberg and surrounding peaks.
Edit: The first indications that this event will develop into a COL (and not just a normal cold-front) will be the arrival of (mid-winter) thundershowers over Gauteng tomorrow afternoon (Sun 24th), as shown by the SAWS thunderstorm probability chart.
The first image shows the expected rainfall for the 5 day period from 00Z on Thurs 21st to 00Z on Tues 26th. Peak falls are likely over the central Free State, Lesotho, Drakensberg, KZN and parts of the Eastern Cape coast.
By 00Z on Sun 24th the freezing-level will be down to 1500m over the majority of the southern half of SA, though moisture in circulation will be limited.
During Sun 24th the cutoff-low will begin to develop, with moist tropical air drawn under the cold-upper air. This will produce instability, and by Mon 24th very heavy showers will form.
As the cutoff-low develops the freezing-level will rise slightly (due to the mixing of air-masses), but extremely heavy snow will fall above 2000m on the highest peaks in the Drakensberg.
Edit: Recent model runs show the COL drifting westwards along the S-Cape coast, and reaching the SW-Cape by Tues 26th. This chart shows possible heavy showers (and snow) over the SW-Cape mountains (during the 6 hours from 06Z to 12Z on Tues 26th).