After a rather dry spell in Cape Town (with a few relatively weak cold fronts), there is a bit of a wait till the next significant storm. A weak cold front will brush past on Wed 18th, but this will mostly affect the southern Cape coast.

Several consecutive GFS model runs (up to 12Z on Sun 15th) show a frontal system making landfall on Sat 21st, accompanied by a deep upper trough, and lingering until Mon 23rd.

As usual this sytem is still developing, and it is not possible to make specific predictions this far in advance (168 hours or 7 days), but this could potentially be a period of very windy, wet and cold conditions, with large swells along the coast.

There is a possibility of snow later on Sun 22nd, but this can only be confirmed much closer to the time.

The front due on Wed 18th will have a limited effect on Cape Town, but looks to have a strong effect on the southern Cape, with a lot of cold, wet and windy weather!

Update: The CSAG Myweather charts show a well devloped front brushing the southern Cape coast on Wed 18th:
www.myweather.co.za/avn/images/slp_synop...mple_20100816-48.png
www.myweather.co.za/avn/images/precip_sy...mple_20100816-66.jpg

The predominant wind will be from the south-west, which will bring very cold air onshore on Wed 18th:
www.myweather.co.za/avn/images/wind_syno...mple_20100816-60.png
www.myweather.co.za/avn/images/t2m_south...mple_20100816-60.png

Daytime temperatures below 10C are likely in the Eastern Cape on Thurs 19th:
www.myweather.co.za/avn/images/t2m_south...mple_20100816-84.png

This morning's GFS model runs (00Z and 06Z on Tues 17th), and the SAWS Aerosport charts indicate that the freezing level will drop below 1500m late on Wed 18th. This makes snow likely over the highest mountains in the southern Cape (Swartberg range).

This front is moving up from the deep south, without the usually precursors, such as north-westerly winds and cirrus, but it certainly looks to pack a punch!

Update: The front can be seen today (Tues 17th) on the SAWS ship synoptic chart, and as a sharp boundary on the EUMetsat airmass animation:
old.weathersa.co.za/ship/ship.gif
oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/SOUTHERNAFRICA/

This morning (Wed 18th) the SAWS has put out a warning confirming conditions in the southern and Eastern Cape on Thurs 19th:

1.Very cold, wet and windy conditions are expected over the Eastern Cape interior.
2.Snowfalls are expected along the Eastern Cape northern mountains.

Update. The SAWS has put out an additional warning:

Valid for 18 Aug 2010:
1.Light snowfalls are possible over the peaks of the southern mountains of the Western Cape tonight.

The GFS models (06Z on Fri 20th) continue to show a significant weather event for Cape Town for the weekend of Sat 21st and Sun 22nd, and persisting into Mon 23rd (update).

Update: Snow is quite likely on the mountains of the Western Cape later on Sun 22nd, and overnight into Mon 23rd. (The exact details will become clearer in the next 36-48 hours).

This morning's Terra Modis overpass (Fri 20th) shows heavy snow over the Southern Drakensberg, and some mountains in the Eastern Cape, in the wake of the cold front that passed on Thurs 19th:
rapidfire.sci.gsfc.nasa.gov/subsets/?sub...010232.terra.721.1km

The first relatively weak cold front passed Cape Town this morning (Sat 21st), brining a sprinkling of rain. Very strong winds preceded the front overnight on Fri 20th.

The main front is expected to make landfall later on Sat 21st:
www.myweather.co.za/avn/images/slp_synop...mple_20100821-18.png

Update: Strong to gale force winds will persist until at least midday on Sun 22nd, moving along the coast later:
www.myweather.co.za/avn/images/wind_syno...mple_20100821-36.png

Heavy rain (10-20mm) is expected early on Sun 22nd:
www.myweather.co.za/avn/images/precip_sy...mple_20100821-30.jpg

Update: Daytime temperatures will remain below 15C over much of the Western Cape interior on Sun 22nd, spreading to the Eastern Cape on Mon 23rd:
www.myweather.co.za/avn/images/t2m_south...mple_20100821-36.png
www.myweather.co.za/avn/images/t2m_south...mple_20100821-60.png

The freezing level will drop below 1500m later on Sun 22nd, and snow is likely over the mountains of the Western Cape, extending into Mon 23rd.

The SAWS reports isolated heavy rain in places in the Western Cape in the 24 hours to 8am on Sun 22nd:
metzone.weathersa.co.za/images/PDF_docs/nr_daily_rai.pdf

Highest were Kirstenbosch 25mm, Paarl 25mm, Elgin 23mm, Worcester 23mm, Bellville 22m, etc.

Update: The landmark Athtlone Cooling Towers are being demolished today (Sun 22nd). Here's what they (used to) look like:
www.capetownskies.com/6027/13_flats_haze_rm_hhc.jpg
Unfortunately weather conditions are not ideal for photography today...

The SAWS reports further isolated heavy rain in parts of the Western Cape in the 24 hours to 8am on Mon 23rd:
metzone.weathersa.co.za/images/PDF_docs/nr_daily_rai.pdf

Highest were Citrusdal 21mm, Paarl 19mm, Algeria (Cederberg) 16mm, Franschoek 15mm, Moorreesburg 14mm, Piketberg 13mm, Ceres 12mm, Clanwilliam 12mm, etc.

The rain is almost over, though some showers will linger along the southern Cape coast during the day.

The freezing level this morning (Mon 23rd) dropped below 1500m near Cape Town, so snow is likely over the highest peaks.

After the clouds lifted overnight on Mon 23rd, I took a drive out on Tues 24th. There was light snow above 1600-1700m, and heavy snow above 1800m.

This morning's Terra Modis overpass (Tues 24th) shows several areas of heavy snow, particularly the Hex River valley, and Matroosberg:
rapidfire.sci.gsfc.nasa.gov/subsets/?sub...10236.terra.721.500m