While the Western Cape continues to see beautiful sunny days
July 20, 2015 at 8:51 pm #93535
Numerous recent GFS model runs (up to 12Z on Mon 20th) show a large and evolving weather system covering most of South Africa later this week (Thurs 23rd to Sun 26th). Widespread rain showers, cold temperatures, and snow over the high ground are all likely (to varying extents) during this event.
Preceding this event, there is already a set of cold-fronts brushing the southern coast of SA, likely to produce moderately heavy rain over parts of the Eastern Cape tomorrow (Tues 21st). The cold-core upper-trough associated with these fronts has detached, and the closed circulation will soon move in over the interior. The current situation can be seen on the EUMetsat Airmass animation: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/SOUTHERNAFRICA/
This upper trough will bring moisture down from the tropics during the first half of the week, setting the scene for the main stage of this thread topic.
Another cold-frontal system is likely to brush the SW-Cape on Wed 22nd. This will have marginal effects on land (initially), due to limited moisture in circulation (in the west). The cold-core upper-trough accompanying this second frontal system will move in over the interior, which will by then have considerable tropical moisture in circulation. The resulting broad-scale cutoff-low will slowly move from west to east over the country, producing widespread showers, and heavy snow over the high ground of the Eastern Cape and Lesotho. The COL will finally exit the country via the coast of KZN on Sun 26th.
It is very difficult to describe all the details of this complex and evolving event. The NCEP GFS 7-day and 6-hourly animations give a good overview of several key parameters:
The exact position and intensity of the cutoff-low is difficult to determine in advance (particularly at longer durations), but the MAG GFS ensemble ‘spaghetti’ charts of the 500hPa isolines show a few key stages:
Attachments:July 22, 2015 at 7:32 am #93541
The current Black South Eastern conditions being experienced over the Western Cape have intensified, and strong winds have reportedly caused damage in several areas overnight (into Wed 22nd). See:
Its hard to summarise the upcoming cutoff-low, but all indications are that it will be a significant weather event for large parts of South Africa between Thurs 23rd and Sun 26th. Widespread (further) heavy rains, very cold conditions and thick snowfalls are all likely in places. All the NCEP animations linked earlier have updated, and show many aspects of this event.
The latest prediction for Giant’s Castle in the Drakensberg shows a cumulative snow total exceeding 1 meter (above 2500m elevation) during Fri 24th and Sat 25th! See: http://www.mountain-forecast.com/peaks/Giants-Castle/forecasts/3280July 22, 2015 at 9:47 am #93545
Several houses in Gordons Bay lost their roofing. Maximum recoded wind gust on the iweather GB station was 135.2km/h.
Also a note the wind had a large easterly component compared to the normal seasonal S, SE.
Moderator: Awesome photo, thanks for posting! Where did the roof land, in one piece, or lots of small bits?
Attachments:July 22, 2015 at 11:42 am #93562
The SAWS reports very heavy rainfall over coastal parts of KZN and the Eastern Cape during the 24 hours to 8am on Wed 22nd: http://www.weathersa.co.za/media/data/climate/nr_daily_rai.pdf
Highest include Virginia (Durban) 207mm, Margate 123mm, Mtunzini 80mm, Port Edward (KZN) 72mm, etc. Also of note were East London 53mm, Port Alfred 43mm, Fort Beaufort 41mm, Grahamstown 37mm, Queenstown 32mm, etc.
Edit: The SAWS reports several new records yesterday, particularly the rainfall at Virginia: http://www.weathersa.co.za/media/data/climate/nr_records.pdf
Relatively mild minimum temperatures (9C) were recorded over the Free State, due to inflow of moist tropical air.
Edit #2: The IOL website has some fairly dramatic photos provided by the Cape Argus, showing wind damage: http://www.iol.co.za/news/south-africa/western-cape/pics-cape-wind-topples-trucks-trees-1.1888862July 23, 2015 at 8:17 am #93565
Edit: The SAWS reports scattered light to moderately heavy rainfall over many parts of SA during the 24 hours to 8am on Thurs 23rd: http://www.weathersa.co.za/media/data/climate/nr_daily_rai.pdf
Highest include Mandini (KZN) 71mm, KS Airport (Durban) 47mm, Mt Edgecombe (KZN) 39mm, Ulundi (KZN) 29mm, Richards Bay (KZN) 28mm, etc. Also of note were Noupoort (NC) 23mm, Kirstenbosch (WC) 19mm, Slangkop/Kommetjie (WC) 19mm, Strand (WC) 18mm, Postmasburg (NC) 17mm, Queenstown (EC) 17mm, De Aar (NC) 16mm, Fauresmith (FS) 16mm, etc.
Today (Thurs 23rd) is a ‘transition’ period between ‘first’ trough over the interior of South Africa, and an approaching cold-frontal system. The gale-force South Easterly wind finally abated over Cape Town late yesterday (Wed 22nd). During the early hours of this morning there were moderately heavy showers over parts of the SW-Cape, produced by scattered moisture and instability.
During today (Thurs 23rd) the existing trough over the interior is likely to shift somewhat eastwards (and weaken slightly), pushed by the cold-front which is due brush the SW-Cape later this afternoon. Both systems are currently visible on the SAWS ship synoptic chart, and EUMetsat Airmass animation:
The ‘main’ event this week will begin when a ‘second’ cold-core upper-trough associated with this cold-front moves in over land tomorrow (Fri 24th). There is already a large amount of tropical moisture in circulation, and this will be absorbed into a developing closed circulation. This combination will form a large cutoff-low, which will slowly intensify over South Africa for the remainder of the week, until exiting via the coast of KZN on Sun 26th.
All of the outcomes mentioned at the beginning of this thread will all be triggered in various ways by this large and evolving weather system. It is always difficult to make specific predictions in advance for this kind of event, and I will post further details closer to the time.July 24, 2015 at 11:06 am #93567
A well developed cold-front passed over the SW-Cape last night (Thurs 23rd into Fri 24th), producing plenty of rain and very cold conditions. Snow was observed in many places above 1400m, including the Hottentots Holland mountains east of Cape Town.
Snow also fell overnight on parts of the Drakensberg, as seen in this early morning EUMetsat image (06Z on Fri 24th).
The SAWS reports widespread light to moderate rainfall during the 24 hours to 8am on Fri 24th: http://www.weathersa.co.za/media/data/climate/nr_daily_rai.pdf
Highest include Kirstenbosch 37mm, Strand 36mm, Jonkershoek 29mm, Cape Point 25mm, Wepener (FS) 22mm, CT Airport (W/O) 21mm, etc.
A few new temperature records were set yesterday, with 10.8C maximum at Clanwilliam. See: http://www.weathersa.co.za/media/data/climate/nr_records.pdf
Today (Fri 24th) is another ‘transition’ day, with the cold-core upper-trough now positioned over the western half of South Africa, but rather limited moisture in circulation. Some rain is likely to fall along the southern Cape coast (Overberg) as the cutoff-low starts to intensify.
The moisture currently over the eastern half of SA is rapidly moving offshore, and today (Fri 24th) should be mostly dry over the central interior of the country (some showers over Gauteng, Mpumalanga and northern KZN).
The potential for snow on Sat 25th and Sun 26th has changed somewhat, with slightly less moisture in circulation over the central interior of SA. It will be as cold as predicted, with the freezing level between 2000-2500m in places. However, the peak rainfall is likely to be over KZN and the eastern Drakensberg (less over the Free State, and not so much over Lesotho). This can be seen on the updated snow-forecast.com outlook for the next 3 days: http://www.snow-forecast.com/maps/static/southafrica/next3days/snow
Attachments:July 25, 2015 at 10:58 am #93570
The latest model runs (GFS up to 00Z on Sat 25th) show significant reduction in the moisture in circulation over the interior of SA later today. Much less snow is likely to fall over parts of Lesotho and the Drakensberg than earlier predicted (though still substantial in places). Parts of KZN are likely to experience heavy rain, and some snow at quite low elevations (around 2000m) early tomorrow (Sun 26th).
Note: The EUMetsat satellite images have not updated since 1300Z on Fri 24th, so it is difficult to get an overview of the evolving COL.
Edit: In the absence of the EUMetsat animation, the SAWS satellite/radar composite image gives an overview of the current situation:
A major cold-front is likely to reach the SW-Cape next week (Wed 29th to Fri 31st). This will produce heavy rain over the Cape Metropole, followed by widespread snow at low elevations (below 1500m), and extending to most of the southern interior of SA (including the Drakensberg). I will post further details in a new thread topic tomorrow.
Attachments:July 26, 2015 at 3:03 pm #93574
Rapidly clearing skies this morning (Sun 26th) allow an excellent view of the fresh snow that fell over widespread areas of Lesotho, the Drakensberg, and parts of KZN and the Eastern Cape. This Terra Modis view uses false-colour to show the snow as pale-blue to help distinguish it from low clouds (shown in white). See: http://rapidfire.sci.gsfc.nasa.gov/imagery/subsets/?subset=Maloti_Drakensberg.2015207.terra.721.500m
A wider view also shows snow over mountains in the central Karoo (Graaff Reinet, etc). See: http://rapidfire.sci.gsfc.nasa.gov/imagery/subsets/?subset=SERVIR_Africa_South.2015207.terra.721.2km
Edit: The Aqua Modis (afternoon) view shows clearer skies, though a lot of the snow at the lowest elevations has melted: http://rapidfire.sci.gsfc.nasa.gov/imagery/subsets/?subset=Maloti_Drakensberg.2015207.aqua.500m
Edit: Compare the view of similar snow a week ago (Fri 17th), covering much the same area, though today’s falls are much ‘thicker’ (particularly at high elevations). See: http://www.stormchasing.co.za/wp-content/uploads/2015/07/Maloti_Drakensberg.2015198.aqua_.500m.jpg
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