As predicted several days ago, the past few days have
June 20, 2015 at 11:59 am #93408
Numerous recent GFS model runs (up to 00Z on Sat 20th) have shown a complex series of weather events covering most of next week, and extending over a significant part of South Africa. The model runs have been rather fickle (particularly at longer durations), and it has only now become possible to give a coherent description of the whole system.
The main driver of this event is a powerful cold-core upper-trough, which will first bring cold-conditions to the Western Cape, with snow on Wed 24th and Thurs 25th. Part of this cold air will detach and form a closed upper circulation far over the interior of South Africa by Fri 26th. During Thurs 25th there will be minimal moisture over the interior, and negligible snowfall. During Fri 26th the developing cutoff-low will start to bring tropical air (down from Mozambique) into circulation. This has the potential to produce snow over Lesotho and parts of the Drakensberg, extending into Sat 27th.
The freezing level during this event has been very difficult to determine, but it seems it will not be as low as initially hoped. During the first stages the SW-Cape will get heavy snow at the highest elevations, but not falling much below 1600-1800m. During the cutoff-low stage the freezing-level will probably be bewtween 2500-3000m, which would produce heavy snow over Lesotho and the Drakensberg.
The NCEP GFS 6-hourly and 7-day animations give a nice overview of the first few stages of this event. See:
The early events are pretty much like a normal cold-front (with snow), so there is no need to post detailed charts at this stage (see the NCEP ones). At longer durations, the scenarios are a bit more speculative, but worth examining. These MAG GFS charts show the potential cutoff-low and snow at 00Z on Sat 27th (168 hours or 7 days ahead).
As usual I will post more specific details as this event unfolds.
Attachments:June 20, 2015 at 12:38 pm #93412
I wasn’t expecting to post an update to this thread so soon, but the GFS model has the ability to throw up bizarre and fascinating outcomes. I don’t expect this particular scenario to actually come true, but the 06Z GFS run (Sat 20th) shows snow over the southern Kalahari Desert in Botswana during the morning of Fri 26th!
Attachments:June 21, 2015 at 8:38 pm #93421
During the last 24 hours the GFS model runs (up to 12Z on Sun 21st) have shown several interesting scenarios, though there is still little consistency at longer durations. Very heavy rainfall (> 50mm/day) is possible over the central and eastern parts of South Africa during the cutoff-low (Sat 27th), but the timing, location and amount are impossible to specify this far in advance.
The passage of the detached cold-core upper trough over the western interior of South Africa on Thurs 25th, has the potential for further freakish outcomes, with the possibility of (very light) snow as far north as Central Namibia (near Windhoek)! This is indicated by both today’s 06Z and 12Z GFS runs, though the amount of moisure in circulation will be limited, and only traces of precipitation (if any) are possible.
Subsequent model runs are likely to flip-flop further, but eventually the outcome at longer durations should become clearer. At that stage I will post further charts covering the development of the cutoff-low, and the potential for snow over the eastern half of South Africa during Fri 26th and Sat 27th.
Attachments:June 22, 2015 at 7:48 am #93424
This morning’s GFS model runs (18Z and 00Z on Mon 22nd) have shifted the location, and considerably reduced the intensity and amount of moisture in the cutoff-low over the interior of SA on Sat 27th (compared to yesterday’s predictions). Moderate to heavy rains (25-50mm) are shown for the normally dry parts of the Northern Cape, Free State and parts of the NW Province, starting on Fri 26th. This scenario would produce much less rainfall over the E-Cape (none over KZN), and significantly less snow over Lesotho (at the highest elevations only). See: http://www.cpc.ncep.noaa.gov/products/african_desk/cpc_intl/safrica/24h_precip.html
There is still time for the models to ‘flip-flop’ back again, but the massive amounts of snow previously predicted are unlikely to occur. These kind of fluctuations are to be expected at longer durations, and COLs are notoriously hard to predict in advance.
There is still scope for snow near Windhoek early on Fri 26h (though not shown on the latest charts), since the slow-moving cold-core upper trough will spend a protracted length time over southern and central Namibia during this event. The NCEP 500hPa teemperature animation gives and excellent overview of this development (and the gradual weakening of the COL). See: http://www.cpc.ncep.noaa.gov/products/african_desk/cpc_intl/safrica/temp500.html
All of the NCEP GFS animations that I posted at the start of this thread should update this morning, giving a revised view of the likely outcome(s), and today’s 6-hourly charts will cover up 24Z on Sat 27th (the final day of the COL).
Beyond the end of this event, another moderate intensity cold-front is likely to reach the SW-Cape on Sun 28th (shown on the 7-day charts). This frontal system has the potential to bring strong winds, rain and some snow (to coastal areas only), but the outlook is still somewhat uncertain. As usual I will post further details later.June 22, 2015 at 9:21 am #93433
thanks Gordon, looks interesting!June 23, 2015 at 6:27 am #93439
This morning’s satellite and synoptic charts (Tues 23rd) show the approaching frontal system, due to make landfall over the SW-Cape later today: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/ANTARCTIC/
Recent model runs (GFS 18Z and 00Z on Tues 23rd) have shown a much more disappointing long-range outlook, compared to the scenario when this thread first started. The detaching cold-core upper-trough is predicted to push far to the north and west on Thurs 25th, when dry air will be in circulation. The system will slowly pick up tropical mositure, and there is a chance of snow over central Nambia early on Fri 26th.
By Sat 27th the developing cutoff-low will have reached moderate intensity over the southern Kalahari Desert, bringing some rain (25-50mm) over parts of this normally dry area. By Sun 28th the trough will fizzle out over the central interior of South Africa. Widespread cold conditions are likely during this period.
At the end of this period another moderate intensity cold-front will reach the SW-Cape (Sun 28th).
I will post further updates, but at present there isn’t much ‘news’ worth showing in the charts.June 23, 2015 at 10:32 am #93441
Here is the SA Weather Office response from PE at 9:24am:
Models have changed again
Was for some snow in Barkley area
Models dropped it now
Daily DispatchJune 24, 2015 at 1:17 pm #93446
The SAWS reports significant rainfall totals in coastal parts of the SW-Cape during the 24 hours to 8am on Wed 24th: http://www.weathersa.co.za/media/data/climate/nr_daily_rai.pdf
Highest include Kirstenbosch 52mm, Franschhoek 30mm, Paarl 30mm, Strand 30mm, Jonkershoek 27mm, etc.
Snow is already likely to be falling this morning (Wed 24th), but only at the highest elevations (above 1800m). The freezing level will decline slowly, reaching 1600m by this evening, though moisture in circulation will be limited. Some snow is likely below 1500m early tomorrow morning (Thurs 25th) over the SW-Cape coastal mountains (Hottentots Holland), as well as a light dusting further inland (Sutherland).
Today is the ‘first’ day of this event, and while rainfall over the SW-Cape will diminish, the system is likely to (slowly) unfold over the interior of South Africa, with a peak intensity on Fri 26th and Sat 27th. I will post further updates closer to the time.June 25, 2015 at 6:57 am #93447
Just a quick update this morning (Thurs 25th). No further snow is likely over the Western Cape during this event (due to lack of moisture), though it will remain bitterly cold for the next several days. A light dusting of snow (down to 1500m elevation) may be visible at first light this morning on the Boland mountains, and further north in the Cederberg.
Edit: No snow at first light (7.15am) on the Sutherland webcams, and unlikely to get any this time.
Edit #2: The predicted maximum temperature for Cape Town today is 14C (the coldest I can remember in a while). It was 6C earlier this morning, and it is still partly cloudy and damp, with the Boland mountains hidden by persistent clouds.
Some technical detail: The temperature at the Dwarsberg high elevation weather station (1215m altitude) is currently 1.2C (6am on Thurs 25th). While traces of snow are certainly possible, no significant precipitation (> 1mm) has been recorded during the last few hours (since 2am). See: http://www.saeon-fynbos.org/#!dwarsberg-high-elevation-weather/clr8
Edit #3: The SAWS reports more widespread but less intense rainfall during the 24 hours to 8am on Thurs 25th: http://www.weathersa.co.za/media/data/climate/nr_daily_rai.pdf
Highest include Tulbagh 26mm, Nieuwoudtville (NC) 21mm, Strand 19mm, Worcester 16mm, Ceres 14mm, etc.
The cold-core upper-trough is currently moving north along the western coast of South Africa, but is almost ‘invisible’ due to lack of moisture in circulation. The cold and dry air is shown as a large area of (cloud free) deep reddish-brown on the EUMetsat Airmass animation: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/SOUTHERNAFRICA/
The cutoff-low will start developing over the southern part of Namibia during Fri 26th, then picking up tropical moisture and producing heavy rains over parts of the central interior of SA during Sat 27th, before finally disspiating as it exits via KZN on Sun 28th (as previously discussed). It will be very cold during this period, but conditions for snow (away from the mountains) are marginal at best.
Snow is highly likely over the high ground of Lesotho and the Drakensberg during Sat 27th, but the freezing levels and amount of moisture in circulation are still difficult to determine in advance. I will post further upodates closer to the time.June 26, 2015 at 8:32 am #93459
Moderator: Interesting that I was looking at the MAG GFS charts when your message came through. The chart of 500hPa isolines (together with vorticity) at 06Z on Fri 26th almost perfectly matches the rotation of the developing cutoff-low shown in the EUMetsat image! There are a lot of charts that I examine daily, and this one is particularly helpful for COLs!
Attachments:June 26, 2015 at 10:19 am #93463
700hpa with the wind and temp overlay also shows it nicely.
quick 24hour giff of the developing col
Attachments:June 27, 2015 at 8:53 pm #93466
The SAWS reports moderately heavy rain in parts of the normally dry Northern Cape during the 24 hours to 8am on Sat 27th: http://www.weathersa.co.za/media/data/climate/nr_daily_rai.pdf
Highest include Vanwyksvlei 25mm, Postmasburg 24mm, Brandvlei 21mm, etc.
Further heavy rain is likely to have fallen over the central Karoo during today. The outlook for snow remained consistent in the last two days forecasts, and snow was reported this afternoon from places (above 2500m) in the Eastern Cape and Lesotho.
As predicted, the cutoff-low passed (fairly rapidly) over the central interior of South Africa. This morning’s Terra Modis overpass gives a nice overview of this large weather system: http://rapidfire.sci.gsfc.nasa.gov/imagery/subsets/?subset=SERVIR_Africa_South.2015178.terra.721.2km
Note: In this false-colour version, cold clouds tops (ice) are shown as pale blue (above the warmer clouds shown in white).June 28, 2015 at 12:11 pm #93467
Thie morning (Sun 28th) conditions have cleared over most of the Eastern half of South Africa, and the snow over Lesotho and the Drakensberg is visible from all directions.
The geostationary EUMetsat visible (false-colour) image was the first to show the widespread snow, though this is low-resolution and only gives a glimpse of the coverage.
The Terra Modis overpass was very poor alignment, but shows a lot of snow! The Aqua Modis (afternoon) overpass should be better alignment and give greater detail, though some of the low-lying snow will have melted by then.
I will post this image when it becomes available. See: http://rapidfire.sci.gsfc.nasa.gov/imagery/subsets/?subset=Maloti_Drakensberg.2015179.aqua.500m
Note: Both images use false-colour, which helps distinguish snow and ice from lingering low-level clouds (shown in white).
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