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July 6, 2015 at 8:23 pm #93473
After a relatively boring start in July, the SW-Cape will experience an extreme see-saw in conditions over the next 7 days. The SAWS aviation TAF (terminal forecast) for CT airport tomorrow (Tues 7th) shows an unseasonably warm maximum temperature of 27C! This warm-snap will be closely followed by a relatively weak cold-front overnight into Wed 8th, which will bring some rain, and a sharp drop in temperature to more wintery levels.
Another much stronger cold-front is likely to make landfall over Cape Town late on Sat 11th, bringing gale-force winds at first, heavy rain on Sun 12th, and snow over the SW-Cape mountains by Mon 13th. It may remain unsettled for most of the following week (up to Wed 15th), though the models are very unclear at this stage.
This upcoming event has been predicted by numerous model runs (GFS up to 12Z on Mon 6th), but only recently has the outlook converged sufficiently to give a consistent forecast. The latest charts show the freezing level below 1500m by 00Z on Mon 13th, which would bring widespread snow (though this may change over the next few model runs).
These MAG GFS charts show the likely scenario at 12Z on Sun 12th, as well as the relatively high concertainty in the GFS ensemble (tight clustering of the 500hPa spagehetti chart isolines).
As usual I will post further details closer to the time. Tomorrow’s NCEP GFS 7 day and 6-hourly animations should give a good overview of this event.
Attachments:July 7, 2015 at 8:52 am #93477
Did you check the 500hPa temperatures Gordon? -32C and very close during the 11th and 12th!
Moderator: I don’t usually look at the 500hPa temperatures, but they will certainly be very cold! See the NCEP links which I’m posting, where the scale only goes down to -24C.July 7, 2015 at 9:12 am #93479
The last 3 GFS model runs (up to 00Z on Tues 7th) consistently show the freezing level below 1500m later on Sun 12th, so we could be in for a pleasant surprise with snow at relatively low elevations!
The NCEP GFS animations give a good overview of this event:
There is also a possibility of heavy rain over Lesotho during Mon 13th, though this is much less certain.July 7, 2015 at 12:42 pm #93481
Just for interest sake. There is a small patch (20%) SE of Cape Town according to the thunderstorm probability chart on the SAWS aviation website.
Is this because of the warm conditions?
Moderator: Yep, there is an upper trough, with lots of altocumulus! There will be showers later, but I don’t expect any drama today (Tues 7th), since CAPE and LI are negligible…
Attachments:July 7, 2015 at 3:13 pm #93484
Some real nice mammatus developing here above century city.
Edit: and 5min later it’s gone. seemed to have been only a small patch.
Moderator: Awesome, thanks for posting! Also patchy in Kenilworth. Some virga (fallstreaks) but no rain yet.
Attachments:July 8, 2015 at 7:55 am #93488
The last few model runs (up to 00Z on Wed 8th) have ‘wobbled’, but continue to show a favourable outcome for the SW-Cape, as well as heavy rain (and snow) over some parts of the E-Cape.
There is still uncertainty in the exact time of arrival, freezing-levels, and amount of (tropical) moisture in circulation (over the E-Cape), but this is definitely going to be an interesting weather event!
Most of the NCEP GFS animations (previously posted) have updated, and give an ongoing overview of this event.July 9, 2015 at 11:17 am #93495
All of the recent model runs (up to 00Z on Thurs 9th) continue to show significant rain and snow for the W & S-Cape during Sun 12th. The outlook for the E-Cape is much less favourable, with rain likely in places, but the freezing level may not be low enough for snow.
The exact freezing-levels over the W-Cape will depend on timing and location, but good falls of snow below 1500m are likely over all of the SW-Cape mountains (Cederberg, Boland and Overberg) early on Sun 12th. The freezing-level will fall further (down to 1200m), and extend inland and eastwards later on Sun 12th. However, limited moisture in circulation will produce patchy snowfalls (Swartberge), and there may not even be any at Sutherland.
The potential development of a (weak) cutoff-low over the S-Cape coast on Mon 13th means that conditions will remain unsettled (cloudy and cold), but it is impossible to make detailed predictions beyond the weekend (at this stage).
All the NCEP GFS animations continue to update, and give further unfolding details of this event.
Edit: The EUMetsat Antarctic sector Airmass animation gives a nice view of the devleoping frontal system, and associated cold-core upper-trough (shown as deep reddish-brown), currently far to the SW of Cape Town. See: http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/ANTARCTIC/
Edit #2: The 1stweather.com forecast for Cape Town (based on the 06Z GFS run on Thurs 9th) shows maximum temperatures of 11C for both Sun 12th and Mon 13th!! See: http://www.1stweather.com/metrocast/capetown/index_extended.shtmlJuly 10, 2015 at 8:27 am #93499
Both the 18Z and 00Z GFS runs (Fri 10th) show increased possibility of snow over the E-Cape and southern Drakensberg on Mon 13th. However, this is critically dependent on tropical moisture flows as well as the exact freezing-level, both of which are difficult to determine 3.5 days (or 84 hours) in advance.
The outlook for snow over the SW-Cape during Sun 12th remains much the same. Finer details should become available tomorrow morning (Sat 11th), when the SAWS Aerosport spot-charts become available (which only show 24-48 hours in advance).
Edit: The SAWS ship synoptic chart shows a single well developed frontal system to the south-west of Cape Town (though not the assocated cold-core upper-trough). See: http://www.weathersa.co.za/media/data/observation/synoptic/ma_sy.gif
Edit: The maximum temperature for Cape Town today (Fri 10th) is likely to be around 24C, making the transtion to colder conditions over the weekend even more pronounced!July 11, 2015 at 1:02 pm #93501
All the recent model runs (GFS up to 06Z on Sat 11th as well as SAWS Aerosport charts) have converged nicely. A favourable outcome for this snow event is likely over both the W-Cape on Sun 12th, and the E-Cape and Drakensberg on Mon 13th.
The freezing-level during the morning of Sun 12th is likely to be down to 1250m, with widespread snow over the mountains close to Cape Town (Boland, Franschhoek, Hottentots Holland), as well around Worcester, Ceres and the Cederberg further north. After the main front passes, it should clear partially from the west, allowing the snow-capped mountains to be seen. During the day he snow will spread inland to southern parts of the N-Cape (Sutherland), and along the southern Cape mountain ranges (Swartberge).
After falling sharply at first, the freezing-level close to Cape Town will rise slightly later on Sun 12th. Another band of rain showers is likely to arrive overnight. These will produce further light snow at high elevations, but will melt any snow below 1400m. It will remain very cold for an extended period (until Tues 14th), but no further snow is likely for the W-Cape during this event.
I will post further details covering the E-Cape later.July 12, 2015 at 9:33 am #93502
This morning (Sun 12th) I awoke to bright sunshine in Kenilworth, and a light dusting of snow visible on the Sutherland webcams. That is a ‘metaphor’ for this event, with the freezing-levels sufficiently low, but rather limited moisture in circulation (even less than I expected). This is fine for snow ‘sightseeing’ today, but not ideal if you want to play in ground-level snow (such as Matroosberg).
Edit: I forgot to check the SAEON Dwarsberg (Jonkershoek) high-elevation weather site, located at 1200m ASL. The current temperature is -0.4C (10am), so its definitely snowing at that elevation!! See: http://www.saeon-fynbos.org/#!dwarsberg-high-elevation-weather/clr8
All the charts show a moderate intensity cold-front made landfall yesterday evening (Sat 11th), with most of the snow falling during the early hours of this morning. It will remain very cold, but only scattered light showers are possible during today. Slightly heavier falls are likely overnight (into Mon 13th), but mostly restricted to the extreme SW-Cape and southern Cape (nothing further north).
The outlook for snow over the E-Cape and Drakensberg tomorrow (Mon 13th) looks mixed, with light falls over the high ground (above 2500m).
The next major frontal system is likely to reach the SW-Cape during Fri 17th and Sat 18th. This looks more like a rain than a snow event (which we do need). I will create a new thread topic closer to the time.July 12, 2015 at 7:04 pm #93504
While not much further snow is expected (at least in the west), clear skies overnight will produce extremely cold conditions by tomorrow morning. This SAWS Aerosport chart for 05h00 on Mon 13th shows temperatures down to -8C over parts of the central Karoo (north of Beaufort West).
Attachments:July 13, 2015 at 9:15 am #93507
Great site btw.
Moderator: Thanks for the reminder about this site.
Moderator: Note that the URL link only points to the date/time selected (and only past events, not future forecasts!) To plot variables you need to select Airmass RGB and T(emp)500 (for example).
Attachments:July 13, 2015 at 5:24 pm #93510
There is still quite a lot of low cloud around today (Mon 13th), but this morning’s Terra Modis overpass gave a very nice view of the heavy snow (above 1500m) around Worcester, Ceres, etc. In this false-colour version, the snow is shown as pale blue (which helps distinguish it from low clouds). See: http://rapidfire.sci.gsfc.nasa.gov/imagery/subsets/?subset=FAS_SouthAfrica2.2015194.terra.721.500m
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