Cold front SW-Cape 4-5 Aug 2011 - Storm Chasing South Africa

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Cold front SW-Cape 4-5 Aug 2011

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TOPIC: Cold front SW-Cape 4-5 Aug 2011

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#5170

Cold front SW-Cape 4-5 Aug 2011 9 Months, 3 Weeks ago

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Several model runs (up to 06Z on Fri 29th) indicate the potential development of a severe cold-frontal system making landfall on the SW-Cape on Thurs 4th and Fri 5th.

As usual it is not possible to make specific predictions this far in advance (7 days or 168 hours), but several model runs indicate the possibility of gale-force winds, heavy rain, and snow.

I will update this thread, as more information becomes available, closer to the expected time.

gordonr

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#5171

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 3 Weeks ago

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All of the recent model runs (18Z and 00Z on Sat 30th), plus the SAWS forecast, consistently show a significant cold-frontal system, and sharp upper-trough, arriving at Cape Town on Thurs 4th.

It is still too soon to make detailed predictions (6 days or 144 hours in advance), but conditions are certain to be very cold, wet and windy. Snow is likely above 1800m (and possibly a lot lower) on Fri 5th.

Edit: The GFS ensemble still shows considerable uncertainty in the position and intensity of the rainfall, but there is no doubt that something will arrive by Fri 5th:

Edit #2: Some of the model runs (up to 12Z on Sat 30th) indicate the possible formation of a cutoff-low on Sat 6th, but this is very sensitive to the position of the South Atlantic High (and cannot be predicted far in advance). This scenario could produce more intense but localised rain, together with less cold and snow.

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gordonr

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Last Edit: 2011/07/30 20:02 By gordonr.Reason: Cutoff-low Sat 6th.

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#5176

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 3 Weeks ago

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The last few model runs (GFS and NOGAPS up to 00Z on Mon 1st), as well as the SAWS forecast, have all started to converge. The most likely scenario is a single moderately strong cold-front reaching Cape Town later on Thurs 4th, and moving fairly quickly along the southern Cape coast on Fri 5th. (The formation of a cutoff-low seems unlikely at this stage).

This GFS chart gives a rough idea(!) of the likely overnight rainfall distribution, as well as the freezing level, with widespread snow likely in the SW-Cape early on Fri 5th (moving east later in the day).

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gordonr

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Last Edit: 2011/08/01 10:11 By gordonr.Reason: Typo/clarify.

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#5188

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 3 Weeks ago	Karma: 8
Been sinking about getting out this weekend. Any update on rain/snow area for the weekend in the West cape? Moderator: I will post a more detailed chart on Wed 3rd, giving freezing-levels, etc, but so far it looks like Xmas in August Expect to see low-lying snow (1500m) during Fri 5th, with high-lying snow (1600-1800m) persisting all weekend (up to Sun 7th).

Hannes

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Last Edit: 2011/08/02 12:28 By gordonr.Reason: Moderator: Followup.

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#5189

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 3 Weeks ago

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The SAWS has just put out the following warnings for the Western Cape (as well as parts of the Northern Cape and Eastern Cape).
www.weathersa.co.za/web/Warns.asp

Valid 03 August 2011: WATCH: ------
1. Heavy falls of rain are expected in places in the Overberg District on Thursday overnight spreading to the Eden District on Friday.
2. Snowfalls are expected on the high grounds on Thursday overnight through to Friday.
3. Very cold conditions are expected on the southern Northern Cape on Friday. ADVISORIES: -----------
1. Very rough seas with wave heights of 4m to 6m are expected between Alexander Bay and Plettenberg Bay on Friday.

gordonr

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#5190

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 3 Weeks ago	Karma: 4
here is the rainfall prediction- looks good - not that we need more rain in the East Cape !!! This image is hidden for guests. Please login or register to see it. The snow forecast also looks good. This image is hidden for guests. Please login or register to see it. better on Friday afternoon This image is hidden for guests. Please login or register to see it.

Gawie

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Last Edit: 2011/08/02 17:59 By Gawie.

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#5191

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 2 Weeks ago

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In the last few hours (up to 06Z on Wed 3rd) the frontal system has started to consolidate (previously it was just a smudge of clouds), and is clear on the EUmetsat Antarctic sector airmass animation (it is not yet on the Southern Africa sector).
oiswww.eumetsat.org/IPPS/html/MSG/RGB/AIRMASS/ANTARCTIC/

Recent model runs (up to 00Z on Wed 3rd) have 'wobbled' slightly, with the trend towards more widespread rain, and slightly higher snow elevation. It is rather difficult for the models to deal with this kind of compact and rapidly evolving frontal system!

The SAWS Aerosport spot-chart for 34S 19E (near Jonkershoek) shows the freezing-level down to 1400m by 24Z on Thurs 4th:
old.weathersa.co.za/Aerosport/ShowSpotGr...ODELD2.jsp?img=-3419

Hannes: Those are very nice snow charts, but it is hard to predict the freezing-level 72 hours in advance! (I am not sure of the source, but they look like something I saw from the SAWS models). The overall trend is very close to what I was expecting, with the terrain layer being very helpful!

Update: The 10Z SAWS Aviation TAF (Wed 3rd) shows the front making landfall in the middle of the day on Thurs 4th.

gordonr

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Last Edit: 2011/08/03 12:15 By gordonr.Reason: Aviation TAF.

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#5192

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 2 Weeks ago

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The 06Z GFS run (Wed 3rd) continues to show widespread rain, with some snow in the SW-Cape overnight on Thurs 4th into Fri 5th. Later on Fri 5th heavy rain is likely along the Eastern Cape coast, with snow on the highest peaks (not as low as the charts Hannes posted).

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gordonr

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Last Edit: 2011/08/03 13:03 By gordonr.Reason: 2nd chart.

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#5194

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 2 Weeks ago

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Hey G, charts were posted by Gawie, and I quite like them

They give me general landmark mountains that I can watch closer.

Give some indication on snowfall expectation around Sutherland plz. My goal is to drive in falling snow flakes (not ice rain) and the only likely place close to Cape Town would be Sutherland.

Moderator: Oops, sorry, wrong person!

I don't know much about conditions in Sutherland. It will certainly be cold enough, but I'm not sure how far north the moisture will go. In any case the rain will have moved east by Friday night, if that is your time-frame.

Moderator #2: The SAWS has put out the following warning for the Northern Cape (which includes Sutherland), so I think this answers your question:
Valid 05/08/2011: Watch: ------
1. Snowfalls are expected on the southern and southwestern highgrounds on Friday.

P.S. If the freezing level is below 1400-1500m there are several roads around Ceres that would get snow (and are closer than Sutherland).

Hannes

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Last Edit: 2011/08/03 21:44 By gordonr.Reason: Moderator: Followups.

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#5195

Re: Cold front SW-Cape 4-5 Aug 2011 9 Months, 2 Weeks ago

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According to the 1stweather.com website, Cape Metropole and Winelands can expect rainfall of between 50mm & 75mm and up to 100mm in the Overberg region for the twenty four hour period ending 02h00 on Friday the 5th August 2011.

How accurate is the rain prediction? If this is true, this will come as a huge relief after a dry July!!

Moderator: The 1stweather.com rainfall charts have a known 'error', particularly when short-term rainfall rates exceed 50mm/day over a 6 hour period.

IMO the cumulative rainfall totals are likely to be in the 25mm range, but potentiallly a lot higher than this near the mountains.

The SAWS was showing 60mm for Grabouw on Fri 5th, but now is showing 32mm. The SAWS is now showing 60mm for Stilbaai and Riversdale (and 55mm for Cape Agulhas).

Moderator #2: 100mm of rain in 24 hours wouldn't be a 'relief', it would be a disaster...

Michael123

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Last Edit: 2011/08/03 17:27 By gordonr.Reason: Moderator: Followup.

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