Rain and storm for Cpt

Gordon_R

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Mould growing on the satellite picture? Interesting thought.
LOL! The daytime satellite images use false-colour, with red and blue as normal, but with green replaced by short-wave infrared. This makes growing vegetation stand out, since it only absorbs certain wavelengths of light, and reflects the rest, as does shallow water. True-colour is rather boring, since most vegetation does not appear strongly in green, since all the light in the image has to pass through the atmosphere, and is dominated by blue haze.

Edit: You can play with other wavelengths here, and look at closer images than the geostationary weather satellites: https://worldview.earthdata.nasa.gov/?p=geographic&l=MODIS_Terra_SurfaceReflectance_Bands143,MODIS_Aqua_SurfaceReflectance_Bands143(hidden),MODIS_Terra_SurfaceReflectance_Bands721(hidden),MODIS_Aqua_SurfaceReflectance_Bands721(hidden),Reference_Labels(hidden),Reference_Features(hidden),Coastlines&t=2019-08-01-T00:00:00Z&z=3&v=11.637184391644837,-37.679766860214855,25.916767724978172,-30.674884047714855
 
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Segg

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LOL! The daytime satellite images use false-colour, with red and blue as normal, but with green replaced by short-wave infrared. This makes growing vegetation stand out, since it only absorbs certain wavelengths of light, and reflects the rest, as does shallow water. True-colour is rather boring, since most vegetation does not appear strongly in green, since all the light in the image has to pass through the atmosphere, and is dominated by blue haze.

Edit: You can play with other wavelengths here, and look at closer images than the geostationary weather satellites: https://worldview.earthdata.nasa.gov/?p=geographic&l=MODIS_Terra_SurfaceReflectance_Bands143,MODIS_Aqua_SurfaceReflectance_Bands143(hidden),MODIS_Terra_SurfaceReflectance_Bands721(hidden),MODIS_Aqua_SurfaceReflectance_Bands721(hidden),Reference_Labels(hidden),Reference_Features(hidden),Coastlines&t=2019-08-01-T00:00:00Z&z=3&v=11.637184391644837,-37.679766860214855,25.916767724978172,-30.674884047714855
Wow that's very interesting, and clever! I was wondering why the different sources I use had such varying images - case closed
 

Gordon_R

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Confirmation of another good rise in the average CoCT dam levels, up 4.7% during the last week to 76.8% on 5 Aug 2019: http://resource.capetown.gov.za/documentcentre/Documents/City research reports and review/damlevels.pdf

Since the Berg River Dam is 100% full, the main laggard is Theewaterskloof, which rose 4.9% to 67.0%.

At current rates, the total is almost certain to exceed 80% by the end of August, and potentially exceed 85-90% by the end of the rainfall year in October.

It has been raining on and off today (Mon 5th), unlikely to have much effect on the dam levels, but just enough to make things miserable. Similar weather is expected on a few days during this week.
 

genetic

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At current rates, the total is almost certain to exceed 80% by the end of August, and potentially exceed 85-90% by the end of the rainfall year in October.
Is this due to good rains or stringent water restrictions. I still feel like it's been a pretty dry winter so far, with a few days of good rain.
 

Gordon_R

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Is this due to good rains or stringent water restrictions. I still feel like it's been a pretty dry winter so far, with a few days of good rain.
A bit of both! If you look at the chart in the link I just posted, the draconian water restrictions through the summer of 2018/19 were enforced long after the critical point was passed, probably in fear of another dry year. This means we started the winter rainfall season at about 45%, the highest average dam levels in 4 years.

Since then anything above an absolute minimum rainfall pushed the totals up to new highs. To go from 45% to 75% during the middle of winter is pretty average.

We did have a relatively late start to winter, but caught up with a few very wet days, mostly over the catchment areas. BTW, the dams count the total inflow, not the number of rainy days (or nights).

Many places away from the mountains have indeed had below average rainfall, and agriculture remains under strain.

Edit: A few other figures: https://www.news24.com/SouthAfrica/News/western-cape-dams-more-than-61-full-the-best-in-4-years-20190805
The total average level for dams across the province is 61.2%, the best it's been in four years.

"Two years ago, on 7 August 2017, the average dam level for the province was 28%. The Theewaterskloof Dam, at that stage, was only 22% full.
Clanwilliam Dam 91.6%. (2018: 99.2%, last week: 71.6%)
 
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Gordon_R

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We had a considerable amount of rain near the mountain last night, with 30mm+ in our gauge in Kenilworth (faulty reading), and 38mm at my cousin's house in Rondebosch.

The SAWS report for the 24 hours to 8am on Tues 6th shows heavy rain for Kirstenbosch (40mm), but very little (< 10mm) elsewhere: http://www.weathersa.co.za/images/data/climate/nr_daily_rai.pdf

Edit: Average dam levels for CoCT keep rising steadily, up 0.7% to 77.5% this morning: http://www.capetown.gov.za/Family and home/residential-utility-services/residential-water-and-sanitation-services/this-weeks-dam-levels

More widespread rain is likely during the middle of the day tomorrow (Wed 7th).
 
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Gordon_R

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A sharp cold-front arrived over Cape Town around 11am this morning, producing a period of very heavy rain, and localised flooding. Our gauge in Kenilworth showed 12mm during a period of less than 2 hours.

Further showers are likely this afternoon, along with gusty conditions, and very cold temperatures as the wind direction switches to south-westerly.

The satellite image shows how well defined the frontal boundary was:

MET9_721-southernAfrica_20190807_0900[crop_25X2].jpg

Source: https://eumetview.eumetsat.int/static-images/MSG/RGB/NATURALCOLOR/SOUTHERNAFRICA/
 

Gordon_R

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After a fine day today (Sat 10th), another moderate intensity cold-front is forecast to pass over Cape Town tomorrow (Sun 11th). I wouldn't normally create a post for such a weak storm, but it is a long weekend, and some people will be disappointed by the weather.

Given all the rain and unsettled weather we have had recently, it will be nice to see some sunshine. Winter is not over yet, but the next 7-10 days are predicted to be significantly drier, with no major cold-fronts in the foreseeable future.
 

air

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Absolutely bucketing here in Gardens/CPT. Jonkershoek already past 20mm for today, long may it last.
What I find exceptional is how quickly Clanwilliam has filled up. Does anyone have an idea on when the construction to increase capacity will complete?
 

Gordon_R

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Absolutely bucketing here in Gardens/CPT. Jonkershoek already past 20mm for today, long may it last.
What I find exceptional is how quickly Clanwilliam has filled up. Does anyone have an idea on when the construction to increase capacity will complete?
Edit: 8.5mm overnight in gauge here in Kenilworth.

Gap in the rain for a few hours, then more showers this afternoon.

The Clanwilliam Dam is a 4 year project, according to this report: https://www.iol.co.za/capetimes/news/clanwilliam-dam-project-to-bring-new-dawn-17485681
 
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Gordon_R

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The updated dam levels for CoCT area just reached 80.6%, up 3.8% last week, even with very limited rainfall:
http://www.capetown.gov.za/Family and home/residential-utility-services/residential-water-and-sanitation-services/this-weeks-dam-levels

Edit: The 5 year and 10 year comparisons are also encouraging: http://resource.capetown.gov.za/documentcentre/Documents/City research reports and review/damlevels.pdf

Theewaterskloof is still lagging, but has reached 70% (compared to 43% last year).

Chart of Berg River Dam inflows for the last 14 days: http://www.dwa.gov.za/Hydrology/Unverified/DetailStageFlow.aspx?Station=G1H076FW&Type=Flow&Rain=N
 
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Gordon_R

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There was some light rain around Cape Town overnight, mostly near the mountains. Our gauge in Kenilworth had 5mm this morning (Wed 14th). The SAWS reports 13mm for Kirstenbosch, but only 2mm for CT airport.

The SAEON Dwarsberg mountain catchment station had significant falls, most of which should run off into the reservoirs. Since the Berg River Dam is already overflowing, incremental increases in the other big dams will be more gradual. I estimate another 1-2% rise this week, in spite of the limited rain.

The outlook for the foreseeable future remains dry, with no significant cold-fronts expected in the next 7-10 days.

Update: Average dam levels up 1.2% to 81.7% during the week to Mon 19th Aug: http://www.capetown.gov.za/Family and home/residential-utility-services/residential-water-and-sanitation-services/this-weeks-dam-levels

Edit: Everything else as previously noted: No rain foreseeable, and Theewaterskloof definitely the last to fill up.
 
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SAguy

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An interesting statistic would be to see the impact of population on total days of water available.
20 years ago 100% full CoCT dams would have probably held enough water for double the amount of days that it does now.
 

air

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After a promising first 2 weeks of August, the remainder of the month has been a damp squib :-(

This week will be the first week in the past 11 weeks where we do not have a gain for the big 6.
Last year, we achieved an approx. 14% uplift in the 5 weeks from late August, hoping we can do the same this year!
 

genetic

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No decent rain expected for the next 10 days. Very dry August it seems.
 

Gordon_R

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No decent rain expected for the next 10 days. Very dry August it seems.
Not much rain (most of us have had enough!), but dramatically cooler conditions than the relatively sunny and mild weather we had last week...
 
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