Rain and storm for Cpt

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Gordon_R said:
Numerous recent forecasts, on various independent models, all indicate a well developed cold-front reaching the SW-Cape on Fri 9th. Some predictions go as high as 20mm, but this may just be a fluke. Probably a good opporunity to replenish (but not fill up) rainwater tanks.

As usual, more rainfall (30-50mm) is always possible over the mountains, due to orographic effects. However, there will be very little runoff from the catchments, due to low soil-moisure content. Welcome yes, but do not expect this to have any substantial effect on dam levels.
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Agree. There seems to be very good chance of that front reaching into the western cape interior and thus the catchment areas. There is also a small chance that a cut off low will also develop ahead of that front.
 
I'm looking forward to the rain on Saturday - and trust me - there will be rain. No mater how dry summer is, it will always rain when it is the 99er cycle event ;)
 
Geoff.D said:
Agree. There seems to be very good chance of that front reaching into the western cape interior and thus the catchment areas. There is also a small chance that a cut off low will also develop ahead of that front.
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I'm still seeing very positive developments for the Cape Metropole (and the dam catchments). However the front looks likely to dissipate on landfall, and I'm not seeing any evidence of a COL developing.

The cold-core upper-trough will enhance orographic rainfall, but this will deplete the front of all its moisture, and unfortunately the interior of the W-Cape will receive nothing.

Potential rainfall during the 24 hour period on Fri 9th:
gfs_africa_120_precip_p24[20180209_0000-2400_120Z_crop].gif
 
Gordon_R said:
I'm still seeing very positive developments for the Cape Metropole (and the dam catchments). However the front looks likely to dissipate on landfall, and I'm not seeing any evidence of a COL developing.

The cold-core upper-trough will enhance orographic rainfall, but this will deplete the front of all its moisture, and unfortunately the interior of the W-Cape will receive nothing.

Potential rainfall during the 24 hour period on Fri 9th:
View attachment 496076
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Just out of interest - what are the signs of a cut off low developing, do I need to look at synoptic charts and have a trained eye to spot them?

Are there more cut off lows than before - I seem to remember them being fairly rare events in the 1980's and 1990's?
 
PeterBee said:
Just out of interest - what are the signs of a cut off low developing, do I need to look at synoptic charts and have a trained eye to spot them?

Are there more cut off lows than before - I seem to remember them being fairly rare events in the 1980's and 1990's?
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You cannot find a cutoff-low on a synoptic chart, since it is an upper-atmosphere development. You need to look at charts that combine both isobars, with 'so-called' 500hPa thickness. Such charts are publicly available, but not easily understood. (You would be better of waiting for someone like me to mention that one is coming, otherwise its all rumours...)

Cutoff-lows are the mid-lattitude equivalent of tropical-storms: relatively rare, potentially powerul, and almost impossible to predict with any accuracy, even 2-3 days ahead.
 
Couple of big drops in Stellenbosch, but hardly enough to wet the ground properly. Did not expect that.
 
Rouxenator said:
Couple of big drops in Stellenbosch, but hardly enough to wet the ground properly. Did not expect that.
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A weak upper trough is passing over the SW-Cape this morning. A smattering of rain is possible in places.

Since the convective cloud base is very high (altocumulus), only large raindrops are able to reach the surface (rather than evaporating).
 
Gordon_R said:
You cannot find a cutoff-low on a synoptic chart....Such charts are publicly available, but not easily understood. (You would be better of waiting for someone like me to mention that one is coming, otherwise its all rumours...)
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Thanks, and I accept your suggestion;)
 
air said:
propeller heads! How is THU/FRI/SAT looking for rain in CT area? :-)
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As per my prediction 10 days ago, the chances of good rain is high to certain. The duration though is not certain.
 
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The outlook for rain in CT on Fri 9th continues to look good. However, there is still significant uncertainty about how much rain will fall, whether it will persist into Sat 10th, and how far inland it will extend.

The event will mostly be a normal cold-front, but a strong upper trough will provide significant enhancement of rainfall, and rapid deepening just before landfall.

At the same time, the relative poistion of the upper trough and the surface front is difficult to predict, hence the lingering uncertainty in the model predictions.

Stay tuned!
 
daveza said:
ATTACH 496888
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Please do not post model charts unless you know what you are doing!

There are 3 things wrong with that chart (apart from the fact that it shows zero rain).
1. The original link shows two sections, the one quoted is for daytime only (8am to 4pm). If you look further down the page, most of the rain will fall after 4pm.
2. Every model, and every model run is different. Without showing the location, date of model run, etc, those kind of single data points, without context, are completely useless.
3. yr.no tends to show numbers at the lower range, compared to other models. In addition, almost none of the models show orographic effects, which are significant in suburbs near the mountains (Newlands). In addition, the catchment areas usually receive 2-3 times the amount of rain at CT airport.

This is a fairly complex weather event, and even 72 hours (3 days) ahead it is impossible to give a consistent outlook. One thing is for sure, I can guarantee that the rainfall will not be zero, so your post is misleading on multiple fronts.

daveza said:
Windy.com

Theewaterskloof.

ATTACH 496892
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Edit: Another misleading image. Theewaterskloof dam is in the rain-shadow of the Hottentots Holland mountain rainge. While the reservoir itself will receive minimal rain, the mountain catchment will receive significant amounts.

Edit: Unfortunately (as I have said before), runoff will be limited, due to low soil moisture (in the middle of summer). Do not expect any significant increase in the dam levels from this (or any similar) event, until late-May or early-June.
 
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Well this is Grabouw - do I dare presume if there is zip rain in Grabouw there's a likelihood of zip entering the dams...

gra.PNG
 
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