Rain and storm for Cpt

[Agent_Smith]

Berg River dam was looking good on Saturday from what I saw at the Bastille Day trail run. Some good (although not torrential) run off into the dam.

 

[MrGray]

According to Windguru it peaked this morning and it will gradually calm down until Monday evening for another round... Sunday might be bearable.

 

[Gordon_R]

The SAWS reports moderate rainfall over various places around the Cape Metropole during the 24 hours to 8am on Fri 19th: http://www.weathersa.co.za/images/data/climate/nr_daily_rai.pdf

Highest include Ceres 52mm, Grabouw 33mm, Paarl 30mm, Jonkershoek 23mm, Kirstenbosch 22mm, Villiersdorp 22mm, etc. Places away from the mountains received much less rain.

There was 20mm in our gauge in Kenilworth this morning.

Inflow into the Berg River Dam overnight was good, around 1% increase up to 8am:
http://www.dwa.gov.za/Hydrology/Unverified/DetailStageFlow.aspx?Station=G1H076FW&Type=Flow&Rain=N
http://www.dwa.gov.za/Hydrology/Unverified/DetailStageFlow.aspx?Station=G1R004FW&Type=Flow&Rain=N

Other CoCT dam levels will be available later: http://www.capetown.gov.za/Family and home/residential-utility-services/residential-water-and-sanitation-services/this-weeks-dam-levels

Edit: Berg River Dam up 1.6% in 24 hours, total dams up 0.7% in 24 hours.

The next major cold-front is set to be a big one, with strong winds, heavy and widespread rain, very cold and significant snow on Tues 23rd!

 

[Gordon_R]

Still some uncertainty in the model runs, but this is pretty much what it will look like on Tues 23th:



Edit: Typo 23rd...

 

[Geoff.D]

Today's pic of the same potential forecasted system for the 23rd:

 

[The_Ogre]

Still some uncertainty in the model runs, but this is pretty much what it will look like on Tues 28th:

Don't you mean 23rd?

 

[Gordon_R]

Today's pic of the same potential forecasted system for the 21st:
View attachment 686047

The storm that has been circulating to the W of CT will almost completely dissipate, and become absorbed in a new larger storm. Snow and severe weather always comes from the SW up from the Antarctic, whereas a system that has lingered for several days over the Atlantic will never amount to much. It is hard to show a full animation of developments, since most of the power for the storm will be a cold-core upper-trough, and no single chart can show all of this detail.

A good intermediate image is this chart from early on Mon 22nd:



Edit: Partial animation showing surface isobars and rainfall: https://www.cpc.ncep.noaa.gov/products/international/cpci/data/00/gfs_mslp_precip_safrica.html
Animation showing the 500hPa temperature profile: https://www.cpc.ncep.noaa.gov/products/international/cpci/data/00/gfs_500mb_tmp_safrica.html

 

[Geoff.D]

How to get the above pics:
Choose this url: https://mag.ncep.noaa.gov/model-gui...odel Guidance&model=GFS&area=AFRICA&ps=model#

Follow the instructions:


Read up on how this all works here:
https://mag.ncep.noaa.gov/help/ModelGuidance.php

 

[Gordon_R]

How to get the above pics:
Choose this url: https://mag.ncep.noaa.gov/model-guidance-model-parameter.php?group=Model Guidance&model=GFS&area=AFRICA&ps=model#

Follow the instructions:

Read up on how this all works here:
https://mag.ncep.noaa.gov/help/ModelGuidance.php

You can turn on animation for those images, but because of the 1 hour time interval you will have to load more than 100 images into your computer memory, which is beyond the scope of older PCs and smartphones.

 

[Geoff.D]

You can turn on animation for those images, but because of the 1 hour time interval you will have to load more than 100 images into your computer memory, which is beyond the scope of older PCs and smartphones.

I have just pushed my PC to its limits to see what would happen. And --- the animation worked! Was rather surprised. Have always in the past ended up "paging through" the images or making guesses. Now I can jst run the animation and stop it where it becomes interesting.

 

[Gordon_R]

For several days the models have predicting the approach of a big storm, but only this morning (Sun 21st) are the main components visible on the satellite image (previously they were halfway around the world). The big low that has been sitting to the west of Cape Town will move harmlessly to the south. The driving factor will a large mass of very cold air pushing up from the Antarctic. During Mon 22nd this will trigger development of a deep surface low and powerful cold-front, which will make landfall during Tues 23rd (as shown on previous charts).



Source: https://eumetview.eumetsat.int/static-images/MSG/RGB/AIRMASS/ANTARCTIC/

 

[genetic]

Been a pretty dry July.

 

[Rouxenator]

Doc you need to break it down into terms a moron like I can understand. So what you're saying is tomorrow I can take the bike but now Tuesday?

 

[Craig_]

Doc you need to break it down into terms a moron like I can understand. So what you're saying is tomorrow I can take the bike but now Tuesday?

My wife said you are crazy the other day when you came past us by the gate in the dark. It was moer cold.

 

[Rouxenator]

These days I have to pick any dry day I can get. But Saturday out on Bottelary Hills was proper cold!

The ride to work is only cold the part into town and that bit from Paradyskloof to Jamestown. The town itself does have a bit of heat. I do layer up and wear a beanie under my helmet and proper MTB gloves too.

 

[Gordon_R]

This morning's satellite image (Mon 22nd) shows the two systems approaching Cape Town. The first is a remnant of the lingering low, which will bring some wind and rain this afternoon. The second is still developing, and will bring much more stormy weather during tomorrow (Tues 23rd).



Source: https://eumetview.eumetsat.int/static-images/MSG/RGB/AIRMASS/ANTARCTIC/

 

[Gordon_R]

The exact timing of the rainfall is still hard to define, but the cumulative total for tomorrow (Tues 23rd) shows a peak distribution over the high ground of the SW-Cape, with lesser amounts further inland:



P.S. The SAWS has put out warnings on Twitter, though not yet on their website:

https://twitter.com/x/status/1152903727948410880

 

[Geoff.D]

My first post on this thread was about the way Lows develop mid-Atlantic and then slide past the country.
The AAO index has been negative for a while now but no significant weather fronts were around until the one in the mid-Atlantic, followed by the southern ocean system. The AAO index is again swinging positive so, this might, in fact, be the last gasp for this years. cape rainy season. Rather a poor showing compared to last year.

 

[AchmatK]

Haven't had any snow on the helderberg mountains this year yet. Still freezing cold in Somerset West though.

 

[Gordon_R]

My first post on this thread was about the way Lows develop mid-Atlantic and then slide past the country.
The AOO index has been negative for a while now but no significant weather fronts were around until the one in the mid-Atlantic, followed by the southern ocean system. The AOO index is again swinging positive so, this might, in fact, be the last gasp for this years. cape rainy season. Rather a poor showing compared to last year.

Another significant cold-front is predicted for Mon 29th.

BTW, I don't place much credence in long-range trends.