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Cape Floods - A Cut-Off Low To Remember

This post will be a mix of meteorological discussions about the system as well as my own personal experience with this storm, being stuck in the hardest hit area myself.

It started with a forum thread posted on Monday the 11th November. A simple post from a user saying "What is the status of the upcoming weather event for Cape Town on Friday, 15th November 2013? Some models indicated a good chance for thunderstorms and the possibility of heavy rains!!" It wasn't long before a discussion was revolving around this topic. Already on Monday the models were indeed painting a picture of a severe weather scenario being set up. Cape Town doesn't often see high lifted indexes and CAPE values, but at this stage models were suggesting LIs of -7 and CAPE in excess of 800 J/kg. LIs of -7 around Cape Town are almost unheard of and seeing the models portray that definitely perked some ears up. Unlike some summer cut-off lows that are driven by warm cores of a more tropical nature, this storm had a strong cold-core driving its energy, though was still bringing in warm moisture from far east.


On Tuesday the 12th, the models were continuing to show high instability and signs of very heavy rain in areas. Though at this stage the GFS model was forecasting the bulk of the rain to occur far east of Cape Town, towards the Garden Route. The forecast then shifted as model data on Wednesday showed the bulk of the rainfall shifted west towards Cape Town. It was later on Wednesday that the South African Weather Service sent out weather advisories and warnings for the weekend. The SAWS sent out the following relating to the weekend:

Moisture Flowing in to Cut-Off Low



WATCHES:
========

 


2. Heavy rain leading to flash flooding is expected in places over the
Western Cape Friday Night and Saturday

3. A gale force easterly to south-easterly wind of 65km/h is expected
in places along the coast between Cape Point and Cape Agulhas Friday
night and Saturday.

SPECIAL WEATHER ADVISORIES:
===========================

2 . Extremely hot temperatures are expected in places over Limpopo
Valley tomorrow(Thursday).

4. An intense cut-off low is expected over the southern and eastern
parts of the Eastern Cape. The public are advised that heavy rain
and localised flooding can be expected Friday night into Saturday.





The models then threw a spanner in the works when later on Wednesday the GFS shifted the main rainfall back towards the east, while the Navgem model did the opposite and shifted it towards the West Coast.


Things finally settled with regards to the models, on Thursday, with both the GFS and Navgem showing extremely high rainfall rates for most of the greater Cape Town area and parts of the Overberg. A forum post on this day included a graphic of the precipitable water values, which demonstrated well how this Cut-Off Low would be pulling in moisture from far to the east, mostly sub-tropical moisture. It is well known that Cut-Off Lows in the Cape pose the biggest risk of flooding, as the flow of moisture is generally longer lasting than a quick moving cold front and the rates of rain associated with the tropical moisture also tends to be heavier.

At this point, the models were still showing bulk of the rainfall occurring on Saturday and not many people were expecting for Friday to be the day with the most rainfall. Friday in Somerset West started off dry and breezy, with signs of towering cumulus around. The rain began here in Somerset West at about 16:00, as I left work I had to run into the store to buy dinner, when I left the rain was still coming down steadily, but there was no cause for alarm yet. I got home at about 16:30, and kept an eye on the rain. At 16:45 it started to bucket down, still no signs of cause for alarm, but I went outside and began filming the initial start of the downpour with my cellphone purely for documentation, with no idea of what was going to unfold over the next two hours.

An hour later, the rain was still coming down at a rate of 60mm an hour or more. And then came the thunder, frequent lightning with about 4 strikes visible per minute. I decided to get in my car and see if there was any flooding occurring. Some of the roads were already 10-15cm under water with water levels higher than the pavements in parts of Somerset West, specifically Lourensford Road. Due to the heavy rain and the road being under water I accidentally rode over a burst water main. It was then I decided to head back home, and it's then I got these shots of the beginning of what would end up being potentially the biggest flooding event this city has seen.

I arrived home just after 18:00 and it was still raining extremely heavily and I was now a victim of the weather - the ceiling of the one side of our house collapsed due to the heavy rain breaking the gutter and we were soon 15cm under water in that part of the house, which in turn tripped the power and left us in the dark.

 


When we got our power back on I logged online and began hearing the stories from friends in other areas of Somerset West and Strand talking about their situations. A friend in Strand had recorded 40mm in the first hour of the rainfall. Others near Bizweni were suffering the results of the Lourens River bursting its banks! Rain continued to belt down until about 10pm, along with the frequent thunder. Soon reports were coming in displaying just how bad the flooding was, the most shocking were the pictures coming from Medi Clinic Hospital near Bizweni, where the flooding has caused them to evacuate. There were other reports of people being stranded on their houses, and seeing some of the pictures that were coming in through social media at the time it wasn't surprising.

On Saturday morning I woke up early and headed out to see what was going on around town, if the water levels were still high. I went towards Bizweni, but the police had blocked the area as a car was stuck in a pool of water from the night before. The houses appeared to have at least 2 feet of water, half way up their windows the night before, but had now subsided to about one foot. Lourens River was still very much over its maximum, and flowing heavily. I then went towards Strand and despite extremely heavy flooding in Beach Road the night before, the water levels had diminished greatly by the time I was there. Flooding was still present in areas though, and I had to change my route when I ran into a large pool of water near Gantz Center. There were police and emergency rescue services throughout the streets. In a 15 hour period from 16:00 on Friday to 08:00 on Saturday, some rain gauge measurements exceeded 200mm!

Luckily for many, the rain was only moderate later on Saturday. But the damage had already been done. It is unknown when Medi Clinic Hospital will be operational again and there is likely tends to hundreds of millions of rands of damage that occurred in Somerset West alone. This will be a storm for the record books and one to no doubt go down in the city's history.


Bridgewater