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Helderberg Fire – 16 April 2011

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3 February 2011 – Cape Town Trough

The models had been showing this possibility for a long time, Gordon Richardson of Capetownskies and I had been discussing the potential about a week before and while always being reluctant to go with long term model runs, things were looking promising. The models had also been fairly consistent in the nature of the system, a sharp trough moving southwards along the west coast, where-after it would close off and become a cut-off low pressure system which would then move SE. I’d noted that once the transition to cut-off low had occurred moisture would then be limited to the central and eastern parts of the Western Cape.

One of the first things that caught my eye with the models was the amazing amount of CAPE it had been forecasting, it’s not often Cape Town sees CAPE values in excess of 1000 with LI indexes of -3. Needless to say there was a lot of instability forecast.

I put in for a days leave 2 days prior to the event because the models had been so consistent and I knew if something did pop off and I was in the office I would be raging at myself.

On the day I awoke at 5am to make sure I don’t miss anything, I went outside and watched the sun rise. The sky then was filled with cirrus and just a few altocumulus. As you most likely know, cirrus clouds are never good for thunderstorm potential, ideally we’d want the cirrus to clear and while the altocumulus do block the view for the most part, they’re much more indicative to instability than the cirrus. Temperatures on the day were near 35’C and humidity was far higher than Cape Town is used to.

By 1pm things were looking slightly better with much of the cirrus gone, but there were no clouds in the sky really. I kept in close contact with Gordon via text message, with him being located in Paarl at the time it could give me an idea of what to expect with the trough moving down as he’d get the weather first, though he was reporting overcast conditions with limited view due to the altocumulus. There was a strong inversion cap present which was limiting any development, for something to start we needed that cap to break, the Skew-T soundings were very borderline on the cap giving in.

Then about an hour later at 2pm it finally happened, I was located on the boundary of the trough, clear visible to the NE were the signs of altocumulus. I headed outside with my camera, video camera and a bottle of water. I sat down at a local park and watched the cumulus begin to develop… It was a lovely feeling at first seeing convection getting going, then 30 minutes later it got frustrating when it was clear that there was just not enough instability yet, the clouds would build rapidly and then fall apart. I continued to watch for another 30 minutes until signs eventually got much better, FINALLY a large towering cumulus had developed just to my south-east and was managing to maintain it’s structure, if one area fell apart another would rapidly take it’s place.

By about 3:30 things were looking good, there was now dark clouds to my NE and to my SE. The first thunder came from the cell to the NE along with some very pleasant and much welcomed rain! A little while later it eased off a bit and I began to look at what appeared to be the core of a cell to the SE, heavy rain curtain in the distance, though I didn’t know whether it would make it towards me… The wind was around 5-10km/h for the most part and was extremely quiet apart from the rumbles of thunder- then I heard a sound in the distance, originally I thought it was just extremely heavy rain approaching but then out of nowhere it hit. What was most likely a gust front, brought with it horizontal rain and winds rarely seen associated with thunderstorms in the Cape Town area. I’m not sure on the exact wind speed, but I’m certain it was over 65km/h perhaps pushing on 80km/h at times. It was difficult to stand and I was blown onto my knees a few times while filming, this lasted for about 5-10 minutes after-which it dropped back down to no wind again.

The gust front mentioned was reported to have moved in a NW direction and impacted drivers on the N2 a bit later. There were also reports of hail from the Stellenbosch area.

After this the lightning had really picked up and I started filming some of the strikes around me, the screams of school children whom were playing on the school fields still filled the air after each flash. The lightning was good cloud to ground lightning strikes and some were in close proximity to me, as can be seen on the one video.

http://www.youtube.com/v/2cxuOaZ1EnI?version=3

On Friday the 4th there was still evidence of the instability around as large cumulonimbus were visible between 80 and 150km to the E. It was a good idea to take the day off because one rarely gets to experience such storms in the Cape often. Cape Town and the Northern Suburbs seem to have also got a bit of thunder, but for once I feel as though I didn’t get the short end of the straw. Finally Somerset West took a real hit from a good storm cell.

http://www.youtube.com/v/xlznkd124b0?version=3

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