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How To Prepare For The Cape Town “Monster Storm” – A Realist’s Guide
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How To Prepare For The Cape Town “Monster Storm” – A Realist’s Guide

So you’ve seen the headlines: “Monster Storm To Hit Cape

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Cape Braces For Massive Winter Storm

Cape Braces For Massive Winter Storm

Last week we published a tentative piece talking about how the forecast models were showing signs of an intense cold front making landfall in the Cape this coming week. Since that publication, the models have been very consistent in forecasting this storm, with every run since showing the potential for a large scale severe weather event. There were occasional ‘wobbles’ in the data, with some runs showing more of a wind & rain event, while some showed a more snow orientated forecast.

The guidance seems to have settled a bit, and as it stands the storm is looking like it will primarily be a wind and rain event, though snow is also likely to accompany the deep low pressure system.

Winds

In terms of strength, this appears to be a real beast and the type of storm the Cape hasn’t seen in quite a few years now, should the forecasts verify. The associated winds are currently forecast to be in excess of 40kt (74km/h) with gusts potentially reaching in excess of 100km/h in places. While we are used to seeing the South Easter rip through the Mother City during summer, it is rare that we see forecast sustained wind speeds exceed 40kt. This value is even greater for Cape Point, where Windguru’s forecast, using the GFS model is showing gusts in excess of 60kt (111km/h).

windguru

Windguru forecast for Cape Point

wind-speed-forecast

Forecast probability for windspeeds exceeding 40kt.

Pressure

This extreme wind forecast is based off the synoptic setup of the storm. A deep low pressure is forecast to move from the SW up towards the country, with a strong high pressure ridge extending along the western side of the storm. This intense low pressure area in combination with the high pressure next to it creates a tighter gradient of pressure, and pressure gradients are the driving force behind the winds.

It is also worth noting that the models show the central pressure in Cape Town dropping below 1000mb. While we often feel the brunt of a strong storm, the central low pressure is generally a few thousand kilometers south of the country and it is rare that we ever see sub-1000mb pressures in the country.

baroclinic

Baroclinic intensification (before landfall). Showing the cold-core upper trough overshooting the surface low, causing rapid deepening of the front.

Rain & Snow

The details for both rain and snow are still changing a bit between model runs, so there is less certainty in the discussion of these aspects. However, there is a high confidence in the likelihood for good falls of rain throughout the Cape. The Boland area in particular seems likely to receive large amounts of rain, especially towards the mountain catchment regions where rainfall is likely to be heavier due to orographic effects. We should be able to expect rainfall measurements anywhere from 50mm to 100mm in the far SW Cape and Boland area. Rain values however can easily change depending on the exact setup of the storm upon arrival. But we are optimistic that this should be a great rain event for Cape Town.

The snow outlook is also looking quite positive for those hoping to do their first snow chase of the year. While freezing level and snow potential have been the most volatile in the forecasts, as it stands there is a strong likelihood for snow along the SW Cape mountains. The 06z forecast of the GFS was particularly bullish on this aspect. However, every run thus far has showed at least some snow occurring during the event. At the time of publication Snow-forecast.com was predicting in excess of 20cm of snow for Matroosberg, with freezing levels around 1500m. We recommend keeping an eye on the forecasts, as this is still the most likely aspect to change before landfall occurs.

24-hour-precip

24 Hour Precip forecast

rain-forecast-25mm


Probability of 24 hour rainfall figures in excess of 25mm

Sea Conditions

While there is a lot of focus on the rain and wind aspect to this storm, the nature of the setup lends itself to the development of maritime damage too. Because the storm has a fairly large area of ‘fetch’, in conjunction with both extremely strong winds and a near full moon — it seems likely that this storm will pose both a risk of extremely large swells and the possibility for storm surge on some beaches. Forecasts for sea swell are in the range of 9m to 10m at this moment and in conjunction with a near full moon, we could easily see damage along some of the beaches during high tide.

This storm has the potential to bring with it dangerous conditions, and we must remind you that although we enjoy updating you with what the latest weather occurrences are, we are by law not able to issue any warnings ourselves, and suggest that for official warnings you please visit the South African Weather Service website, who have already started issuing long range (day 3 to 5) warnings on the system.

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