This is a quote from a report this past weekend by
A cut-off low pressure system developed earlier in the week over the northern parts of South Africa and has since been deepening (intensifying) and moving southwards towards the Western Cape. This low pressure system brough strong winds and good rainfall to the Western Cape through the night of the 1st into the 2nd of June, as well as throughout the day of June the 2nd. On Wednesday the 3rd, the COL was positioned over Cape Town in the morning, which has lead to partly cloudy skies with a fair amount of sunshine. However, this is set to change soon as the low continues to move south eastward and by doing so, will bring the flow of moisture over Cape Town.
Cape Tonians can expect heavy rains and possibly flooding from around mid-day today into the night, as this moisture band will be capable of producing in excess of 100mm of rain within the next 18 hours. Though it’s important to understand the nature of how cut-off lows work in order to understand how easy it is for the rainfall totals predicted to either be far less or far more than what is expected (explained below).
Apart from heavy rainfall, the position of the COL later today could result in strong winds as well as heavy swell in places along the coast. This combined with today’s Spring high, could cause a bit of coastal surge later today, with high tide at 16:00 for Cape Town.
What Are Cut-Off Lows & Why Are They Tough To Forecast
A cut-off low is an area of low pressure which is not in a linear form, unlike troughs. They are usually formed when a trough deepens to the point where it then becomes a cut-off low and unlike most troughs which originate in the northern parts of Southern Africa which move from west to east, a cut off low will then travel whether it is guided by the surrounding high pressure systems (ridges). A COL will typically move towards the south, and be pulled poleward — that is unless a ridge is strong enough to prevent it from doing so, and instead will move along the top of the ridge, like as if on a conveyer belt.
Depending on the position the effects of the COL can be different. For the COL that is currently affecting the Western Cape, it was in a position yesterday whereby it was drawing in a lot of moisture from the warmer tropical rainfall areas to the north, while also bringing in cooler moisture from the west. In cases like this it’s very possible that the COL will be associated with thundershowers.
Cut-off Lows will generally have a band of moisture that it wraps around the low pressure center, where the center of the low often sees very little rain in comparison to where the main flow of moisture is taking place. In the current COL, the position on the 2nd of June meant that the Cape was experiencing moisture on the right hand side of the low, coming from a tropical nature, while today June 3rd, the low pressure will be on the other side, and so the rainfall expected will be coming from the south.
The challenges with predictions is easy to understand when you look at how the moisture flow works. If the COL were to be just 100km further left than forecast, it would see the bulk of the predicted rainfall not affecting land at all and rainfall totals would be much less than predicted. At this stage, the models are suggesting however, that the Cape Town area will lie directly in the path of the highest likely rainfall area within the moisture band — but don’t be surprised if things change without warning.