Intense surface and cut-off low over South Africa

Intense surface and cut-off low over South Africa

13 July 2012 18:00 UTC—14 July 10:00 UTC

Intense surface and cut-off low over South Africa
Intense surface and cut-off low over South Africa

An intense cold front and associated cut-off low passed over South Africa between 13 and 14 July 2012. These systems, particularly the cut-off low, brought very cold conditions, heavy rains, snow and strong winds.

Last Updated

22 October 2020

Published on

13 July 2012

by Mmathapelo Makgabutlane (SAWS )

Development and movement of system

 Airmass RGB image of cold front and cut-off low with 500hPa geopotential heights on 13 July at 18:00 UTC
Figure 1: Airmass RGB image of cold front and cut-off low with 500hPa geopotential heights on 13 July at 18:00 UTC

A cold front was located along the south-east coast of South Africa on 11 July.

It moved over the south-western interior of the country on 12 July, and was over the central interior by 13 July.

In the mid-levels the low had already began to be cut off from the westerly flow, as shown in Figure 1.

Therefore, the cut-off low had already started developing to the west of the surface trough.

 

As the cut-off low was deepening the upper level trough of the cut-off low, which was orientated to the west of the surface trough, caused convergence and enhanced vertical motion.

This can be seen in the increase in the size of the cold cloud complex south of the south coast and over the western interior of South Africa, between Figure 2 on 13 July at 18:00 UTC and Figure 3 on 14 July at 10:00 UTC.

 
 Met-10, 13 July 2012, 18:00 UTC
Figure 2: Met-10, 13 July 2012, 18:00 UTC
IR image of cold front and cut-off low at 18:00 UTC
 Met-10, 14 July 2012, 10:00 UTC
Figure 3: Met-10, 14 July 2012, 10:00 UTC
IR image of cold front and cut-off low at 10:00 UTC
 850hPa (purple) and 500hPa (black and green) geopotential heights on 14 July at 12:00 UTC
Figure 4: 850hPa (purple) and 500hPa (black and green) geopotential heights on 14 July at 12:00 UTC

By 14 July the cut-off low had moved eastwards to such an extent that the centres of the cut-off low and the cold front were so near each other they were almost indistinguishable as separate centres on satellite imagery.

The close proximity of the centres of the lows is shown in Figure 4, where the closed lows found at 850hPa (cold front) and 500hPa (cut-off low) are shown to be almost vertically aligned.

Figure 4 also shows that the high pressure to the west of the country was bringing in air from the higher, colder latitudes, thereby resulting in the cold temperatures that were subsequently experienced in the western half of the country.

The cold front had started to weaken and the clouds associated with it had merged with the clouds forced by the cut-off low.

 
 Natural Colour RGB image with ColourGlobe overlay of cold front and cut-off low on 14 July 2012 at 07:00 UTC.
Figure 5: Natural Colour RGB image with ColourGlobe overlay of cold front and cut-off low on 14 July 2012 at 07:00 UTC.

Another reason why it is difficult to distinguish the centres of the two systems from each other is that the amount of convergence and uplift caused by the cut-off low resulted in much cloud development.

Figure 5 shows that there was a large cloud shield, mainly of thick cloud, over the southern interior and south of the the south coast; this was where the two systems were superimposed on each other.

Full Resolution version of Figure 5 , Met-10, Natural Colour RGB, 14 July 07:00 UTC.

 

Figure 6 shows that the omega values within the vicinity of the upper trough 500hPa were mostly negative, indicating that upward motion was taking place there. These clouds had a high vertical extent.

The Water Vapour 6.2 channel shows that there was moisture at higher levels in the troposphere; the cloud pattern can be seen in Figure 7, indicating high level moisture.

 500hPa (black and green) geopotential heights and omega values (blue) at 500hPa on 14 July at 06:00 UTC
Figure 6: 500hPa (black and green) geopotential heights and omega values (blue) at 500hPa on 14 July at 06:00 UTC
 Water Vapour 6.2 channel with 500hPa geopotential heights on 14 July at 06:00 UTC
Figure 7: Water Vapour 6.2 channel with 500hPa geopotential heights on 14 July at 06:00 UTC

Figure 7 also shows that over the north-western boundary of the country extending down over the central interior there was a boundary between the moist air and dry air.

This was the likely location of the upper air jet stream, which would cause divergence in the upper levels, further enhancing convergence in the low and mid-levels.

By 15 July the surface front had moved off the interior of the country. At the same time the cut-off low was no longer cut off from the westerly flow.

The surface high was still oriented in such a way as to bring cold air into the southern and central interior of the country. This caused the resulting cold weather in these regions.

Associated weather and effects

 Natural Colour RGB with areas of snow indicated on 14 July 2012 at 07:30 UTC.
Figure 8: Natural Colour RGB with areas of snow indicated on 14 July 2012 at 07:30 UTC.
Full Resolution

During 13 and 14 July parts of the Free State, Northern, Western and Eastern Cape experienced very cold conditions — maximum temperatures of less than 10 °C — and heavy rain — at least 50mm over 24 hours. People died from drowning and exposure to very cold conditions.

The Eastern Cape also experienced strong winds which caused structural damage to electricity power lines.

The cut-off low also resulted in heavy snow in certain areas of the affected provinces.

Figure 8 shows satellite imagery of the snow on the morning of 14 July, that was not obscured by any cloud shields.

It is likely that there was more snow on the ground, especially over parts of the Eastern Cape, however, it cannot be seen due to cloud cover.

References