On the Easter weekend of 19-22 April 2019, a cut-off low (COL) weather system caused severe floods in southern parts of South Africa.
By Kgolo Mahlangu (SAWS)
The cut-off low pressure system produced intense precipitation, with the aid of a high pressure system at the surface, over the Easter weekend in South Africa. It resulted in significant damage to the infrastructure and property, and more than 80 deaths in and around the city of Durban, KwaZulu-Natal Province, in south-eastern South Africa.
Significant amounts of rainfall were recorded in that Province, due to the system being semi quasi-stationary. The areas were subjected to heavy rainfall for prolonged periods of time, which led to localised flooding. The NWP models were able to pick up the system and the heavy rains over the affected areas. The model data, coupled with the satellite imagery, enabled SAWS to issue alerts to relevant stakeholders.
The COL was fully developed by 21 April, with its centre located over the south-western interior of South Africa (Figures 3 & 4). The resulting floods caused a lot of damage along the south-east coast and the adjacent interior of KwaZulu-Natal.
The four Meteosat-11 Natural Colour RGB images indicate the typical ‘comma’ structure of a COL. There is a blend of convective and strati-form clouds which all contributed to the flooding.
Figures 1 to 4, indicate the life span of the system from day 1, 19 April, where the upper trough was still located just south west of the country, before gradually moving towards the western interior.
Also at the surface there was a ridging high pressure system below the COL (Figure 5), advecting moisture into the south-eastern parts of the country. Coupled with the jet stream aloft, to the east of the COL core, convection was highly favoured in that area, embedded in the low level clouds that were due to the advection of low-level moisture.
The enhanced IR10.8 imagery (Figure 6), shows that the COL consisted mostly of deep convective clouds and the low-level clouds, depicted by the bright cyan clouds and the grey clouds, respectively.
The coldest cloud top temperatures are seen over the KwaZulu-Natal, which corresponds to the thick precipitating clouds; depicted by the deep red clouds from the Day Microphysics imagery (Figure 7).
The Natural Colour RGBs show that the thick cloud remained stationary over the province for three days, leading to flooding in areas along the south coast.
The 24-hour accumulative precipitation model picked up the heavy rain, that was expected in parts of the country (Figure 8). The dark red and purple colours depict heavy rainfall — the legend to the right of the image indicates rainfall amounts between 50 mm and 200 mm. Note: 50 mm of rain within 24 hours is the threshold for heavy rain. In this case most places surpassed the threshold.
The table below (Figure 9) shows the accumulative rainfall amounts over the Province documented on 23 April, reporting rainfall amounts from 22 April. The highest amount was recorded at Paddock — 234.6 mm of rain. Paddock is located on the south coast, near Port Shepstone. The other towns that reported significant amounts are also circled in red (table 1) — Durban, Mt Edgecombe and Virginia with 165.0 mm, 161.8 mm and 162.8 mm, respectively.