GARDEN ROUTE | KAROO NEWS - An intense cold front/mid-latitude cyclone is forecasted to bring a significant drop in atmospheric pressure and strong winds, leading to high waves and storm surge along the west and south-east coastline of South Africa, in places between Alexander Bay and Gqeberha, on Sunday, 7 July and Monday, 8 July.
What is storm surge?
A storm surge is an abnormal seawater rise, over and above predicted astronomical tides, generated by a storm or intense weather system. A surge can be either positive (storm surge) or negative (negative storm surge). A positive surge can occur when strong near-surface on-shore winds along a coastline significantly increase water levels. In addition, it may intensify with Rapid Cyclogenesis (intensification), which is when the central sea-level pressure deepens by greater than or equal to 1 hPa per hour, generally over a period of 12 hours or longer.
Storm surge is calculated as the difference between the expected astronomical tide and the actual water level (including the effects of weather). Strong onshore winds and waves can further increase water levels at the coast. When these conditions occur during high tide or spring tide, the impact on the coastline can be exacerbated.
Conversely, if they occur during low tide, the impacts can be minimal or negligible. The SAWS Wave and Storm Surge (SWaSS) model shows a storm surge along the west and southeast coastline of South Africa (Figure 1). The intense mid-latitude cyclone (intense cold front) is forecast to undergo intensification, with a drop in atmospheric pressure of 20 hPa expected over the next 24 hours. This drop in pressure and other favourable weather conditions will lead to a significant storm surge in places along the west and southwest coastline.
In addition, wave heights of 6-8 meters are expected, potentially reaching up to 10 meters in the southwest, along with gale to strong gale force north-westerly to westerly winds. The system will result in heavy rainfall, with potential impacts such as damage to coastal infrastructure and disruptions of daily activities. Recreational areas at or near beaches are also expected to be at risk.
To better understand this event, the time-series graphs from two national ports are presented below (Figure 2). The time when the positive storm surge will occur is indicated by a black arrow, namely early Monday morning, between 02:00 and 08:00.
The SAWS has issued two impact-based warnings for the west and south-east coastlines for Sunday into Monday, 7-8 July 2024.
Please see the warning below: 1. Orange level 6 warning: Storm surge leading to localised flooding in low-lying areas and disruptions to beachfront activities is expected along longer stretches of coastlines between Cape Columbine and Cape Agulhas tomorrow, (Sunday, 7 July 2024), spreading to Port Alfred on Monday morning (8 July 2024).
2. Yellow level 3 warning: Storm surge leading to localised flooding of low-lying areas is expected between Cape Columbine and Alexander Bay tomorrow (Sunday, 7 July 2024) until Monday morning (8 July 2024).
The public is urged and encouraged to regularly follow weather forecasts on television, radio as well as social media platforms. Updated information in this regard will regularly be available at www.weathersa.co.za as well as via the SA Weather Service X account (@SAWeatherServic).
Compiled by: Lauren Smith, Stella Nake and Elani Heyneke Edited by: Stella Nake and Dr Ramontsheng Rapolaki
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