The 24th July 2024 marked the breaking of the record for the wettest month at the Steenbras Catchment Office (~1.2 km southwest of the N2 national road, just as one drives over the top of Sir Lowry’s Pass and Steenbras Dam), since rainfall records started at the office in January 1970 (about 55 years ago).
The prior highest rainfall month was June 1994 with 380 mm in total, and this was surpassed on 24th July 2024, which had a monthly total of 383 mm on this date. Another big cold front passed through the southwestern Cape at the end of July, which pushed the July 2024 rainfall at the Steenbras Catchment Office to a new monthly record of 455 mm. In comparison, the long-term July monthly average at the office is 156 mm, therefore rainfall this July has been almost 3 times the long-term average (LTA) at Steenbras.
There are even higher July 2024 rainfall amounts that have been recorded in other parts of the southwestern Cape as of the 30th July – Newlands has had 848 mm (versus the July LTA of 290 mm), Wynberg 608 mm (versus the July LTA of 186 mm) and Wemmershoek 611 mm (versus the July LTA of 173 mm). This record July rainfall (after below average rainfall from October last year to June this year) seems to potentially be related to a “Sudden Stratospheric Warming” (SSW) event over the Antarctic, which may be forcing frontal systems further northward, hitting the southwestern Cape and southwestern Australia with more frequency.
The end result of this record July rainfall, other than flooding across the southwestern Cape? The Western Cape Water Supply System (WCWSS) total dam storage, which was looking perilous at ~68% on the 1st July 2024, jumped to just over 99% on the 22nd July 2024. That is a 31% total dam storage increase, which is the equivalent of almost 280 million cubic metres (or billion litres) of water (just over 110 000 Olympic-sized swimming pools as a current relevant visual comparison) entering the six WCWSS dams (Berg River, Lower Steenbras, Upper Steenbras, Theewaterskloof, Voelvlei, and Wemmershoek) in three weeks.
On the Steenbras Catchment Office rainfall figure below, the different coloured lines represent cumulative annual rainfall from 2012 to present, with current 2024 cumulative rainfall until the end of July (934 mm) being indicated by the dashed red line, the 54-year average cumulative rainfall by the dashed grey line, and the 54 year mean annual precipitation (MAP) of 1070 mm by the horizontal dashed black line. The blue bars show the 54 year average rainfall amount for each month, whereas the pink bars show 2024 rainfall for each month.
