In November 2020, 60.5mm rainfall was recorded over 16 raindays, which is good seeing that Makhanda is still in drought. The long-term average and long-term median for November are 86.4mm and 63.5mm respectively.
With one month to go, 659.5mm has been recorded so far for the year. This is significantly better than 2019 and not far below the long
term annual average of 700mm. The long term average for raindays in November is 10 (maximum 20; minimum 4).
Ten raindays have occurred (25.7% of the 35-year-old record) with 12 times (34.3%) above 10 raindays and 14 times (40%) below. There were three days during the 16 days of rain in which there was in excess of 7mm of rain; therefore it is suspected that there was some runoff/seepage into the water courses feeding the Howison’s Poort Dam.
The Ocean Nina Index (ONI) continues its downward trend and in August/September/October (ASO) was at -1.0°C. That means it is now two months it has been below -.05°C. Should it remain below -0.5°C in December we would have entered a La Nina phase which is very likely to be the case.
Back to the “Great 100-year Flood”.
Although people call it a 100-year flood, according to the two previous occurrences, October 1823 and November 1922, it is actually 99 years between the two events.
So could it occur in 2021? It is still likely to occur in either October or November, as these are the two months in which Grahamstown’s highest rainfalls have been recorded on a regular basis. Once a total for the current year’s rainfall has been established, at the end of December, I’ll have a relook at the data.