As Nelson Mandela Bay’s rainfall season approaches and the metro’s dam levels remain critically low – the South African Weather Service has painted a bleak picture, saying long-term forecasts do not indicate any major rainfall in the coming months, “statistically or meteorologically”.
A spokesman for the Port Elizabeth branch of the SA Weather Service, Garth Sampson, said Port Elizabeth experienced its rainfall season between March and August, while the dams in the catchment areas experienced various other rainfall seasons.
He said Kareedouw’s rainy season was between May and October, while Uitenhage was between April and October. The Patensie area received its rainfall between March and November.
“So generally the trend is from autumn into spring. However the seasonal outlook does not look good. The meteorological outlook is that we have a bit of an El Niño situation happening, and this means slightly above to slightly below rainfall, which is not going to help things at all.”
He said South Africa generally received below-normal rainfall during El Niño years, which were associated with a band of warm ocean water that developed in the central and east-central equatorial Pacific temperatures, which influenced atmospheric circulation, and consequently rainfall and temperature in specific areas around the world.
He said he had also started his own statistical desktop study in order to ascertain the historical periods between events and the highest likelihood when they would re-occur to replenish the dams.
“I am looking at how many days and months there are between rainfall events of 50mm in 24 hours. However, looking at historical statistics, it does not look good for June until October.
“Also, I have found that the amount of rainfall in 24 hours that constitutes a flood or mini flood is also decreasing. This is due to urbanisation which among other things is aggravated by the amount of cemented areas. After only 20mm to 30mm what does Newton Park look like?”
Sampson said the metro’s dams had been filled to capacity only 18 months ago. He said between 1955 and 1965 there had been a 10-year “dry spell”.
“Our rainfall has not decreased or increased drastically. There are just more people who rely on it than there were then.”
However, he said total rainfall for this year to date was at 132mm, which was well below the mid-year average of 222mm.
“Last year we had a total of 450mm, but in 2015 we had 700mm so it varies. In 2012 we had over 1 000mm.”
He said because Port Elizabeth relied on a cut-off low system to replenish its dams, these events brought rainfall events of 50mm or more in 24 hours. However they did not happen often and there needed to be certain other factors in place for them to produce enough rainfall.
“These cut-off low systems were also responsible for the floods of 1968, 1981 and various other floods,” he said.
“In the history of PE we have serious events once every four years, very serious events once every seven years, absolute disasters every 27 years and catastrophic events every 36. This includes the floods of 1908, 1968 and 1981.
“This does not however mean that we will get a severe event at exactly 36 years but on average it is 36 years.”