December could see above normal rainfall
Most local and international forecasting systems indicate southern Africa’s summer rainfall areas can expect wetter conditions during the mid-summer (December, January, February) periods.
Uncertainty of these forecasts remains however since factors such as the possible development of a La Niña (increased chances of wetter conditions) have not yet strongly manifested, and this usually casts doubt on the accuracy of the forecasting systems.
The South African Weather Service has issued a statement saying given the expected positive conditions mentioned above, the public are reminded that SA and many of its neighbours are still firmly in the grasp of severe drought conditions.
The forecasting system indicates the possibility of well above normal rainfall conditions which, given the current drought effects, may cause negative impacts if flooding does occur. “It is very important to keep monitoring the developments that may alter or strengthen the current expectations for the summer season,” the report states.
Observations show that ENSO (El Niño Southern Oscillation) shows a tendency of strengthening toward a weak La Niña during October. However, most prediction models indicate that the likelihood for (weak) La Niña development toward summer 2016/17 is further reduced. According to these models, the expected weak La Niña starts to decay before it gains its maturity or attains the status of a La Niña episode.
Furthermore the ENSO state should remain below the threshold value (-0.5 C) for three consecutive months to be qualified as a La Niña episode.
“This may render ample room for uncertainty that may weaken the expected wet spell projected by the climate models for the coming seasons,” according to the report.
The Indian Ocean Dipole (IOD) is still expected to remain in a negative phase up to the end of spring and is expected to reduce to a neutral state during the beginning of the summer season (December). A negative phase of the IOD usually tends to prevent the transport of moisture from the Indian Ocean to the southern Africa region and may not have a noticeable contribution to the overall moisture budget hovering over the region.
“However, as observed during October, the tendency of cooling over the south western Indian Ocean, south of Madagascar, may create favourable conditions for rainfall activities,” the report states. The forecasting system shows enhanced chances for above-normal rainfall conditions for the coming season.
“The likelihood for extreme wet conditions is also highly favoured at least for the beginning of the summer season. Irrespective of the uncertainty associated with the model forecasts which mainly arises from those governing systems that steer the direction of the season, this unusual wet indication should be cautiously monitored particularly over the summer rainfall region of South Africa. It is known that most of historical flooding events recoded in South Africa are associated with La Niña induced weather patterns,” the report concludes.