Seasonal Climate Watch November 2016 to March 2017

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Moses Henderson
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1 Seasonal Climate Watch November 2016 to March 2017 Date: October 20, Advisory Most local and international forecasting systems are indicating that South Africa s summer rainfall areas can expect wetter conditions during the early (NDJ) and mid-summer (DJF) periods. The expected warmer conditions from the previous forecast have also subsided somewhat, which may be further evidence of wetter conditions in the seasons mentioned above. Uncertainty of these forecasts remains a concern 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. 2. Recommendation Given the expected positive conditions mentioned above, the public is reminded that the country is still firmly in the grasp of severe drought conditions. Added to this, the forecasting system also 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 3. State of Climate Drivers 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 La Niña episode. Furthermore the ENSO state should remain below the threshold value (-0.5 o 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. 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 2016, the tendency of cooling over the south western Indian Ocean, south of Madagascar, may create favourable conditions for rainfall activities.

2 4. Climate Forecast Details 4.1 Rainfall The forecasting system shows enhanced chances for above-normal rainfall conditions for the coming seasons. The likelihood for extreme wet conditions is also highly favoured at least for the beginning of the summer season as shown in Figure 1. 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 is associated with La Niña induced weather patterns. Forecast quality for total seasonal rainfall is indicated in figure A1. Figure 1: Rainfall forecasts for the three overlapping seasons valid for the period of November 2016 to March 2017 and extreme forecasts for November 2016 to January 2017 season (right panel).

4.2 Minimum and Maximum Temperatures The north-eastern and western part of South Africa show better chances for above-normal and below-normal temperature conditions

This tendency is presumably more consistent with the direction of the rainfall forecast expectation noted above.

3 4.2 Minimum and Maximum Temperatures The north-eastern and western part of South Africa show better chances for above-normal and below-normal temperature conditions respectively. However, the likelihood for abovenormal temperature conditions is largely reduced comparing to last month s outlook. This tendency is presumably more consistent with the direction of the rainfall forecast expectation noted above. Forecast quality for average seasonal temperature is indicated in figure A2. Figure 2: Probabilistic minimum (left panel) and maximum (right panel) temperature forecasts for the three overlapping seasons valid for the period of November 2016 to March 2017.

4 Contributing Institutions All the forecasts are a result of an objective multi-model prediction system developed at the South African Weather Service. This system consist of long-range forecasts produced by the following institutions: 5. Appendix Figure A1: The skill of the forecasting system in discriminating wet or dry events during the forecasting period as shown in the caption of each plot. Those regions with no shades imply that the forecasts are not better than chance.

5 Figure A2: The skill of the forecasting system in discriminating hot or cold events during the forecasting period as shown in the caption of each plot. Those regions with no shades imply that the forecasts are not better than chance.

IGAD CLIMATE PREDICTION AND APPLICATION CENTRE

IGAD CLIMATE PREDICTION AND APPLICATION CENTRE CLIMATE WATCH REF: ICPAC/CW/No.32 May 2016 EL NIÑO STATUS OVER EASTERN EQUATORIAL OCEAN REGION AND POTENTIAL IMPACTS OVER THE GREATER HORN OF FRICA DURING