CUMULUS Report May 2019 - South Africa

The current spate of warm and sunny days over the interior should make way for cloudy conditions with showers or thundershowers over most of the western to central interior by mid-week. However, towards the weekend, conditions will clear again and warmer, sunny conditions should return according to current forecasts. Furthermore, there are no current indications of severe cold or widespread frost during the next few days over the interior. Over the winter rainfall region, current forecasts indicate very little rainfall, with most of the area expected to receive below-normal rainfall.
The following is a summary of weather conditions during the next few days:
• General:
o Little to no rain is expected over the northern parts of the country.
o The southern half of the country may experience normal to above-normal rainfall.
o Below-normal rainfall is expected over the winter rainfall region.
o Except for partly cloudy to cloudy conditions over the southern parts during the middle part of the week, the area should be fine and dry during the rest of the period.
o Temperatures over the interior will on average remain in the normal to above-normal range, with largest positive deviations over the central to western interior.
o There is no indication of severe frost given the expected weather conditions during the next few days.
o Fresh to strong southeasterlies will occur over the southwestern parts from Sunday.
• Rainfall:
o Most days will be sunny and dry over the interior.
o Scattered showers and thundershowers will develop over the western parts of the Northern Cape and interior of the Western Cape on Tuesday (7th), moving eastwards during the day.
o Scattered showers and thundershowers will move from the west over the eastern parts of the Northern and Western Cape Provinces, the Eastern Cape, Free State, southern North West and KZN on Wednesday (8th), clearing from the west.
o Light showers are possible over the northeastern parts of the country, especially along the eastern escarpment, on Thursday (9th) and Friday (10th).
• Temperatures:
o The central to eastern interior will become cooler following the rain by Wednesday.
o The interior will warm up gradually from Friday (10th) onwards.
o Light frost is possible in isolated areas over the Highveld and along the Drakensberg as well as parts of the Free State in areas where a long frost season is typical, from Thursday (9th) to Saturday (11th).
o It will become warm to hot over the western to northwestern interior from Friday (10th)
o Hot, Berg-wind conditions are possible along the West Coast from Friday (10th).

o Maximum temperatures over the western maize-production region will range between 21 and 28 °C while minimums will be in the order of 5 – 12°
o Maximum temperatures over the eastern maize-production region will range between 18 and 24°C while minimums will be in the order of 4 – 8°C.
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Seasonal overview
El Niño and seasonal forecasts
According to the Australian Bureau of Meteorology the chances for the strengthening of El Niño during the next few months are diminishing while according the International Research Institute for Climate and Society, there remains a reasonable likelihood of an El Niño during 2019. In general, both these outlooks are less optimistic for the strengthening of a warm event and expect a short-lived event:
ENSO (updated 30 April): The Bureau's (Australian Bureau of Meteorology) ENSO Outlook remains at El Niño ALERT. This means the chance of El Niño developing in 2019 is approximately 70%. Climate models indicate that if El Niño does develop it is likely to be short-lived.
Although the surface of the tropical Pacific Ocean remains warmer than average, water below the surface of the ocean has been gradually cooling. A cooling of water at depth can lead to a cooling of the ocean surface, which may reduce the length of an event if one develops. Most climate models indicate surface warmth in the Pacific Ocean will remain at El Niño-like levels at least through May. The longer the ocean surface warmth remains, the more likely it is that the atmosphere will respond, and El Niño will develop.

Based on the weak El Niño, forecast models forecasted dry conditions for South Africa during late summer, following (according to earlier forecasts) a wetter early to mid-summer period over the interior. While seasonal forecasts tended to indicate drier conditions towards late summer, this was a weaker signal than what is sometimes associated with El Niño summers, possibly at least in part due to the weakness of the event. Forecasts from global forecasting groups such as the IRI remained negative also for autumn with regards to rainfall over South Africa towards late summer while above-normal temperatures are forecasted to remain in place. The following are the latest seasonal forecasts for Africa, from the IRI, for late summer (February - April) and autumn/early winter (April-June) respectively.

During late December and early January, it may once again be drier – basically during the period when the mid-summer drought usually occurs. If this dry period develops, it will most likely not be as severe as during 2017/18. From late January, conditions may very well improve again, and then even more so from early February. There is a possibility that large parts of the summer rainfall region could receive normal to above-normal rainfall during February and/or March, while globally the indicators should start signaling the possibility of a La Niña towards 2019/20. Should the wet conditions develop in the north, there is also an enhanced likelihood of tropical systems (such as tropical depressions/storms/cyclones) influencing the region.

Seasonal outlook: Summary
Based on the state of El Niño, it was safe, by mid-summer, to assume that there would be a tendency towards drier and warmer conditions at least in part during the summer. However, both Global Coupled Models and forecasts based on the decadal variability in the climate system suggested a weak negative influence. The only difference was that the predictions based on decadal variability (issued here) suggested increasing wetness towards the end of the summer, with a drier start, while Global Climate Models suggest wetter conditions earlier, drying somewhat towards late summer.

Overview of expected conditions over South Africa during the next few days
An upper-air trough will result in scattered thundershowers over the southern half of the country on Tuesday (7th) and Wednesday (8th). Light showers are also possible in the northeast, related to the system, by Thursday and Friday (9th, 10th). A frontal system may result in showers over the winter rainfall region by Wednesday (8th). The rest of the period should see anticyclonic conditions dominating, with pleasant autumn conditions and little in the way of rainfall. The anticyclonic flow will result in hot berg-wind conditions over the western parts, especially the West Coast, from Friday (10th).
Conditions in main agricultural production regions (7 - 13 May)
Maize production region: Sunny and mild to warm conditions will prevail for the most part with little to no indication of frost. Temperatures will be somewhat lower from Thursday (9th). Scattered thundershowers are possible over the southern parts on Wednesday (8th). Light showers or thundershowers are possible over the eastern parts on Thursday (9th) and Friday (10th). The rest of the period should be mild, sunny and dry. Maximum temperatures over the western maize-production region will range between 21 and 28 °C while minimums will be in the order of 5 – 12°. Maximum temperatures over the eastern maize-production region will range between 18 and 24°C while minimums will be in the order of 4 – 8°C.
Cape Wine Lands and Ruens: Rainfall during this period is expected to remain below normal over the region. It will be mild for the most part, but hot towards the north and along the West Coast later. A frontal system may bring light showers and fresh westerly winds to the region on Wednesday (8th). It will become warm to hot along the West Coast from Friday (10th) onwards. Strong south-easterly winds are possible in the southwest on Sunday (12th) and Monday (13th).During this time, light showers are possible along the Garden Route.

According to current model projections (GFS and CCAM atmospheric models) of weather conditions during the coming week, the following may be deduced:
• Cold, wet and windy conditions are possible over the southern parts of the country on Wednesday (8th) and Thursday (9th). These may have negative impacts on small stock.
• Strong southeasterlies are expected over the southwestern parts during the weekend and early next week. These may be conducive to the development and spread of wild fires where vegetation is dry.