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Cumulus Report April 23 - South Africa

Gradual clearance will however set in from the west, and by Thursday most of the country should experience sunny conditions. It will warm up gradually through the week towards the weekend, when maximum temperatures will eventually recover to normal whilst minimum temperatures are expected to remain in the normal to above-normal range. Drier, warmer conditions towards the weekend will be a very welcome change for late-planted crops following an above-normal number of cloudy, rainy days during April. There are no current indications of severe cold event and very little indications of widespread frost given the current and expected weather conditions.
The following is a summary of weather conditions during the next few days:
• General:
o Above-normal rainfall is expected over the central to southeastern parts.
o Especially the western maize-production areas are expected to receive further above-normal rainfall.
o The southwestern extremes of the country, including the winter rainfall area, will be dry for the most part and may receive below-normal rainfall.
o Temperatures over the interior will on average remain in the normal to above-normal range.
o Temperatures over the coastal areas in the south and east will be below normal.
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 until Wednesday (24th) and again by Friday and Saturday (26th/27th).
• Rainfall:
o Rain and thundershowers are still possible on Tuesday (23rd) over the central to southern parts of the Northern Cape as well as the Western and Eastern Cape (except for the West Coast).
o Rain and thundershowers will continue over the central to southeastern parts until Wednesday (24th).
o Heavy falls are still possible over the northwestern/northern Free State into southern North West on Tuesday (23rd).
o Widespread and heavy rain along the KZN coast should clear later on Tuesday (23rd).
o Little to no rain is expected over the country from Thursday (25th).
o The Lowveld and eastern escarpment may experience cloudy conditions with light showers during the weekend (27th / 28th).
• Temperatures:
o Maximum temperatures over the western maize-production region will range between 19 and 27 °C while minimums will be in the order of 6 – 14°C.
o Maximum temperatures over the eastern maize-production region will range between 18 and 25°C while minimums will be in the order of 4 – 12°C.
o Minimum temperatures across the interior (including the maize-production region) will decrease somewhat in the clear conditions following the rain, especially by Thursday (25th) to Saturday (27th). During this period, minimum temperatures will be below 10°C, with light frost possible in isolated preference areas.
o Maximum temperatures will gradually increase over the interior throughout the period.
o It will become hot along the West Coast and Karoo by Sunday and Monday (28th/29th).
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

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. Maps below...
Above-normal rainfall is more likely to occur over the eastern parts of the summer rainfall region during early to mid-summer (left – OND – October, November, December), while the west is likely to remain drier than normal. Towards late summer (right – JFM – January, February, March), there is a strong indication that above-normal rainfall may develop over the northeastern parts of the country, spilling also into the central parts. The western parts will still be more likely to receive below-normal rainfall. These maps are similar to the maps published previously, but here the tendency for below-normal- and for above-normal rainfall is shown instead of the tendency for normal-to-above-normal rainfall.
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.

The atmospheric circulation patterns favored the northeastern parts of the country for near-normal to above-normal rainfall during the period October – December 2018. Frequent anti-cyclonic upper-air conditions as well as persistent westerly winds at the surface kept the central to western parts dry. Most of the forecasts indeed indicated some kind of west – east improvement in expected conditions. The relatively wet area was focused more strongly further south, over the Eastern Cape, in the forecast issued by the IRI. The forecast issued by SAWS had a stronger positive signal over the Lowveld instead of the northern Highveld. The forecast issued in Cumulus also indicated the positive signal in the northeast, somewhat closer to the area where rainfall was near normal to above normal. Drier conditions in the west were resolved fairly well by all three forecasts.

The following is a very concise summary of weather patterns over the summer rainfall region, intended to provide an overview of observed conditions in the light of the seasonal forecast:
November 2018 was an extremely hot and dry month over especially the central parts of the country. During December, above-normal rainfall occurred over the northeastern parts, but it remained dry over the central parts for the most part until the 27th. From 27 December to 8 January, widespread rain occurred over large parts of the summer rainfall region.
Mid-summer drought conditions occurred from 10 to 25 January. Wet conditions replaced the dry weather from late January and lasted until about 20 February. Towards the end of this wet period, a tropical low resulted in heavy falls along the northern escarpment.
Another warmer and dry situation then developed from the 20th of February and lasted until 8 March. From 8 March, and especially around the 11th, widespread rain and thundershowers occurred over the interior, focusing especially on North West and the Free State. Most recently, thundershowers (mostly isolated to scattered) located mostly over the western to central parts, moving to the extreme east from today (19 March).
The observations above indicate that the latter part of summer so far was wetter than the early summer, in accordance with the earlier outlook provided in Cumulus, and based on the decadal climate variability signal over South Africa. Together with somewhat above-normal rainfall, tropical system had a direct impact over the extreme north-eastern parts of the country.

Overview of expected conditions over South Africa during the next few days
The cut-off low pressure system that resulted in widespread rain over many parts will start moving out east/southeastwards, resulting in clearance. Together with this, the high-pressure system to the south, which was responsible for advection of large amounts of moisture into the country, will weaken. As the system moves out, clear conditions will set in, with maximum temperatures expected to slowly increase towards the weekend. Apart from the tropical cyclone expected to influence the northern parts of Mozambique, conditions over South Africa is expected to remain much more stable from Thursday onwards, with pleasant daytime temperatures and abundant sunshine – following weeks of excessive rainfall and cloud cover over especially the central to southeastern parts. A high-pressure system ridging around the country by the weekend will however cause cloudy conditions with light showers over the Lowveld and escarpment. With the high towards the east during the weekend, the flow will become off-shore over the western to southern parts, with warm to hot conditions expected there.
Conditions in main agricultural production regions (23 - 29 April)
Maize production region: Initial cloudy and rainy conditions over most of this region will clear gradually and it should be sunny and warmer during the day from Thursday (25th) onwards. Widespread rain and thundershowers will still dominate, especially over the western areas, until Wednesday (24th). Given the upper-air dynamics and atmospheric temperatures, thundershowers may also result in hail in places, but these should, for the most part, remain small.
While maximum temperatures are expected to recover to the mid and uppr-mid-20s, minimum temperatures will initially decrease somewhat in the clear conditions following the rain, with temperatures in the single digits from Thursday (25th) to at least Saturday (27th). In preference areas, light frost is possible. Cloudy and somewhat cooler conditions may set in over the eastern areas on Sunday (28th).
In general, maximum temperatures over the eastern maize-production region will range between 18 and 25°C while minimums will be in the order of 5 – 12°C. Over the western parts, maximum temperatures will range between 19 and 27°C while minimums will be in the order of 7 – 14°C.
Cape Wine Lands and Ruens: This region will be mild to warm the most part while little to no rain is expected except for possible showers or thundershowers over the interior (mostly Karoo) and along the Garden Route on Tuesday (23rd). It may become hot along the West Coast and over the Karoo by Sunday (28th) and Monday (29th). Fresh to strong south-easterly winds are expected over the southwestern parts until Wednesday (24th) and again by Friday to Saturday (26th/27th).


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