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Cumulus Report: 14 December 2018- South Africa

 
Large parts of the summer rainfall region, particularly the northeastern interior, received significant rainfall during the last few days. Since Friday (7th), many locations over northeastern South Africa received in excess of 50 mm of rain. For some areas, this was the first significant rainfall for the 2018/19 rainy season.

The winter rainfall region also received rain during this period as the upper-air trough moving across the country was associated with a strong cold front at the surface, resulting in rain over the winter rainfall region and large parts of the southwestern to southern interior. The system that was responsible for most of the rain (upper-air trough and some upper-air perturbations ahead of it) is currently exiting the country in the east. The result is a sharp decline in rainfall potential across the country. The circulation across the country is anticyclonic currently while there is only a marginal influx of moisture into the east - this influx isn't of tropical/subtropical origin and has a very low potential for promoting rainfall.

Without an indication of a strong weather system that may lead to rainfall during the next few days, it will once again be somewhat drier with temperatures increasing. Towards the latter part of the week, maximum temperatures will soar over the northern parts. While conditions will largely be unfavourable, isolated to scattered thundershowers will start developing again in a diagonal band across the country, including the North West, Free State, KZN, Gauteng, Mpumalanga and the northeastern parts of the Eastern Cape from Friday onwards. These will be heat-induced thundershowers that will develop by the afternoons and will generally move from a southerly direction. While some may produce totals in excess of 10 mm, weak upper-air support will preclude the development of widespread rainfall while day-time temperatures will be high, leading to large evaporative demand.

The following is a summary of weather conditions during the next few days:

Rainfall during the next few days should be below normal over most parts, but normal to above normal over the Highveld and most of KZN.
Temperatures will on average be above normal over the interior and near normal along the coast and adjacent interior.
A cold front will result in light showers over the southern parts of the winter rainfall region on Wednesday (12th).
Hot and windy (westerly winds) conditions are expected over the central parts of the country on most days from Wednesday (12th) to Monday (17th). The areas to be affected include much of the Northern Cape interior, western Free State, western North West and northern parts of the Eastern Cape.
Isolated to scattered thundershowers will occur from Friday (14th) on most days over the central to eastern parts of the Free State, central to eastern North West, southwestern Limpopo (mostly Waterberg region), Gauteng, Mpumalanga and KZN.
Temperatures will increase from Tuesday (11th). It will become hot with maximum temperatures in the mid- to upper 30s over the northern interior (most of the Northern Cape, northern and western Free State, North West (especially western parts) and Limpopo from Thursday (13th) onwards.
Little to no rain is expected over the winter rainfall region and the Garden Route.
Cooler weather with light showers are expected along the coasts of the Eastern Cape (eastern parts) and KZN on Friday (14th) and Saturday (15th).
Strong southeasterly winds are expected over the southwestern parts of the winter rainfall region by Friday (14th), continuing until Monday (17th).
It will become hot over the West Coast, Swartland and Karoo on Sunday (16th) and Monday (17th).
Seasonal overview
El Niño and seasonal forecasts

ENSO Sea surface temperatures in the tropical Pacific Ocean have now exceeded El Niño thresholds for more than a month. However atmospheric indicators - such as trade winds, cloud patterns, and the Southern Oscillation Index (SOI) - have not reached El Niño levels. This indicates that the tropical ocean and atmosphere are not reinforcing each other and remain 'uncoupled'. This coupling is required to establish and sustain any ENSO event, and is what drives widespread Australian and global impacts.

Recently, trade winds in the western Pacific have weakened in association with the Madden-Julian Oscillation. Some models suggest they may remain weakened for at least the next fortnight.

According to the International Research Institute for Climate and Society, there is a high likelihood for an El Niño during summer 2018/19.

The Southern Oscillation Index has been trending negative since early this year, an indication of a negative atmospheric response to warmer SSTs, signalling a trend towards El Niño-like conditions. However, recent values are in the neutral range - signalling weak coupling as mentioned above. Australian Bureau of Meteorology - http://www.bom.gov.au.

Based on the developing El Niño, forecast models lean towards a tendency for drier conditions by late summer, while early to mid-summer is expected to be relatively wet over much of the interior. Coupled with the dry signal towards late summer, there is also a concomitant indication of warmer than usual conditions. The positive temperature anomalies are also indicated for early-to mid-summer. It is worth noting that, while seasonal forecasts tend to indicate drier conditions towards late summer, this is a weaker signal than what is sometimes associated with El Niño summers, possibly at least in part due to the weakness of the event. The following are the latest seasonal forecasts for Africa, from the IRI, for mid (December - February) and late (January - March) summer respectively.

There are indications that the summer rainfall region may experience above-normal rainfall during early to mid-summer with a very slight indication of possible warmer-than-normal conditions 

Towards late summer, seasonal forecast models suggest somewhat drier-than-normal conditions over much of the interior, with a stronger indication of the development of a warm anomaly, centred towards the northwest of South Africa

Expected rainy season progression, associated with decadal variability (Issued 22 October 2018)
Since summer 2017/18, in terms of decadal climate forcing, there has been a push towards El Niño conditions. Global Climate Models therefore predict the onset of El Niño conditions during the next few months. The negative forcing experienced during the last few months, giving rise to the development of weak El Niño conditions, will be replaced by a positive influence on the climate system during the next few months. By late summer, there should be a strong push towards La Niña conditions - this may result in wetter than normal conditions over large parts of the summer rainfall region by late summer. Conditions therefore, during most similar summers as 2018/19, are usually somewhat drier in early to mid-summer, but wetter towards late summer. This is somewhat different to the typical El Niño signal as is forecast by climate models. Given the El Niño-like conditions present currently, it may be safe to assume a tendency towards drier conditions with above-normal temperatures during large parts of the summer. However, based on conditions in similar years in the past, the January-March period may turn out quite favourable.

The early rain experienced over the summer rainfall region since late September should largely be replaced by relatively dry conditions into November. This may likely be interspersed by a short wet period in early November. From late November, there is likely to be a resurgence of relatively wet conditions over the summer rainfall region, possibly lasting into December. Again, by late December / 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. Based on the tendency in previous similar years, 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.

Normal to 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.

Seasonal outlook: Summary
Based on the current state of El Niño, it is safe to assume that there will 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 suggest a very weak negative influence. The only difference here is that the predictions based on decadal variability suggest 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.
Rainfall (% of long-term mean): November 2018

Parts of the Eastern Highveld, the northern coastal belt of KZN and southeastern Northern Cape received above-normal rainfall during October. The rest of the country was mostly drier than normal.

It is clear that the western to central maize-production region, including the southwestern parts of the eastern production region, experience vegetation stress related to low rainfall. The far eastern areas received good rain, and vegetation activity seems above normal.

Overview of expected conditions over South Africa during the next few days
There is no indication currently of a strong weather system that could cause significant rain during the next few days. A weak upper-air trough will however move into the southern parts of country by Friday. With some upper-air perturbations to the north of the trough, as well as the advection of moisture due to an anti-cyclonic circulation to the east over the Indian Ocean, isolated to scattered thundershowers will develop over the central to eastern parts, especially the Highveld, from Friday to Monday. A cold front will result in some light showers over the winter rainfall region on Wednesday. The cold front will also result in lower temperatures over the western and southern parts of the country on Wednesday (12th) and Thursday (13th).

Conditions in main agricultural production regions (11 - 17 December)
Maize production region: The period will start off with partly cloudy and mild conditions. Temperatures will increase gradually during the week and it will become hot over the western parts of the production region by Thursday and into the weekend. Isolated to scattered thundershowers will occur from Friday (14th), with a fairly good distribution by Friday - but to a lesser extent also on the subsequent days.

Cape Wine Lands and Ruens: A cold front will bring cooler conditions with a westerly wind on Wednesday (12th) and Thursday (13th). Some light showers are expected, mainly in the southwest, on Wednesday (12th). It will gradually become warmer from Thursday (13th) onwards. It may become hot over the Swartland, West Coast and Karoo on Sunday (16th) and Monday (17th). Strong southeasterlies may develop along the southwestern coastal areas from Friday (14th) to Monday (17th).

Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) - http://Wxmaps.org

Possible extreme conditions - relevant to agriculture
The South African Weather Service issues warnings for any severe weather that may develop, based on much more information (and in near-real time) than the output of one single weather model (GFS atmospheric model - Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) - http://Wxmaps.org) considered here in the beginning of a week-long (starting 11 December) period. It is therefore advised to keep track of warnings that may be issued by the SAWS (www.weathersa.co.za) as the week progresses.
According to current model projections (GFS and CCAM atmospheric models) of weather conditions during the coming week, the following may be deduced:

Warm to hot and windy conditions over parts of the Northern Cape and into the western Free State and western North West may be conducive to the development and spread of wild fires from Wednesday (12th) on most days until Monday (17th).
Thundershowers over the central to eastern parts on Friday (14th) may become severe.
Strong southeasterly winds over the southwestern parts of the Western Cape from Friday (14th) onwards may be conducive to the development and spread of wild fires.


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