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Cumulus Report: Summary of the Week- South Africa


Widespread rain expected over the summer rainfall region
Conditions will become very favourable for summer rainfall over the northeastern half of the country, including much of the Free State, during the next few days. Scattered thundershowers are expected from Wednesday (5th) according to current projections. Conditions will become especially favourable for widespread thundershowers and rain from Friday (7th) over much of the central to northeastern parts (from the central Free State northeastwards).

Widespread showers are also projected over the winter rainfall region and along the Garden Route and into the Karoo, from Thursday to Saturday. The hot conditions currently experienced over much of the interior will give way to cooler conditions with extensive cloud cover, according to current projections, by the weekend. The wetter conditions over the northeastern parts may come to an end by Monday (10th) according to current projections, after which temperatures should start a gradual increase.

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

Rainfall will be above normal from the central Free State and central North West eastwards over the northeastern parts of the country.
The southwestern parts of the country is expected to experience below-normal temperatures with cool westerly winds dominating.
Temperatures over the interior should decrease gradually through the period.
Rain is also expected over the winter rainfall region.
It will be hot over the Limpopo River Valley and Lowveld on Wednesday (5th) and Thursday (6th).
It will be hot and windy over the eastern parts of the Northern Cape into the western Free State and western North West on Wednesday (5th) and Thursday (6th).
Cold, windy and rainy conditions will set in over the southwestern parts of the Cape on Thursday, spreading up the West Coast and western to central Northern Cape as well as along the Garden Route and Karoo until Saturday.
Temperatures over the southwestern to central parts will fall by +/- 10° C compared to the previous day by Thursday.
Cool to cold, wet and windy conditions are expected according to current projections along the southern escarpment and most of the Western to Eastern Cape interiors from Thursday to Sunday.
Isolated thundershowers are expected over the central to northeastern parts on Wednesday (5th).
Scattered thundershowers are expected over the central to northeastern parts on Thursday (6th).
It will become partly cloudy to cloudy with scattered to widespread thundershowers over the central to northeastern parts by Friday, with wet conditions expected to persist until Sunday according to current projections.
Some thundershowers over the summer rainfall region may become severe, especially those developing over the central parts.
According to current projections, some parts of the Highveld and into southern Limpopo may receive significant daily rainfall totals from Saturday to Sunday.
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.

Australian Bureau of Meteorology - http://www.bom.gov.au 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. 

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 (Forecasts by the IRI - http://iri.columbia.edu/).

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 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 (top - OND - October, November, December), while the west is likely to remain drier than normal. Towards late summer (above - 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
A dominating factor of weather conditions during the next few days will be the slow development of an upper-air trough over the southwestern parts of the country while upper-air perturbations will be present to the northeast, over the northern to central and northeastern parts of the country. A large anticyclonic circulation pattern to the east of the country, over the Indian Ocean will contribute moisture to the central to eastern interior. With available moisture and upper-air instability during much of the period, especially from Thursday, scattered to widespread rainfall should occur over the northeastern and central parts of the country according to current projections.

Frontal systems will cause cooler conditions over the western to southern parts, while contributing rain over the southwestern to southern coastal areas and interior (into the southern to central Northern Cape) from Thursday into the weekend.

Conditions in main agricultural production regions (4 - 10 December)
Maize production region: Isolated thundershowers are expected on Wednesday to Thursday (6th), becoming scattered to widespread and continuing over the weekend over the entire area. Thundershowers may become severe, especially over the western to central parts. Some thundershowers, according to current projections, may result in significant daily totals on Saturday (8th) and Sunday (9th) over the central to eastern parts. Temperatures will be near normal, but somewhat above-normal in the beginning of the period (Tuesday and Wednesday 4th-5th). Temperatures are expected to decrease throughout the period, with maxima in the low 20s over the eastern parts by the weekend.

Cape Wine Lands and Ruens: It will be mild to cool throughout the period. Rain should occur in the form of showers, from late Thursday to midday Friday over most parts, continuing over the Karoo and Boland on Saturday. The wind will be moderately southwesterly to northwesterly from Tuesday (4th) to Friday (7th), becoming moderate to fresh southeasterly to easterly from Saturday (8th) onwards. It will become warm on Monday (10th) over the Swartland and West Coast according to current projections.

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 4 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:

It will be very hot over the Lowveld and most of the Limpopo River Valley on Tuesday (4th) and Wednesday (5th).
A sharp drop in temperatures Thursday (6th) with moderate to fresh winds and rain during the night may affect small stock negatively over the southern to southeastern interior (up to the southern escarpment over the Western Cape and Eastern Cape). These conditions may persist on Friday (7th).
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 on Wednesday (5th) and Thursday (6th).
Thundershowers over the central to eastern parts may become severe from Thursday to Saturday, especially over the central parts where conditions may be more unstable.
Significant daily rainfall totals over the Highveld (Gauteng, eastern North West, western and southern Mpumalanga and northern Free State) may lead to flash flooding on Saturday and Sunday according to current projections (these are still quite far ahead of time - and therefore this outlook is more uncertain).


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