Cumulus Report: 20 November 2018- South Africa

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The summer rainfall region can expect more widespread thundershowers after an extended hot spell. The winter rainfall region will remain mostly dry.
Wetter conditions following heat and drought
After more than a week of hot and dry conditions, a much more favourable situation is now becoming established. Fairly widespread thundershowers will occur during the next few days over the summer rainfall region, excluding the western parts. Some thundershowers may become severe. This is the result of the typical unstable conditions related to upper-air dynamics and surface air interactions that usually occur in early summer.

The early maize-production region over the northern Free State and Mpumalanga will especially benefit from the developing conditions and thundershowers will occur on most days over these areas. This is very good news for especially the northeastern parts of the Free State and southwestern Mpumalanga. Here, most of the region should receive above-normal rainfall.

The southern parts of the country will be cool due to an on-shore flow for most of the time, while some showers are expected especially along the Garden Route towards the west (Tuesday, 20th) and east (Wednesday, 21st). The winter rainfall region is expected to remain dry during the period except for some initial showers by the 20th.

While cool to cold air will initially invade the country, especially the western to central parts, it will become hot over the western interior from the 23rd (Friday).

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

Normal to above-normal rainfall is expected over the eastern to northeastern summer rainfall region during the next few days, with some thundershowers also possible at times over the central parts.

The western interior (mostly Northern Cape) will remain dry during the next few days.
Cool, dry air will spread over much of the western, southern and central interior during Tuesday (20th) and Wednesday (21st).
Minimum temperatures over the central to southern interior will be relatively low on Wednesday (21st) and Thursday (22nd) morning.
It will be significantly colder (+/- 10° C drop in temperatures) over the southern interior (most of the Western Cape and Eastern Cape) on Tuesday and Wednesday (20th and 21st), accompanied by strong southerly to westerly winds. The wind should be especially strong along the coast.

Small areas of frost may occur over the southern escarpment on Wednesday and Thursday morning.
Thundershowers will occur over most of the northeastern parts on Tuesday (20th). Some of these may become severe, with strong winds and hail possible in places especially during the late afternoon.
Rain and showers will occur from the western winter rainfall region and western Garden Route (Tuesday 20th) moving through to the eastern Garden Route (Wednesday 21st) before clearing.
Most of the country will be dry on Thursday (22nd).
Thundershowers will redevelop over the northeastern parts from Friday. Scattered thundershowers should occur from the Free State and North West eastwards during the weekend. These should continue into early next week.
Some thundershowers during the weekend and early next week will become severe, including parts of the Free State and North West.
There are early indications of more significant rainfall over the summer rainfall region by Tuesday (27th) with rain also indicated over the southern coastal belt and adjacent interior. These projections specifically are somewhat far ahead of time and not necessarily considered reliable.
It will become hot and dry over the western interior from Friday (23rd).
Strong southeasterlies are expected over the southwestern coastal areas most of the time. Where vegetation is dry, this could enhance the probabilities for the development and spread of wild fires.

Seasonal overview
The tropical Pacific continues to meet some, but not all, El Niño criteria. The Bureau's ENSO Outlook remains at El Niño ALERT, meaning there is at least a 70% chance of El Niño fully forming in 2018.

Sea surface temperatures in the tropical Pacific Ocean now exceed El Niño thresholds. However, atmospheric indicators - such as trade winds, cloudiness, pressure patterns and the Southern Oscillation Index (SOI) - have yet to show consistent or sustained signs of El Niño. This clearly indicates that the tropical ocean and atmosphere are not currently reinforcing each other and remain 'uncoupled'. This coupling is required to not only fully develop and sustain an El Niño, but is what drives widespread global weather and climate impacts.

International climate models predict sea surface temperatures to remain above El Niño levels in the coming months. By February, two of the eight surveyed models dip just below El Niño thresholds.... - Australian Bureau of Meteorology -

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. Australian Bureau of Meteorology -

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 -

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 (Forecasts by the IRI 

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 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): October 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.

Vegetation activity is above normal over the northern to eastern parts of the eastern maize-production areas due to a normal start to the rainy season over much of the area. However, the northern parts of the Free State and southwestern parts of Mpumalanga experience drought stress and need rain soon. The projected rainfall from the weekend onwards may favour these areas. Over the winter rainfall region, grain-production areas are also still experiencing above-normal vegetation activity following a normal to above-normal rainy season. There are still indications of drought stress over the northern parts of the West Coast, extending into the Great and Little Karoo and the western parts of the garden Route. A slow start to the rainy season is noted over much of Limpopo, where vegetation activity is also below normal. Another area where some drought stress is noticeable is the eastern parts of the Northern Cape and northwestern Free State extending into much of central North West.

Overview of expected conditions over South Africa during the next few days
Upper-air troughs and perturbations moving across the interior, together with the Atlantic Ocean Anticyclone ridging strongly around the country on two occasions, will result in a much more favourable situation over the summer rainfall region for rain compared to the last few weeks.

The first upper-air trough will move across the interior on Tuesday (20th), and out eastwards by Wednesday (21st). At the surface, an influx of cool, dry air will result in clearance from the west, after initial rainfall over the winter rainfall region. The influx of cooler air will be felt over the southern parts with some showers and windy, cold conditions on Tuesday (20th) and Wednesday (21st). The invading dry air may also result in a tendency for severe storm development where scattered thundershowers occur over the summer rainfall region.

It will be dry over most parts on Wednesday and possibly Thursday (22nd), but upper-air perturbations moving into the central to northern parts and a renewed influx of moisture as an anticyclonic circulation pattern strengthens to the east of the country will result in a renewed development of thundershowers over the northeastern to central parts from Friday. Interaction between moisture and dry air, together with upper-air dynamics, may result in some thundershowers becoming severe.

During early next week, an approaching upper-air trough and renewed ridging of the Atlantic Ocean Anticyclone, may result in more significant and widespread rainfall over the interior - given the long lead time, this outlook is still uncertain.

Conditions in main agricultural production regions (20 - 26 November)
Maize production region: Scattered thundershowers will occur over the region on Tuesday (20th). Some of these may become severe. It will be dry on Wednesday over most parts. From Thursday, scattered thundershowers will occur, continuing through the weekend. There may be an improved distribution of thundershowers over the western parts compared to earlier in the period. Temperatures will on average be in the normal ranges.

Cape Wine Lands and Ruens: Following some showers initially (Tuesday 20th), it will become fine and mild for the most part. It will be warm over the West Coast, Swartland and Karoo from Thursday (22nd) to Saturday (24th). Winds along the southwestern coastal areas will be strong southeasterly most of the time.

According to current model projections (GFS and CCAM atmospheric models) of weather conditions during the coming week, the following may be deduced:

Thundershowers over the central to northeastern parts may become severe on Tuesday (20th).
A sharp drop in temperatures on Tuesday (20th) with strong 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).
Thundershowers over the central to northeastern parts during the weekend and early next week may again become severe with strong winds and hail possible.
Strong southwesterly winds are possible along the southeastern coastal areas (mostly Eastern Cape) by Tuesday (20th).
It will be very hot over the Lowveld and most of the Limpopo River Valley on Tuesday (20th) and Wednesday (21st).
Strong southeasterlies are possible over the southwestern coastal areas on most days. Where vegetation is dry, this may be conducive to the development and spread of wild fires.
Hot conditions will develop over the Northern Cape from Friday (23rd) to Sunday (25th).
There is a possibility of significant rainfall over the eastern to southeastern parts by Tuesday (27th). Given the long lead time, this forecast is still very uncertain.

Agricultural Research Council - Institute for Soil, Climate and Water (ISCW) - Climate Data Bank. Data recorded by the automatic weather station network of the ARC-ISCW.