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Cumulus Report: November 2018- World Report- South Africa

Heatwave conditions will prevail over the western and northern interior with isolated thundershowers in the eastern to northeastern interior until the weekend. Thundershowers in the northeast will become more widespread from Sunday.


Dry conditions expected to continue until the weekend
The recent dry conditions are expected to continue for the next few days, into the weekend. Temperatures will be on an increasing trend as anticyclonic circulation continues to dominate. The western to central interior will be especially hot, while the southern to western coastal areas will be mild due to an on-shore flow during most of the period over these areas. Heat-wave conditions are possible over large parts of especially the western to northern interior. Isolated thundershowers, mostly heat induced, will occur over the eastern to northeastern interior, focusing on the eastern Highveld and Drakensberg. Precipitation will therefore be in the form of short-lived downpours and should not be well distributed.

Intense convection and dry air in the region may cause isolated storms to become severe, but given the generally unfavourable conditions for rainfall, such instances will be few and far between. Storms will generally move from the south/southeast, around the center of the high-pressure system over the interior.

Winds during the next few days over much of the interior will have a fairly strong westerly component, feeding dry air into the country.

The prolonged dry conditions experienced currently may be seen as somewhat extreme, and affect certain areas negatively in terms of the planting window nearing the end. However, the dry period following wetter conditions earlier in October, is a feature that is frequently associated with the decadal climate signal, as discussed in the seasonal forecast section.

Thundershowers over the northeastern parts will become more widespread from Sunday, when large parts of the Eastern Highveld should receive rainfall. Thundershowers will also spread more generally over the northeastern half of the country, including the Free State, by Monday (19th). 
The winter rainfall region is expected to remain dry during the period.

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

The next few days will be dry and hot over most of the interior.


Temperatures will on average be between 2 and 7 °C above the long-term mean over much of the interior, focusing especially on the central interior.


Isolated thundershowers are expected over the northeastern parts during the week, focusing mostly on the Highveld and Drakensberg, moving southeast-northwest.
No rain is expected over the central parts during the week.
It will be hot, dry - and sometimes windy over the central to northern and northwestern parts until Sunday (18th).
It will also be very hot over the Lowveld from Thursday to Saturday.
It will become cooler over the southern to eastern coastal areas and adjacent interior from Saturday (17th).
Temperatures will decrease over the southern, eastern to northeastern parts of the country from Sunday (18th).
It will become cooler with scattered thundershowers over the eastern to northeastern parts from Sunday (18th).
Thundershowers over the northeastern parts on Sunday (18th) may become severe. This tendency may spread to cover the entire Free State and North West by Monday (19th) according to current projections.
Strong southeasterlies are expected from Friday (16th) over the southwestern coastal areas.
Hot conditions over the interior will spread to the West Coast and Swartland by the weekend.


Seasonal overview El Niño and seasonal forecasts

ENSO The ENSO Outlook remains at El Niño ALERT, meaning there is approximately a 70% chance of El Niño occurring in the coming months; about triple the normal likelihood. Sea surface temperatures in the tropical Pacific Ocean have warmed to El Niño levels over the past fortnight. However, atmospheric indicators (such as the Southern Oscillation Index - SOI) of El Niño are largely near normal, suggesting that the ocean and atmosphere are not yet re-inforcing each other, or 'coupled'. This re-inforcement is critical in any El Niño developing and becoming self-sustaining.

International climate models suggest further warming of the tropical Pacific Ocean is likely, increasing the possibility of coupling occurring in the coming months. Seven out of eight climate models suggests sea surface temperatures will remain above El Niño thresholds until at least March 2019 - Australian Bureau of Meteorology -

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.

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 early (November - January) 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

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 (OND - October, November, December), while the west is likely to remain drier than normal. Towards late summer (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 Condition Index: 1 - 10 November 2018

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
Hot and dry conditions will prevail for the most part as a high-pressure system in the upper-air will dominate across the interior until at least the weekend. Isolated, south-north moving heat-induced thunderstorms will however develop on all days in the typical preference areas (Drakensberg, Eastern Highveld and Waterberg).

While most of the interior will be hot, the southwestern coastal areas and at times the southwestern interior will enjoy an influx of cooler Atlantic air, keeping temperatures in the normal ranges.

Conditions for thundershowers over the northeastern to central parts will improve from the weekend.

Conditions in main agricultural production regions (13 - 19 November)
Maize production region: It will be sunny to partly cloudy and hot. Very little precipitation is expected over most areas during the week until Friday. Isolated thundershowers are however expected over the eastern areas, mostly from Gauteng towards the east, favouring the eastern Highveld through the week.

From Saturday (17th), thundershowers will become more widespread over the central to eastern parts of the region, while temperatures will be lower. The thundershowers will spread over the entire region by Sunday (19th) and early next week.

Cape Wine Lands and Ruens: It will be partly cloudy to fine and mild through most of the period with light to moderate westerly to southerly winds. Light showers are possible initially on Tuesday (13th) at first. Winds will become strong southeasterly along the southwestern coastal areas from Friday (16th) into the weekend when it may become warm to hot over the West Coast into the Swartland. Little to no rain is expected over the region except for the initial wet conditions in the southwest.

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 13 November) 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:

Hot and dry conditions will dominate most of the time over the central to northeastern parts of the Northern Cape, most of the Free State, North West, Gauteng, Limpopo and the entire Lowveld for the entire week until at least Saturday, continuing over the central to western parts into early next week.
Hot, dry and (at some times) windy conditions over the northern and central to western interior through most of the period may be conducive to the development and spread of wild fires where vegetation is present. These conditions may spread as far west as the West Coast and Swartland by the weekend.

Some thundershowers over the eastern parts (especially over the Drakensberg and Eastern Highveld) may become severe by Sunday (18th). Current projections indicate the possibility of severe storms also by Monday (19th) over much of the Free State, North West, southern Limpopo, Gauteng, Mpumalanga and western to northern KZN (forecasts this far ahead of time are not necessarily considered reliable).
Strong southeasterlies are possible over the southwestern coastal areas from Friday (19th). Where vegetation is dry, this may be conducive to the development and spread of wild fires.
Sources:

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.

Coarse Resolution Imagery Database (CRID), ARC-ISCW.

Australian Bureau of Meteorology - http://www.bom.gov.au
Climate Prediction Center - http://www.cpc.ncep.noaa.gov 
International Research Institute for Climate and Society- http://iri.columbia.edu/
The Annular Mode Website - http://www.atmos.colostate.edu/ao/index.html

NOAA Climate Prediction Center - http://www.cpc.ncep.noaa.gov

CSIR NRE (National Resources and the Environment) 
'CSIR NRE produces forecasts on an experimental basis, doesn't guarantee the accuracy of the daily forecasts and cannot be held accountable for the results of decisions taken based on the forecasts'
Weather Underground - http://www.wunderground.com
Cooperative Institute for Meteorological Satellite Studies (CIMMS) - Tropical Cyclone Group - http://tropic.ssec.wisc.edu/
Tropical Cyclone Centre La Reunion - http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/index.html

Information on drought conditions over the USA: 
NOAA National Weather Service - http://www.weather.gov
United States Drought Monitor - http://droughtmonitor.unl.edu

Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) - http://Wxmaps.org 
'COLA and IGES make no guarantees about and bear no responsibility or liability concerning the accuracy or timeliness of the images being published on these web pages. All images are generated by COLA and do not represent the actual forecasts issued by the National Weather Service. These products are not a substitute for official forecasts and are not guaranteed to be complete or timely. The underlying data are the direct product of the various operational forecast models.'


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