Wetter conditions following heat and drought
Some rain occurred since the 20th over large parts of the northeastern interior, including some of the maize-production areas. This followed a period of almost complete dryness over the summer rainfall region since early November - and in some places since late October.
The next few days will see a continuation of scattered thundershowers over the summer rainfall region, spreading also over the central areas at times. It should be a week with relatively extensive cloud cover, with an indication of clearance by the latter part of the weekend and early next week.
The southern parts of the country will remain cool due to an onslaught of cool, dry air from the south/southwest. The strongest event is expected on Tuesday (27th) and Wednesday (28th). The on-shore flow will again result in fairly widespread showers and rain along the Garden Route and (to a lesser degree) into the Karoo, spreading eastwards over the area on Tuesday (27th) and Wednesday (28th).
Thundershowers over the central to eastern areas will shift northeastwards from Tuesday (27th), moving back to the central interior by Friday (30th). Again, these should clear from the west and move into the eastern to northeastern areas during the weekend, clearing by early next week when there is some indication of a renewal of wetter conditions over the far-eastern parts according to current projections.
The following is a summary of weather conditions during the next few days:
Rainfall over most of the summer rainfall region should be near normal during the next few days.
The western interior is expected to be dry.
Cool, dry air will spread over much of the western, southern and central interior during Tuesday (27th) and Wednesday (28th).
The northeastern to eastern parts of the country, especially the Lowveld and northeastern KZN, will be very hot on Tuesday (27th).
The northeastern parts of the country, especially the Limpopo River Valley, will be very hot on Wednesday (28th).
Widespread thundershowers are possible over the eastern parts of Limpopo and Mpumalanga on Wednesday (28th), with significant falls possible along parts of the escarpment.
Strong westerly winds are possible over the eastern parts of the Northern Cape and northern parts of the Eastern Cape on Tuesday (27th).
Rain and showers will occur from the western winter rainfall region and western Garden Route (Tuesday 27th) moving through to the eastern Garden Route (Wednesday 28st) before clearing.
It will be significantly colder (+/- 10° drop in temperatures) over the southern interior (most of the Western Cape and Eastern Cape) on Tuesday and Wednesday (27th and 28th), accompanied by strong southerly to westerly winds. The wind should be especially strong along the coast.
Strong southwesterly winds are expected along the Garden Route on Tuesday (27th), spreading along the coast towards the north coast of KZN on Wednesday (28th).
Scattered to widespread thundershowers are expected over the central parts on Friday (30th), moving into the northeastern parts by Saturday where scattered falls are expected.
Thundershowers over the central parts on Friday (30th) may have a tendency to become severe with strong winds and hail possible.
Strong southeasterlies are expected over the southwestern coastal areas from Wednesday (28th) to Friday (30th).
Seasonal overview
El Niño and seasonal forecasts
ENSO El Niño-level SSTs were observed in the October average, and the subsurface waters also continued to be markedly warmer than average. However, the atmospheric variables showed mainly ENSO-neutral patterns. Only lower-level wind anomalies averaged weakly westerly in the eastern Pacific - a suggestion of El Niño. The official CPC/IRI outlook calls for an 80% chance of El Niño prevailing during winter, and a 55-60% chance of continuing into spring 2019.
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 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
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 favorable.
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 Condition Index: 11 - 20 November 2018
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.
Two upper-air troughs moving over the summer rainfall region together with ridging high-pressure systems will result in fairly normal distribution of thundershowers for this time of the year during the next few days. The first high-pressure system ridging around the country will also cause an influx of cool, dry air over the southern to western and central parts on Tuesday (27th) and Wednesday (28th). At the surface, a cold front will result in some showers over the winter rainfall region on Tuesday (27th), while the on-shore flow will result in fairly widespread showers along the Garden Route, moving eastwards (27th / 28th). The ridging high will also result in cooler conditions in the east as it moves around the country. The undercutting cool, moist air in the east will also result in fairly widespread thundershowers over the extreme eastern to northeastern areas by Wednesday (28th).
A second upper-air trough will traverse the country from Friday (30th) to Saturday (1st), resulting firstly in fairly widespread thundershowers over the central parts, including most of the Free State and North West. When moving east on Saturday, scattered to isolated thundershowers will again spread into the eastern to northeastern parts. It may become hot and windy over the central parts following the system from the 30th to Sunday (2nd).
An influx of moisture from an anticyclonic flow to the east of the country, together with some instability, may result in scattered thundershowers over the far eastern parts by early next week.
Conditions in main agricultural production regions (27 November - 3 December)
Maize production region: Isolated to scattered thundershowers will occur on the 27th, confined mostly to the eastern parts by Wednesday (28th) with a fairly good distribution over the far-eastern parts. It will be cooler on Thursday (29th), with scattered thundershowers still possible in the east, with isolated falls spreading over the entire area. Scattered to widespread thundershowers will move into the western to central areas by Friday, spreading eastwards while becoming more isolated on Saturday. Thundershowers will be confined to the far-eastern parts by Sunday (2nd), with somewhat more significant showers indicated by Monday according to current projections, over the eastern parts.
Cape Wine Lands and Ruens: Light showers are expected over the southwestern parts and eastwards along the Garden Route on Tuesday (27th). The wind will be strong southeasterly in the southwest from Tuesday to Thursday (29th). Temperatures should remain in the near-normal range, but the West Coast and Swartland may become hot on Thursday (29th), with warmer conditions spreading also into the Karoo by Friday (30th) when westerly winds may cool down the northwestern areas. Apart from the initial showers, the period should be fairly dry (and mild).
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 27 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.
It will be very hot over the Lowveld and most of the Limpopo River Valley on Tuesday (27th) and Wednesday (28th).
A sharp drop in temperatures on Tuesday (27th) 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).
Strong westerly winds over parts of the Northern Cape and into the Eastern Cape may be conducive to the development and spread of wild fires on Tuesday (27th).
Thundershowers over the Lowveld and along the escarpment by Wednesday (28th) may have the tendency to become severe in places.
Strong southwesterly winds are possible along the southeastern coastal areas (mostly Eastern Cape) by Wednesday (28th), spreading along the KZN coast later.
Thundershowers over the central parts (Free State, North West) may become severe in some places with strong wind and hail by Friday (30th).
Strong southeasterlies are possible over the southwestern coastal areas, especially from Tuesday (27th) to Thursday (29th). These may be conducive to the development and spread of wild fires.
Hot, windy conditions over most of the Northern Cape interior and surrounds from Thursday (29th) to Monday (3rd) may be conducive to the development and spread of wild fires where dry vegetation is available
