More rain is expected over especially the central to southern parts during the next few days and the entire maize-production region should receive normal to above-normal rainfall.
Conditions for more rain over the central parts remain in place.
Scattered to widespread thundershowers occurred over large parts of the interior during the last few days, including also the central parts. Upper-air circulation was generally favourable and the Atlantic Ocean Anticyclone ridged south of the country on numerous occasions, keeping lower-level flow favourable over the central to eastern parts while maintaining strong south easterlies over the south western Cape.
During the weekend, as an upper-air trough moved through and dry air invaded the western parts, thundershowers had an enhanced tendency to become severe over the central and eastern parts. Widespread showers and thundershowers also occurred along the Garden Route and parts of the winter rainfall region and Karoo due to a surface low in that area on Saturday (2nd) associated with the upper-air system. By the 4th, activity was confined to the north eastern parts.
More rain is expected over especially the central to southern parts during the next few days as upper-air troughs will redevelop over the south western areas and a weak tropical low will make its way westwards over Botswana, dragging moisture into Namibia from where it will spill into the western and central parts of South Africa ahead of the trough in the southwest. During the weekend and towards early next week, parts of the western and Eastern Cape provinces, including the Karoo, may receive widespread rain.
Once again, wetter conditions over large parts of the summer rainfall region will be associated with frequent ridging of the Atlantic Ocean Anticyclone resulting in strong south easterlies expected over the south western parts of the winter rainfall region on several days.
The following is a summary of weather conditions during the next few days:
• Rainfall over most of the interior will on average be normal to above normal. This excludes the northern parts of the winter rainfall region and the north eastern parts (mostly Limpopo) according to current projections.
• The entire maize-production region should receive normal to above-normal rainfall.
• On average, it will still remain warmer than normal over the south western and western interior.
• On average, temperatures over the eastern parts of the country should be near normal.
• It will be hot and dry with north westerly winds over the interior of the Northern Cape and as far east as the western Free State on Wednesday and Thursday (6th and 7th).
• Very hot, windy berg-wind conditions are expected over the Garden Route on Thursday (7th) and Eastern Cape on Friday (8th).
• Hot and dry conditions will remain in place over most of the interior of the Northern Cape, the West Coast, Swartland and Karoo until Thursday (7th).
• Thundershowers are expected initially over the north eastern parts of the country, but will gradually be replaced south westward. By Friday (8th), scattered thundershowers will occur over the western, central and eastern areas.
• Scattered to widespread thundershowers will occur over the central, western and southern to south eastern parts of the country (including the Karoo) by the weekend and early next week.
• The western maize-production region may receive scattered to widespread thundershowers from Friday (8th) onwards.
• Thundershowers will occur relatively far west on Saturday (9th) to Monday (11th), reaching the western escarpment, covering the entire interior of the Northern Cape and parts of the Western Cape and possibly even the West Coast on Monday (11th) according to current projections (somewhat long lead time).
• Hot and humid conditions are expected over the Limpopo River Valley and Lowveld on Sunday and Monday (10th/11th).
• Thundershowers may produce significant daily rainfall totals over the southern to eastern Free State, Drakensberg and north eastern parts of the Eastern Cape by Sunday (10th).
• Widespread rain and showers with significant falls in places are possible over the Garden Route, Karoo and most of the Eastern Cape according to current projections by Sunday and Monday (10th and 11th).
• Moderate to strong south easterlies will occur over the south western parts on most days, reaching maximum strength on Wednesday (6th) and Sunday (10th).
• The winter rainfall region will be dry initially until the weekend when showers and thundershowers are expected along the Garden Route and Karoo by Sunday (10th) and Monday (11th), possibly reaching the southern parts of the West Coast by the 11th according to current projections.
Seasonal overview
El Niño and seasonal forecasts
The El Niño-Southern Oscillation (ENSO) is currently neutral. The Tropical Pacific sea surface temperatures have cooled slightly in the past fortnight but remain warmer than average. Importantly in the central to western Pacific, warmer than average sub-surface waters persist. This is often a precursor to El Niño events. While most atmospheric indicators of ENSO are neutral, trade winds have been weaker than average, or even reversed, across the central and western Pacific in recent weeks, leading to sub-surface warming. Westerly wind anomalies over the western tropical Pacific are forecast to persist a little longer, which is likely to assist further warming of the tropical Pacific.
The majority of climate models suggest ENSO-neutral sea surface temperatures will continue through autumn. However, the current ocean warmth, and the forecast for warmer than average sea surface temperatures later in the year, means the possibility of El Niño remains. Three of eight models suggest that El Niño may establish by mid-2019, with another three models on the warm side of neutral. It should be noted that model outlooks that span the southern autumn period tend to have lower accuracy than outlooks issued at other times of the year. This means outlooks beyond May should be used with some caution...Australian Bureau of Meteorology - http://www.bom.gov.au
According to the International Research Institute for Climate and Society, there remains a high likelihood for an El Niño during summer 2018/19.
The Southern Oscillation Index was trending negative during most of last 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 to positive range - signalling weak coupling as mentioned above.
Based on the weak El Niño, forecast models lean towards a tendency for drier conditions by late summer, following (according to earlier forecasts) a wetter early to mid-summer period over 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. 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. Recent seasonal forecasts (except for that of SAWS) are somewhat more negative with regards to rainfall over South Africa towards late summer.
During late December and 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 signalling 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.
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 north eastern parts of the country, spilling also into the central parts. The western parts will still be more likely to receive below-normal rainfall. These maps are similar to the maps published previously, but here the tendency for below-normal- and for above-normal rainfall is shown instead of the tendency for normal-to-above-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.
The atmospheric circulation patterns favoured the north eastern parts of the country for near-normal to above-normal rainfall during the period October - December 2018. Frequent anti-cyclonic upper-air conditions as well as persistent westerly winds at the surface kept the central to western parts dry. Most of the forecasts indeed indicated some kind of west - east improvement in expected conditions. The relatively wet area was focused more strongly further south, over the Eastern Cape, in the forecast issued by the IRI. The forecast issued by SAWS had a stronger positive signal over the Lowveld instead of the northern Highveld. The forecast issued in Cumulus also indicated the positive signal in the northeast, somewhat closer to the area where rainfall was near normal to above normal. Drier conditions in the west were resolved fairly well by all three forecasts.
Expected rainy season progression, associated with decadal variability - update
Following wetter conditions earlier, there has been a drying trend over the central parts of the country while precipitation in the northeast has also become less widespread. Based on typical patterns related to decadal variability, wetter conditions may return from late January, after what - at this stage- may resemble a mid-summer drought over the central parts.
If wetter conditions do develop from late January and in February, it will most likely be related to a repositioning of tropical systems. Currently, the presence of a tropical low towards the northeast of South Africa has a negative effect on rainfall locally - or at best no impact at all. The positioning of the low-pressure area to the northeast is however also related to the mid-latitude anomalies present lately. Persistent westerlies over the central to western interior, related to a northward displacement of the Atlantic Ocean Anticyclone, will need to cease in order to have a positive rainfall pattern established later. This may happen with more frequent ridging of high-pressure systems around the country.
(29/01/2019) Most recently, there was a significant change in atmospheric circulation patterns. Above-normal rainfall is expected over large parts of the interior as the Atlantic Ocean High ridges south of the country more frequently and hot, dry westerly winds are expected to weaken / disappear over the central parts. Together with these changes, enhanced convection to the northeast of South Africa has also ceased.
(05/02/2019) The favorable conditions for rainfall over the interior is expected to continue during the early part of February.
The most recent VCI map shows above-normal vegetation activity over the central to eastern maize-production region while the western parts (Bothaville/Welkom westwards) experiences the effect of dry weather during most of December and Middle-January. Recent rains since late January will have a positive impact on vegetation activity in these western areas.
Overview of expected conditions over South Africa during the next few days
Large-scale circulation patterns will remain fairly favourable for rainfall over the interior, especially from Friday onwards. At first a weak tropical low is expected to move westwards across Botswana from Limpopo, resulting in scattered thundershowers spreading westwards into Namibia from Limpopo. With a building trough in the westerlies over the south western parts from Friday (8th), intensifying through the weekend, scattered to widespread thundershowers will spread over most of the interior while clearing in the northeast. Apart from the tropical moisture introduced from the north over the central to western parts, a large anticyclonic circulation feature will strengthen over the Indian Ocean during the period and will contribute moisture from the east over the country. The deepening trough in the southwest together with some dry air moving in over the south western parts by Sunday (10th) and Monday (11th) may result in a tendency for more severe thundershowers over some of the southern to south eastern parts of other country.
Conditions in main agricultural production regions (5 - 11 February)
• Maize production region: Temperatures should remain in the near-normal range over the region while total rainfall is expected to be normal to above normal for this time of the year. Isolated thundershower will occur over the northern half on Tuesday (5th). It should be mostly dry on Wednesday (6th). Scattered thundershowers are once again expected over the central to northern and eastern parts of the region by Thursday (7th). From Friday (8th), scattered to widespread thundershowers may occur over the entire region, clearing somewhat in the extreme northeast on Saturday (9th).
• Cape Wine Lands and Ruens: Southeasterlies will once again dominate. These will be moderate to strong for the most part over the southwestern parts, with strongest winds indicated by Wednesday (6th) and Sunday (10). The dominant southeasterly to easterly flow over the interior will result in hot conditions over the Swartland and West Coast as well as the Karoo and Boland, reaching maximum intensity by Thursday (7th). Some thundershowers may occur over parts of the interior, especially further east (Karoo and Garden Route) by Sunday (10th) and Monday (11th). Showers along the Garden Route may be widespread on Sunday (10th). Showers and thundershowers may possibly spread to the West Coast, to include the entire region, by Monday (11th) according to the latest projections.
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 - Centre 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 5th February) 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 until Thursday (7th) over parts of the Northern Cape and into the western Free State at times may be conducive to the development and spread of wild fires where (if) vegetation is dry.
• Berg-wind conditions over the Karoo and Garden Route of the Western and Eastern Cape from Wednesday (6th) to Friday (8th) may be conducive to the development and spread of wild fires.
• Some thundershowers may produce significant rainfall totals:
o Over the southern Drakensberg on Saturday and Sunday (9th and 10th).
o Over the southern to eastern Free State and interior of the Eastern Cape on Sunday (10th).
o Over the Karoo and Garden Route on Sunday and Monday (10th and 11th).
• Some thundershowers may become severe (heavy downpours, strong gusts and in isolated cases hail):
o over the central to eastern parts of the Northern Cape and interior of the Eastern Cape (Sunday, 10th)
• Moderate to strong southeasterlies over the southwestern parts of the Cape during most of the period (reaching a maximum on Wednesday (6th) and Sunday (10th) may be conducive to the development and spread of wild fires where vegetation is dry.
