Widespread rain occurred during the period 11 to 16 February over much of the central to eastern parts of the country. Cloudy conditions with rain and thundershowers dominated especially from the 13th to the 15th. Rainfall over the central parts was somewhat under-predicted. Due to the presence of the tropical system in the north, heavy rain also occurred over some of the northern parts of the country, especially along the northeastern escarpment where isolated areas received daily rainfall totals approaching 300 mm. Conditions cleared by the 17th, with sunny to partly-cloudy conditions prevailing coupled with relatively low night-time temperatures.
The next few days will see a gradual build-up of moisture levels while temperatures will increase towards Thursday. Cooler conditions with scattered thundershowers will set in over the eastern and northeastern parts from Thursday or Friday (22nd). Current projections indicate fairly favorable upper-air conditions and a continuation of an influx of moisture, resulting in scattered thundershowers from the northern Free State and central North West eastwards and northwards over the northeastern parts of the country. Maximum temperatures will also decrease slightly from Friday (22nd) due to cloud cover and the on-shore flow at the surface.
Once again the Atlantic Ocean Anticyclone ridging around the country will result in strong southeasterlies over the southwestern parts of the winter rainfall region on several days, especially early in the period and again from Sunday (24th) onwards.
The following is a summary of weather conditions during the next few days:
• General:
o Rainfall will on average be normal to above normal over the eastern to northeastern interior as well as the eastern to southern coastal belt and adjacent interior.
o The central to eastern maize-production region should receive normal to above-normal rainfall.
o The central to western interior should receive below-normal rainfall. Very little rain is expected over these areas according to current projections.
o The winter rainfall region as well as the interior of the Western Cape can expect below-normal rainfall for this time of the year.
o On average, it will remain warmer than normal over the southwestern, western interior and central interior.
o The eastern to southeastern coastal areas and adjacent interior will remain mild to cool for the most part with regular showers.
o Very little to no rain is expected over most of the interior until Thursday (21st).
o Thundershowers will occur over the eastern to northeastern parts from Thursday (21st) onwards.
o Moderate to strong southeasterlies will occur over the southwestern parts on most days, especially until Thursday (21st) and again by Sunday (24th) into early next week.
• Rainfall:
o Scattered thundershowers are possible over the eastern parts, including the eastern maize-production region, from Thursday (21st).
o Scattered thundershowers are possible over the northeastern half of the country, including the central to eastern maize-production region, from Friday (22nd).
o No rain is expected over the western interior according to current projections.
o Showers are possible along the eastern to southeastern coastal belt and adjacent interior on most days.
o Thundershowers over the northeastern parts may become severe from Friday (22nd) to Sunday (24th).
o Light showers are possible over the Garden Route and Karoo, accompanied by cooler conditions, by Friday and Saturday (22nd and 23rd).
o The winter rainfall region should receive very little rain during the next few days and will be dominated by warm to hot and dry conditions for the most part.
• Temperatures:
o Hot and dry conditions with westerly winds will dominate over the Northern Cape with high maximum temperatures especially during the rest of the week.
o Hot, berg-wind conditions will occur over the Karoo and coast of the Eastern Cape on Tuesday (19th) and spread to the coast of KZN on Wednesday (20th).
o Very hot conditions will develop over the West Coast and Swartland from Tuesday (19th) to Thursday (21st) and possibly again by early next week.
Seasonal overview
El Niño and seasonal forecasts
ENSO The El Niño-Southern Oscillation (ENSO) remains neutral. However, the Bureau's ENSO Outlook remains at El Niño WATCH, meaning there is approximately a 50% chance of El Niño developing during the southern hemisphere autumn or winter, twice the normal likelihood.
Tropical Pacific sea surface temperatures have warmed slightly in the past fortnight. In the sub-surface, weak warmth extends down to 175 m depth. Recent weakening of the trade winds in the western Pacific means that further warming of the equatorial Pacific is likely in the coming weeks to months.
Five of eight climate models indicate the central Pacific is likely to reach borderline or weak El Niño levels during autumn, with four models remaining above threshold levels into winter. El Niño predictions made in late summer and early autumn tend to have lower accuracy than predictions made at other times of the year. This means that current forecasts of the ENSO state 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.
International Research Institute for Climate and Society- http://iri.columbia.edu/
Australian Bureau of Meteorology - http://www.bom.gov.au
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. The following are the latest seasonal forecasts for Africa, from the IRI, for late summer (January - March) and early autumn (February - April) respectively.
With the weak El Niño developing in the Pacific, seasonal forecast models, such as those used by the IRI, indicate hot and dry conditions for January to March over southern Africa (Forecasts issued in 2018-12 by the IRI - http://iri.columbia.edu/).
Towards late summer, seasonal forecast models suggest a continuation of warmer and drier-than-normal conditions over much of the interior (Forecasts issued in 2019-01 by the IRI - http://iri.columbia.edu/).
Expected rainy season progression (Jan - March), associated with decadal variability
(Issued 22 October 2018)
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. 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 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.
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 (issued here) 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 favored the northeastern 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 seasonprogression, associated with decadal variability - update
(15/01/2019) 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.
(11/02/2019) Rainfall still continues over the summer rainfall region, with a tropical low contributing large amounts of moisture and affecting parts of Limpopo directly until 15 February.
Overview of expected conditions over South Africa during the next few days
The first few days, until Thursday (21st) will be characterized by a dominating upper-air high over the interior while an upper-air trough is located to the east of the country. This type of circulation pattern results in dry conditions over the interior, as experienced since the 16th over most areas. The center of the anticyclonic circulation will be located over Botswana, resulting in hot westerlies especially over the western to central interior. From Friday, the steepening of an upper-air trough over the southern parts of the country, together with upper-air perturbations developing over the northeastern parts of the country will support the development of thundershowers over the eastern to northeastern parts, as far west as the northern parts of the Free State according to current projections. Ridging of the South Atlantic Anticyclone will result in advection of moisture over the eastern parts, resulting in lower temperatures by the weekend but also supporting thunderstorm development. As the system ridges around the country, the advection of moisture and favorable upper-air conditions may also result in showers over the southern parts of the country, including much of the Karoo, during the weekend. Dry westerlies will dominate the western to central areas until at least Friday. Interaction of dry air from the west and moisture in the east will cause some thunderstorms over the northeastern parts to become severe. There are early indications of another upper-air trough developing over the western parts by Monday/Tuesday next week.
Conditions in main agricultural production regions (19 - 25 February)
Maize production region: It will be sunny to partly cloudy and mild to warm, with day-time temperatures increasing towards Thursday when maximum temperatures will be in the mid-thirties over the western parts of the region. From Thursday, isolated to scattered thundershowers will start occurring over the eastern parts, spreading over the entire region by Friday when temperatures will decrease somewhat. Some thundershowers over the region may have a tendency to become severe from Friday and during the weekend, when the main focus area will shift eastwards.
Cape Wine Lands and Ruens:Current projections indicate a continuation of warm to hot conditions over most of this region until Friday (22nd). The wind should be southerly to southeasterly and moderate to strong over the southwestern parts where it may contribute to a high fire danger. The southerly to easterly component in the wind will result in relatively cooler conditions over the Garden Route while hotter conditions will occur especially over the West Coast and Swartland together with (at times) the Karoo. There is a small chance, according to the current outlook, for showers during the weekend - especially over the southern parts (Garden Route and southern Peninsula as well as possibly the southern to eastern Karoo). Southeasterlies in the southwest may become strong by Monday (25th).
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 19th 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:
Thundershowers occurring over the eastern to northeastern parts of the country from Thursday (21st) to Sunday (24th) may have a tendency to become severe.
Moderate to strong southeasterlies over the southwestern parts of the Cape during most of the period, reaching a maximum on Thursday (21st) and Sunday/Monday (24th/25th), may be conducive to the development and spread of wild fires where vegetation is dry.
Warm to hot and windy conditions (westerly winds) are possible over the Northern Cape interior and western Free State from Wednesday (20th) to Friday (22nd). This may be conducive to the development and spread of wild fires where (if) vegetation is dry
Hot, berg-wind conditions will occur over the Karoo and coast of the Eastern Cape on Tuesday (19th) and spread to the coast of KZN on Wednesday (20th).
Sources of information
Rainfall, temperature and wind maps over South Africa for the past week:
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.
