The next few days will see widespread rain and thundershowers over the central to eastern parts of the country (also in the west by the weekend). This will include the western and eastern maize-production regions, according to current outlooks. Several rain-producing systems are expected to develop over the central and western parts, resulting in widespread thundershowers expected on any given day somewhere over the interior. The South Atlantic Anticyclone will be quite prominent and frequent ridging will promote the development of upper-air troughs over the interior, advect moisture over the summer rainfall region and keep the southern parts of the country cool and wet. It will also result in strong southeasterlies at times over the southwestern Cape. There are no indications of early frost according to current forecasts.
The following is a summary of weather conditions during the next few days:
• General:
o Most of the interior will receive above-normal rainfall except for some areas in the southwest and some parts of Limpopo. The southern half of the winter rainfall region and Garden Route will receive near-normal rainfall.
o Normal to below-normal rainfall is expected over the far-western interior and parts of the winter rainfall region.
o Thundershowers will occur frequently over the summer rainfall region.
o It will be cooler with more cloud over the summer rainfall region than during the previous few weeks.
o There is no indication of early frost given the expected weather conditions during the next few days.
o On average, temperatures will be slightly above normal over the interior, but normal to below normal over the southern to southeastern and eastern coastal areas as well as the eastern and southern escarpment.
o The eastern parts of the country, especially the eastern Highveld, will be cloudy, wet and cool on Tuesday (2nd) and Friday (5th).
o The southern parts of the country will be cool to cold, wet and windy on Thursday (4th) and Friday (5th).
o The southeastern to eastern coastal region and adjacent interior will be cloudy and windy with showers on Friday (5th) and Saturday (6th).
o A cold front will result in cold, wet and windy conditions over the winter rainfall region on Thursday (4th).
o Fresh to strong southeasterlies will dominate the southwestern parts on Tuesday (2nd) and again from Friday (5th) continuing during the weekend.
• Rainfall:
o Scattered thundershowers are possible over the central parts of the country, including the western maize-production region, from Tuesday (2nd) to Friday (5th) and possibly again from Saturday (7th) when widespread rain is indicated (current projections at a long lead time) especially by Sunday (8th).
o Partly cloudy to cloudy and cool conditions with rain and thundershowers are possible over the eastern summer rainfall region, including the eastern maize-production region, until Wednesday (3rd) and possibly again by Friday (5th).
o Thundershowers should occur over the eastern parts, including the eastern maize-production region, on most days until the weekend.
o Rain and showers are possible over the winter rainfall region and the Garden Route on Friday (5th), moving to the southeast on Saturday (6th).
o Widespread rain and thundershowers are possible over the central to western interior by Sunday (7th) and Monday (8th) according to current projections, covering most of the Northern Cape and western to central North West and the Free State.
• Temperatures:
o Maximum temperatures over the western maize-production region will range between 22 and 28 °C while minimums will be in the order of 13 – 17°C.
o Maximum temperatures over the eastern maize-production region will range between 16 and 26°C while minimums will be in the order of 11 – 17°C.
o Temperatures will fall by about 10°C over the southern parts of the country where it will be cold and windy on Thursday (4th) and Friday (5th).
o It will be warm to hot over the West Coast, Swartland and northern to western parts of the Northern Cape on Tuesday (2nd), Wednesday (3rd) and possibly again by the weekend.
Seasonal overview
El Niño and seasonal forecasts
According to the Australian Bureau of Meteorology and the International Research Institute for Climate and Society, there remains a high likelihood of an El Niño during 2019:
Most international climate models predict tropical Pacific Ocean sea surface temperatures will remain at El Niño levels at least to mid-year. This would increase the likelihood of the tropical Pacific atmosphere and ocean reinforcing each other, and developing into a full El Niño, with the resultant changes in Australian and…. global weather patterns.
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. After being positive for some time, the index fell sharply during the last few weeks, possibly indicating some coupling between the ocean and atmosphere. The index is expected to become weaker during the next few days while the monsoon is active over northern Australia.
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 (February - April) and autumn/early winter (April-June) respectively.
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. Maps below...
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 northeastern 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.
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.
Mid-season review of seasonal outlook
Top left: Observed rainfall (% of long-term mean, based on satellite rainfall estimates). Top right: Rainfall forecast for OND – SAWS – issued in September 2018. Bottom left: Forecast based on decadal climate variability – issued September 2018. Bottom right: Rainfall forecast for OND – issued in September 2018 by the IRI.
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.
Observed rainfall (% of normal) for October to December 2018. Rainfall as per satellite rainfall estimates.
Forecast – equivalent version of forecast published in Cumulus – issued September 2018.
Rainfall forecast for October to December 2018. Issued by the IRI in September 2018.
Forecast for OND issued by SAWS – September 2018.
Expected rainy season progression, associated with decadal variability – update
The forecast above (published in Cumulus), issued in October 2018, indicated a drier early part of summer and wetter conditions in the second half of summer, particularly in February and/or March.
The following is a very concise summary of weather patterns over the summer rainfall region, intended to provide an overview of observed conditions in the light of the seasonal forecast:
November 2018 was an extremely hot and dry month over especially the central parts of the country. During December, above-normal rainfall occurred over the northeastern parts, but it remained dry over the central parts for the most part until the 27th. From 27 December to 8 January, widespread rain occurred over large parts of the summer rainfall region.
Mid-summer drought conditions occurred from 10 to 25 January. Wet conditions replaced the dry weather from late January and lasted until about 20 February. Towards the end of this wet period, a tropical low resulted in heavy falls along the northern escarpment.
Another warmer and dry situation then developed from the 20th of February and lasted until 8 March. From 8 March, and especially around the 11th, widespread rain and thundershowers occurred over the interior, focusing especially on North West and the Free State. Most recently, thundershowers (mostly isolated to scattered) located mostly over the western to central parts, moving to the extreme east from today (19 March).
The observations above indicate that the latter part of summer so far was wetter than the early summer, in accordance with the earlier outlook provided in Cumulus, and based on the decadal climate variability signal over South Africa. Together with somewhat above-normal rainfall, tropical system had a direct impact over the extreme north-eastern parts of the country.
Rainfall (% of long-term mean): March 2019
The southern half of the maize-production region received normal to above-normal rainfall while the northern parts received below-normal rainfall during March.
Rainfall (mm): 1 – 31 March 2019
Large parts of the Free State received between 50 mm and 100 Less rain occurred over Mpumalanga.
Vegetation Condition Index: 11 – 20 March 2019
Vegetation activity remains near normal to above normal over most of the maize-production region.
Overview of expected conditions over South Africa during the next few days
The next few days will be characterized by a succession of upper-air troughs traversing the subcontinent from west to east, coupled with frequent ridging of the Atlantic Ocean Anticyclone which will feature prominently to the southwest and south of South Africa. These systems will create favorable conditions for rainfall over the interior, providing low-level moisture from the east together with upper-air instability. The combination of systems will especially favor the central to eastern summer rainfall region for widespread precipitation while relatively low temperatures should occur over the southern parts of the country. One of the upper-air systems may develop into a cut-off low by the weekend, resulting in widespread rain over the central to southern and southwestern interior according to current projections.
With the frequent ridging action to the south, fresh to strong southeasterlies are expected in the southwest at times.
Conditions in main agricultural production regions (2 - 8 April)
Maize production region: The region will receive above-normal rainfall during the next few days. It will be noticeably cooler than the previous week with abundant cloud cover. Maximum temperatures over the western maize-production region will range between 22 and 28 °C while minimums will be in the order of 13 – 17°C. Here, thundershower are possible on most days, with indications of widespread rain by late Thursday (4th) and again by Saturday/Sunday according to current projections. The weekend may also be mostly cloudy over this region coupled with the expected rainfall.
The eastern parts of the maize-production region will be cooler, with cloudy and cool conditions expected by Tuesday (2nd) and Friday (5th). Maximum temperatures over the eastern maize-production region will range between 16 and 26°C while minimums will be in the order of 11 – 17°C. During the weekend, this area should be relatively rain-free, while thundershowers and rain is still expected in the west.
Cape Wine Lands and Ruens: This region will be mild to cool for the most part. It will be warm to hot along the West Coast and Swartland on Tuesday and Wednesday (2nd / 3rd) and possibly during the weekend. Cloudy conditions with fresh to strong north-westerly winds and showers are expected over the entire region on Friday (5th) when a cold front will move over. The wind will be strong south-easterly over the southwestern parts on Tuesday (2nd) and again by Friday (5th) and during the weekend.
Possible extreme conditions - relevant to agriculture
• Some thundershowers over the interior may have the tendency to become severe, producing strong gusts and hail:
o Northern Cape interior and Western Cape interior (Sunday and Monday (7th and 8th)).
• Small stock farmers should take note that it will be cool to cold, wet and windy on Thursday (4th) and Friday (5th) over the interiors of the Western and Eastern Cape Provinces.
• Strong southeasterlies are expected over the southwestern Cape, especially on Tuesday (2nd) and again from Friday (5th) continuing into the weekend. These may be conducive to the development and spread of wild fires where vegetation is dry.
