Cumulus Report March 2019 - South Africa

Thundershowers developed over the central to southern and eastern parts of the country during the last few days, in many cases resulting in precipitation over the normally arid areas towards the west and south. From time to time, thundershowers also spread into the Free State and North West. Warm and dry conditions dominated for the most part. On Monday (18th), thundershowers (mostly isolated in nature) shifted to the northeast while the central to western interior became dry.
The next few days will initially be characterized by warm and dry conditions over the interior, but scattered thundershowers will set in over most of the central and eastern summer rainfall region from Friday onwards. Sunny and warm to hot conditions over large parts will be replaced by cooler, partly cloudy to cloudy conditions during the weekend.
The following is a summary of weather conditions during the next few days:
• General:
o The central to eastern parts of the summer rainfall region may receive above-normal rainfall, but the southwestern parts of the country will receive below-normal rainfall.
o On average, temperatures will be above normal over the interior, but normal to below normal over the southern to southeastern and eastern coastal areas as well as the eastern escarpment.
o Warm to hot conditions over large parts of the interior initially will be replaced by somewhat cooler conditions during the weekend and early next week when thundershowers are expected over large parts.
o Little to no rain is expected over the winter rainfall region except the southern parts where showers are possible from time to time.
o Moderate to strong southeasterlies will dominate the southwestern parts from Wednesday (20th) to Friday (22nd) and again early next week.
o There are indications (still fairly uncertain) of widespread rain over the interior by early next week.
• Rainfall:
o Thundershowers are expected over the extreme northeastern and eastern areas on Tuesday (19th). While isolated in nature, some storms may become severe.
o Showers or thundershowers will be confined to KZN and Mpumalanga on Wednesday (20th) and Thursday (21st).
o Thundershowers over KZN and central to eastern Mpumalanga on the 20th and 21st may have the tendency to become severe.
o Scattered thundershowers will occur over the northeastern parts of the country, including the central to eastern maize-production region, on Friday (22nd).
o Scattered thundershowers will occur (according to current projections) over the central to eastern and southeastern parts of the country, including the western and eastern maize-production regions, from Saturday (23rd) into early next week.
o Light showers are expected over the Garden Route on Wednesday (20th) and Friday (22nd).
o Rain and showers are possible over the southern parts of the winter rainfall region and Garden Route as well as the Little Karoo by Monday (25th) according to current projections. The extent and intensity of rainfall is still uncertain with this long lead time.
• Temperatures:

o Maximum temperatures over the western maize-production region will range between 26 and 34 °C while minimums will be in the order of 15 – 19°C.
o Maximum temperatures over the eastern maize-production region will range between 24 and 32°C while minimums will be in the order of 11 – 19°C.
o The warm to hot conditions over many areas will be replaced by cooler conditions from Friday.
o Hot and dry conditions with westerly winds will dominate over the central to northern parts of the Northern Cape on Tuesday (19th) and again by the weekend.
o It will be hot over the Limpopo River Valley and the Lowveld until Thursday (21st).
o Cool conditions with showers are expected along the southern to eastern coastal areas and adjacent interior from Thursday (21st) until Saturday (23rd) starting in the south, moving up the coast.
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:
ENSO (updated 19 March): ……..the chance of El Niño developing in 2019 has increased to approximately 70%, around triple the normal likelihood.
Tropical Pacific sea surface temperatures have touched on El Niño thresholds for the past three weeks, while waters below the surface are also slightly warmer than average. Signs of El Niño in the atmosphere are less clear. While values of the Southern Oscillation Index (SOI) are currently within El Niño bounds, the index is likely to weaken in the coming days. Large swings in the SOI are not uncommon during the southern hemisphere monsoon season. Additionally, trade winds have been closer to normal over the past fortnight after a period of weakened trades in the western tropical Pacific.
Most international climate models suggest sea surface temperatures in the central tropical Pacific Ocean are likely to remain at El Niño levels into winter. Sustained warmer than average ocean waters would increase the likelihood of coupling between the atmosphere and ocean, which would typically cause changes in Australian and global weather patterns. However, current outlooks have less skill for the period beyond May, and therefore predictions for the latter months should be viewed with some caution......Australian Bureau of Meteorology -
SSTs in the tropical Pacific increased from a borderline to a weak El Niño level in February and early March, while subsurface waters became more strongly warmer than average. Patterns in the atmosphere suggested El Niño more than was the case one month ago. 

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.

Towards autumn (April – June), seasonal forecast models suggest a continuation of warmer and drier-than-normal conditions over much of the interior (Forecasts issued in 2019-03 by the IRI -

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.
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 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): February 2019
Most of the summer rainfall region received normal to above-normal rainfall during February. Most of the rain over the central parts occurred before the 18th.

Rainfall (mm): February 2019
Widespread rain occurred over the summer rainfall region until mid-February, with most of the maize-production region receiving in excess of 75 mm and large areas more than 100 mm for the month.

Vegetation activity over the western maize-production region improved markedly following rainfall during late January and early to mid-February. This was followed by widespread rain around 11 March. Activity remains above normal over the eastern production region.

Overview of expected conditions over South Africa during the next few days
It will be warm to hot and dry initially over the interior due to upper-air anticyclonic circulation. Towards the extreme east and northeast, thundershowers are expected due to availability of moisture and some instability to the east of the interior anticyclone. By Thursday to Saturday and again early next week, the Atlantic Ocean Anticyclone will ridge strongly around the country. Upper-air perturbations will strengthen over the interior from Friday while moisture levels will rise due to the ridging action towards the south and east. This will result in scattered thundershowers over large parts of the country. Again, by early next week, the ridging of the second anticyclone and the movement of an upper-air trough over the western to central parts will support further thundershowers over the interior. Current projections indicate the possibility of widespread rain depending on the position and strength of the upper-air trough in the west. Due to the on-shore flow (associated with frequent ridging) over the southern to eastern areas, temperatures over these parts will on average be suppressed while showers or thundershowers will often occur. A cold front, associated with the upper-air trough on Sunday, will result in some showers over the winter rainfall region on Sunday and Monday (24th and 25th). 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 (19 – 25 March)
Maize production region: Following a relatively dry start, thundershowers will spread across the entire region during the weekend. Temperatures will be somewhat above normal, but maximum temperatures will generally not exceed the lower 30s. Maximum temperatures over the western maize-production region will range between 26 and 34°C while minimums will be in the order of 15 – 21°C. Maximum temperatures over the eastern maize-production region will range between 24 and 33°C while minimums will be in the order of 11 – 18°C. In general, the higher temperatures will occur during the early part of the period while lower temperatures (especially maximums) are expected from the weekend onwards. Thundershowers over the eastern parts of the region from Tuesday (19th) to Thursday (21st) may have a tendency to become severe.
Cape Wine Lands and Ruens: This region will be dry for the most part, but there are indications of some widespread showers early next week. Light showers are expected over the southern parts and along the rest of the Garden Route on Tueasday and Wednesday (19th and 20th). It will become hot over the western to northern areas especially on Friday and Saturday (22nd and 23rd) with no rain over the region. Strong Southeasterlies are expected in the southwest on these days, starting Thursday. By Sunday, it will become cloudy and cooler over the southwestern parts, spreading somewhat north and along the Garden Route on Monday according to current projections. This forecast has a long lead time and the extent of the rainfall will probably change somewhat closer to the actual rainfall event.

Strong southeasterlies will again dominate from Monday (25th) in the southwest. Where vegetation is dry, the windy conditions may be conducive to the development and spread of wild fires.

Possible extreme conditions - relevant to agriculture

According to current model projections (GFS and CCAM atmospheric models) of weather conditions during the coming week, the following may be deduced:
• Some thundershowers over the interior may have the tendency to become severe, producing strong gusts and hail:
o Over the extreme eastern and northeastern parts (Tuesday 19th).
o KZN, Mpumalanga (Wednesday and Thursday (20th and 21st)).
• Warm to hot and windy conditions (westerly winds) are possible over the northern parts of the Northern Cape on Tuesday (19th) and again by the weekend. Where vegetation is available and dry, this may be conducive to the development and spread of wild fires.
• Strong southeasterlies are expected over the southwestern Cape, especially from Thursday (21st) until Saturday (23rd) and again early next week. These may be conducive to the development and spread of wild fires where vegetation is dry.




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