CUMULUS Report South Africa - 12th March 2019

From the 8th until the 11th, the following totals (mm) were recorded over the western maize region: Lichtenburg (53), Potchefstroom (41), Wesselsbron (38), Bothaville (49), Kroonstad (64), Welkom (71). Cloudy and cooler conditions enhanced the effectiveness of the rainfall. The upper-air system responsible for the widespread rainfall over the summer rainfall region was a cut-off low, situated over the Western Cape. The position of the upper-air low together with a strong high-pressure system south of the country also resulted in widespread rainfall over the winter rainfall region and Garden Route where some areas received large rainfall totals leading in some cases to flash floods.

The next few days will feature a dominant tropical cyclone (Idah) over the Mozambique Channel, expected to make landfall over Mozambique later this week and resulting in further floods in that country. Current outlooks favor a westward track into Zimbabwe, from where the system is expected to move back east with minimal effect on South Africa’s weather except perhaps resulting in drier conditions over the northern parts and possibly light showers in the extreme northeast.
Over South Africa, drier conditions are expected with only isolated to scattered thundershowers over the central parts, spreading northeastwards later. Temperatures will be somewhat higher than what we saw during the wet spell of the last few days.
The following is a summary of weather conditions during the next few days:
• General:
o Most of the country will receive normal to below-normal rainfall.
o On average, temperatures will be above normal over the interior, but normal to below normal over the southeastern parts and eastern escarpment.
o Tropical cyclone Idah is expected to make landfall late Thursday (14th) and move westwards into eastern to central Zimbabwe. The position of the system, according to current forecasts, will be too far north to result in significant rainfall over the northeastern parts of South Africa. The movement of these type of systems is difficult to predict and the expected track may change over the next few days, with concomitant changes in rainfall distribution.
o Partly cloudy to cloudy and mild conditions may occur over the northern Lowveld and eastern parts of the Limpopo River Valley from Thursday to Saturday (16th) with isolated showers or thundershowers due to the proximity of Tropical Cyclone.
o Inclement conditions may remain in place over parts of the winter rainfall region until Wednesday (13th).
o Moderate to strong southeasterlies will dominate the southwestern parts from Wednesday to Thursday (14th) and Sunday (17th).
• Rainfall:
o Scattered thundershowers will develop over the southern Free State and eastern parts of the Northern Cape, into the Eastern Cape, on Tuesday (12th). These will have a tendency to become severe.
o Showers (more isolated than earlier) may remain in place over the Garden Route until Friday (15th).
o Scattered thundershowers are expected over the central to southern Northern Cape and interior of the Western Cape and Eastern Cape on Thursday (14th) and Friday (15th).
o Isolated to scattered thundershowers are possible over the central to western interior on Tuesday (12th) to Thursday (14th), clearing in the west by Friday but remaining in place over the Free State and adjacent areas.
o Scattered thundershowers will occur over the Eastern Cape and Drakensberg on Friday, spreading northwards into Gauteng and Mpumalanga, eastern Free State and eastern North West by Saturday (16th).
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o Thundershowers over the eastern Highveld on Saturday and Sunday (16th/ 17th) and Limpopo later may have the tendency to become severe (and produce strong winds and hail), above the norm for this time of the summer.
o Thundershowers will locate over the northern to eastern extremes from Sunday (escarpment, Lowveld, Limpopo) and possibly North West.
• Temperatures:
o Hot and dry conditions with westerly winds will dominate over the central to northern parts of the Northern Cape from Friday (15th) onwards.
o An on-shore flow will keep the far eastern to southeastern parts relatively cool with average temperatures in the near-normal category.
o Maximum temperatures over the western maize-production region will range between 27 and 33°C while minimums will be in the order of 15 – 21°C.
o Maximum temperatures over the eastern maize-production region will range between 25 and 33°C while minimums will be in the order of 12 – 18°C.
o It will be hot and humid over the western parts of Limpopo from Thursday (15th) onwards.
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 5 March): The El Niño–Southern Oscillation (ENSO) is currently neutral, but sea surface temperatures in the tropical Pacific Ocean have continued to warm. 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 over the past fortnight, and have touched on El Niño thresholds. This rapid warming was largely due to a period of weakened trade winds, associated with an active period of the Madden–Julian Oscillation (MJO) and the impacts of tropical cyclone Oma and typhoon Wutip. These trade winds have now returned to near average as the MJO has moved eastwards, suggesting that ocean temperatures may also ease over the next fortnight.
Five of eight climate models indicate the central Pacific is likely to reach near borderline or weak El Niño levels during autumn, with four models remaining above threshold levels into winter. However, El Niño predictions made in early autumn tend to have lower accuracy than predictions made at other times of the year.

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.
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, 

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. 
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, 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.
(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.
(26/02/2019) Thundershowers continue over the northeastern parts, but the rest of the country is hot and dry.
(05 - 12/03/2019) Widespread rain and thundershowers occurred over the central parts of the country.
The observations above indicate that the latter part of summer 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.

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 Condition Index: 21 – 28 February 2019
Vegetation activity over the western maize-production region improved markedly following rainfall during late January and early to mid-February. Activity remains above normal over the eastern production region.

Overview of expected conditions over South Africa during the next few days
The upper-air cut-off low responsible for much of the rain over the interior and also the winter rainfall region and Garden Route during the last few days is now shifting out southeastwards. Another (weak) upper-air trough will move over the southwestern parts from Wednesday (13th) to Friday (15th), supporting thundershowers over much of the Northern Cape, Western Cape interior and moving into the Eastern Cape and Free State. While the flow over the interior may become anticyclonic during the weekend, the upper-air trough (now to the southeast) will deepen to the east, with upper-air perturbations possibly moving over the eastern to northern parts, supporting thundershowers over the eastern to northern areas towards early next week. The ridging Atlantic Ocean Anticyclone will support thundershower development over the eastern parts together with the upper-air perturbations. With dry air to the west and the southwesterly winds in the upper-air, thundershowers over the eastern to northern parts may have an enhanced tendency to become severe.
All the while, the tropical cyclone (Idah) making landfall on Thursday (14th), is expected to penetrate as far west as eastern Zimbabwe, where after it is expected to slide back eastwards during the weekend as the steering flow becomes eastwards due to the interaction between the developing high-pressure system over the central parts and the deepening trough to the south/southeast.
With the persistent ridging action to the south, southeasterlies will dominate in the southwest while temperatures over the southern parts of the country will be relatively low.
Conditions in main agricultural production regions (12 March)
Maize production region: This week will be drier than the previous week, but thundershowers are still expected over the entire area at times. 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 27 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 25 and 33°C while minimums will be in the order of 12 – 18°C. Isolated thundershowers are expected over the western parts on Thursday (14th) to Saturday (16th). Thundershowers may occur over North West, Gauteng and Mpumalanga on Sunday and Monday. During this time (Sunday and Monday), some thundershowers will have a tendency to become severe and my produce strong winds and hail.
Cape Wine Lands and Ruens: Some residual showers or thundershowers will linger over mostly the southern to southwestern areas on Tuesday to Wednesday (12th/13th). The wind in the southwest will become fresh to strong south-easterly on Wednesday to Thursday (13th/14th). Scattered thundershowers are possible over the interior (mostly Karoo) on Thursday and Friday (15th/16th). Thundershowers over this area may easily produce strong gusts and hail. Showers are also possible along the Garden route on these days. Temperature wise, the region will start out cool, but it will gradually become warmer towards Thursday (15th), after which maximum temperatures should remain in the upper 20s or lower 30s for the most part, with minimum temperatures in the mid-teens.

Possible extreme conditions - relevant to agriculture

• Some thundershowers over the interior may have the tendency to become severe, producing strong gusts and hail:
o Southern Free State, eastern Northern Cape, central to eastern parts of the Eastern Cape (Tuesday 12th).
o Eastern Cape, western KZN, eastern Free State, southern Mpumalanga, Gauteng (Saturday 16th).
o Northeastern parts (Sunday and Monday 17th / 18th).
• Warm to hot and windy conditions (westerly winds) are possible over the Northern Cape interior from Friday (15th).
• Strong southeasterlies over the southwestern Cape, especially from Wednesday (13th) to Thursday (14th) 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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