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Cumulus Report- 26 February -Rain should return by the weekend- South Africa

Thundershowers were particularly widespread on Friday (22nd) when most places to the east and north of Bloemhof / Bothaville received at least some rain with totals varying between 5 and 30 mm.

Some thunderstorms became severe resulting in wind or hail damage. Thundershowers remained in place over the northeastern parts until yesterday.
The next few days will see warm to hot and dry conditions continuing over much of the interior. Dry westerlies over the interior will compliment several frontal systems moving over the southern parts, keeping temperatures relatively low over the southwestern parts of the country. Westerly to southerly winds along the coast will keep temperatures in the normal to below normal category along the southern to eastern coastal belt and adjacent interior. Thundershowers will, according to current forecasts, once again remain confined to the northeastern parts until at least early next week. Frontal activity in the southwest will be associated mostly with westerly winds over the winter rainfall region. Strong southeasterlies are only expected by the weekend.
The following is a summary of weather conditions during the next few days:
• General:
o It will be drier than normal over most of the interior.
o The eastern to northeastern parts, including the central to eastern maize-production region should receive isolated to scattered thundershowers especially from Thursday.
o The central to western interior should receive no rainfall according to current forecasts.
o The winter rainfall region may receive some showers associated with several cold fronts expected to move over the area.
o It will be warmer than normal over the interior.
o The southwestern parts of the country as well as the eastern to southeastern coastal areas and adjacent interior will remain mild to cool.
o Moderate to strong southeasterlies may occur over the southwestern parts by the weekend.
• Rainfall:
o Isolated to scattered thundershowers are possible over the eastern parts, including (on some days) the central to eastern maize-production region.
o No rain is expected over the western to central interior including the western maize-production region according to current projections.
o Showers and thundershowers may become widespread over the central to northern parts of KZN as well as southern Mpumalanga during the weekend.
o Thundershowers over the northern and eastern parts of Limpopo and eastern parts of Mpumalanga may become severe from Wednesday (27th) to Friday (1st).
o Showers are possible along the eastern to southeastern coastal belt and adjacent interior on most days.
o Showers are possible along the Garden Route on Tuesday (26th) and Friday (1st).
o Showers are possible over the winter rainfall region on Tuesday (26th) and Sunday/Monday (4th/5th).
• Temperatures:
o Hot and dry conditions with westerly winds will dominate over the central to northern parts of the Northern Cape and western North West as well as western the Free State.
o It will be hot to very hot over the western Bushveld on most days.
o It will be warm to hot over the Limpopo River Valley and northern Lowveld on most days.
o The Karoo, winter rainfall region, Garden Route, southern to eastern parts of the Eastern Cape and southern parts of KZN will be cooler than normal for most of the period.

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.


According to the International Research Institute for Climate and Society, there remains a high likelihood for an El Niño during 2019:
SSTs in the tropical Pacific cooled to a borderline El Niño level in January and early February, while subsurface waters continued to be warmer than average. However, some atmospheric patterns of El Niño that had been lacking, finally developed in late January and February. Collective forecasts of models show a return to weak El Niño-level SSTs into summe


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 days, 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, for late summer (January – March) and early autumn (February - April) respectively.


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. 


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 (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.

Rainfall (% of long-term mean): January 2019
The northeastern parts of the country received near-normal rainfall during January. While most of the month was dry, widespread rain did occur in the beginning and again by the end of the month over the summer rainfall region. The western parts of the country were mostly dry.

Rainfall (mm): 21 January – 10 February 2019
Widespread rain occurred over the summer rainfall region from late January to mid-February, with most of the maize-production region receiving in excess of 50 mm.

Vegetation Condition Index: 11 – 20 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

A semi-stationary longwave trough to the south/southeast of South Africa together with the surface Atlantic Ocean Anticyclone positioned somewhat further north will result in dry conditions over most of the interior except for the east and northeast. Interaction of dry air with storms in the east will cause some storms to become severe. A surface high-pressure system directly to the east will keep moisture available to the far eastern areas, enhancing the chances for precipitation. The position of the upper-air trough coupled with moisture from the east will result in rainfall totals in the far east within the normal category.


Westerlies to the south of the country will be positioned somewhat north, reflected in several cold fronts that will move close to the country or influence the southern to southwestern parts.
Conditions in main agricultural production regions (26 February – 4 March)
Maize production region: It will be warm to hot over the western to central parts of this region, with maximum temperatures in the mid-thirties for the most part. Westerly winds should dominate over these areas. The eastern parts should experience near-normal temperatures, with thundershowers from time to time over the far eastern parts. Thundershowers may spread westwards into the rest of the region by early next week.
Cape Wine Lands and Ruens: It will be mild to cool for the most part while the wind will have a westerly component. Southeasterlies are possible in the southwest by the weekend. According to current projections, cold fronts will result in showers over the southern to western parts on Tuesday (26th), Friday (1st) and Monday (5th).


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