CUMULUS REPORT- 5th March 2019 South Africa

The last week may certainly go through as one of the driest weeks during late summer/early autumn over the central parts of the country.

Atmospheric circulation was exceedingly unfavorable with a trough situated to the southeast of the country while surface and upper-air anticyclonic circulation dominated the central interior.

Scattered thundershowers occurred over the northeastern areas, mostly confined to Mpumalanga, Limpopo and parts of KZN. Again, some thundershowers became severe (due to interaction with dry air to the west as well as – at times – relatively strong upper-air winds).
The next few days will see a return to normal late summer conditions, with scattered to widespread thundershowers expected over especially the central parts of the country, including the western maize-production region. Weather systems responsible for the expected rainfall may however result in a greater incidence of severe storms than the norm this late in summer. Strong and persistent southeasterlies over the southwestern coastal region will be evidence of a semi-permanent ridging action by the Atlantic Ocean Anticyclone to the west, forcing a more favorable circulation pattern and much-needed rain over the central parts.
The following is a summary of weather conditions during the next few days:
o The central to eastern and southeastern interior as well as the Garden Route will receive above-normal rainfall for this time of the year.
o The Limpopo River Valley and Northern Cape is expected to receive below-normal rainfall.
o The winter rainfall region, entire coast and adjacent interior as well as the eastern escarpment will be cooler than normal for this time of the year.
o Scattered to widespread thundershowers will dominate the central to eastern parts from later this week, possibly continuing during the weekend.
o It will be warmer than normal over the interior.
o Temperatures over the central to northern interior, where hot conditions occurred during the last few days, will moderate especially from Thursday (7th).
o The Garden Route as well as the eastern to southeastern coastal areas and adjacent interior will remain mild to cool, cloudy and wet from Thursday onwards.
o Moderate to strong southeasterlies will dominate the southwestern parts from Thursday (7th).
• Rainfall:
o Isolated to scattered thundershowers are possible over the eastern parts, including the central to eastern maize-production region, on Tuesday (5th).
o Isolated to scattered thundershowers are possible over the central parts of the country, including the western maize-production region, from Wednesday (6th).
o Scattered to widespread thundershowers are possible from Friday over much of the central to eastern and southeastern parts, including the entire maize-production region.
o Thundershowers over the interior may have a tendency to become severe (and produce strong winds and hail), above the norm for this time of the summer.
o Showers are possible over the winter rainfall region (especially the southwestern to southern parts) on Thursday (7th).
o Cloudy conditions with rain and showers will dominate the Garden Route from Thursday (7th), spreading along the southeastern to eastern coastal belt and adjacent interior where cloudy conditions with showers may persist through the weekend.
• 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 until Thursday (7th).
o Hot, berg wind conditions are expected over the Eastern Cape (especially southeast) on Wednesday (6th).
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 from the 7th.
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 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.
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.
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.
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
Towards late summer, seasonal forecast models suggest a continuation of warmer and drier-than-normal conditions over much of the interior
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.
Mid-season review of seasonal outlook
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.

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.

Overview of expected conditions over South Africa during the next few days
An upper-air trough developing over the southwestern parts of the country during the period, possibly deepening into a cut-off low near the West Coast during the weekend, will result in unstable conditions over much of the central parts, especially from Thursday (7th) onwards. Together with the developing upper-air trough in the southwest, upper-air perturbations moving from the northeast southwestwards over the northeastern parts of the country will also support thundershower development early in the week (Tuesday (5th) to Thursday (7th)) over Limpopo, Mpumalanga, Gauteng and North West. An interesting feature which will drive circulation changes over South Africa, according to current model projections, is a strong Atlantic Ocean Anticyclone that will strengthen southwest of the country, and slowly ridge around the country during the period, with a persistent ridging action indicated until the weekend. The influx om moisture from the east and weakening of the upper-air high over the interior will result in lower daytime temperatures, with maximum temperatures retreating to the lower 30’s or upper 20s over many areas dominated by hot conditions recently.
To the northeast, over the Mozambique Channel, a tropical depression is present. The system may move into central Mozambique and eastern Zimbabwe or Malawi on Tuesday and Wednesday (5th and 6th). Current projections indicate an eastward track from Thursday, back into the Mozambique Channel by the latter part of the week. At this stage, the system is not expected to influence South Africa, except possibly causing some drier conditions over the far northeastern parts during the second half of the week.
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 (5 March)
Maize production region: Rain over the area is especially indicated for most of the week but especially from Friday and into the weekend over the western parts while maximum temperatures will moderate already from Tuesday (5th) and Wednesday (6th). It will be hot in the west initially while scattered thundershowers are possible over the eastern to central parts on Tuesday (5th). Thundershowers will redevelop somewhat further west, over the central to western parts on Wednesday when maximum temperatures will be somewhat lower compared to the previous days. Winds in the west during the day should become more northerly from Thursday (7th) when the chance for thundershowers will improve further over the western parts. Scattered to widespread thundershowers will persist in the west on Friday and Saturday (8th and 9th) while spreading northeastwards into the eastern parts. Scattered thundershowers may persist into Monday according to current forecasts, over the entire region, but should clear later during the week. Thundershowers during this period will have an enhanced tendency to become severe with the potential for hail.
Cape Wine Lands and Ruens: Following some rain on Monday (4th), another frontal system will result in some showers on Thursday (7th). This will be preceded by warm to hot conditions until Wednesday over most parts of this region. The western parts of this region will otherwise be dry. Towards the south, mostly along the Garden Route, the cloudy conditions with showers may persist into the weekend, coupled with southerly to southeasterly winds. The wind over the southwestern parts is expected to become moderate to strong southeaterly from Thursday (7th), persisting into the weekend and possibly early next week. An upper-air low that may develop in the vicinity could cause unstable conditions with showers or thundershowers by early next week.
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 interior may have a tendency to become severe. These could be associated with hail and strong winds.
• Warm to hot and windy conditions (westerly winds) are possible over the Northern Cape interior, western Free State and western North West until Thursday (7th). This may be conducive to the development and spread of wild fires where (if) vegetation is dry.
• Strong southeasterlies over the southwestern Cape, especially from Thursday (7th), may be conducive to the development and spread of wild fires.


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