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Cumulus Report: 13 February 2019


Scattered to widespread rain and thundershowers once again occurred over the country during the last few days.

For the first time this summer, there was also significant westward penetration of rainfall with daily totals in excess of 50 mm in over some of the central to northwestern parts of the Northern Cape. Expected rainfall over the western parts of the winter rainfall area didn't materialise, but large parts of the Karoo did receive scattered thundershowers as a the upper-air system responsible for some of the rain over the central to northern parts also caused thundershowers over these southern parts of the country.

The central to eastern parts of the country, including much of the maize-production region, also experienced partly cloudy to cloudy periods with scattered rain and thundershowers. Large amounts of moisture flowing into the interior from the north were indicative of a favorable second half of summer.

The next few days will see a continuation of the favorable situation over the interior. The presence of a tropical low over the northern parts of Botswana will maintain the availability of large amounts of deep tropical moisture over the interior. This will increase the rainfall potential, supported also by favorable upper-air conditions in the form of an upper-air trough moving across the country. Current projections keep the rain-producing systems over the country until the weekend, when some clearance is indicated from the west. The most-favored areas include North West, Gauteng, Mpumalanga and the northern to eastern Free State.

The tropical low to the north is expected to move over the northern extremes of the country, supporting rainfall over the northern and eastern to central parts until Friday (15th). It should move out into the Mozambique Channel during the weekend. It is still too long before the time to make any statement regarding the system as it moves into the Channel, but most projections currently moves it out of our region in a southeasterly direction, with minimal impact over the interior.

The wetter conditions over large parts of the summer rainfall region will be associated with frequent ridging of the Atlantic Ocean Anticyclone resulting in strong south easterlies expected over the southwestern parts of the winter rainfall region on several days, especially early in the period.

Summary of weather conditions during the next few days:
• General:

Rainfall will on average be normal to above normal over the central to eastern parts of the country. 
The entire maize-production region should receive normal to above-normal rainfall.
The Northern Cape and winter rainfall region as well as the interior of the Western Cape and southwestern parts of the Eastern Cape (areas such as the Garden Route and Little Karoo) can expect below-normal rainfall for this time of the year.
On average, it will remain warmer than normal over the southwestern and western interior. 
Until Friday (15th), cloudy and mild conditions will often occur over the eastern parts of the country with temperatures near- to below normal. 
Rainy conditions over the interior should clear during the weekend when cloud cover and rain relocates to the east.
 Moderate to strong southeasterlies will occur over the southwestern parts on most days, reaching maximum strength on Tuesday to Wednesday (12th - 13th). 
• Rainfall: 
Scattered thundershowers are possible over the central interior, including the western maize-production region, from Wednesday (13th) to Friday (15th). 
Scattered to widespread rain and thundershowers will occur over the northeastern parts of the country, including the central to eastern parts of the maize-production region, from Tuesday (12th) to Friday (15th). 
Thundershowers causing significant daily rainfall totals may occur over the southern Lowveld and escarpment of Mpumalanga on Wednesday (13th). 
o Significant daily rainfall may occur over the central to eastern North West, Gauteng and northern Free State on Thursday (14th) and Friday (15th). 
Significant daily rainfall may occur over the northern parts of Limpopo on Friday (15th) and Saturday (16th).
Cumulative rainfall during the remainder of the week, following earlier rain, may enhance the probability for flash floods after rainfall events in central to eastern North West, northern Free State, Gauteng and western Mpumalanga until Friday (15th). 

 While mostly dry, isolated to scattered thundershowers may develop over the central to southeastern parts of the Northern Cape and northern parts of the Western Cape on Thursday and Friday (14th and 15th). 
The winter rainfall region should receive very little rain during the next few days and will be dominated by warm to hot and dry conditions for the most part.
• Temperatures:
Hot and dry conditions with westerly winds will dominate over the Northern Cape on several days, especially on Tuesday (12th) and again from Saturday (16th). 
 Hot, humid conditions are expected over the Limpopo River Valley and Lowveld on Tuesday and Wednesday (12th/13th).
 Very hot conditions will develop over the West Coast and Swartland from Wednesday (13th) to Friday (15th). 
 Very hot conditions will develop over the Karoo and interior of the Northern Cape on from Friday (15th) to Monday (18th).

More rain expected over the central to eastern parts
The most important feature during the next few days will be the presence of a tropical low pressure system over Botswana, introducing large amounts of moisture over the summer rainfall region. The system will move slowly westwards, causing widespread rain over northeastern South Africa, including most of the maize-production region.

Together with the tropical system in the north, favorable upper-air conditions in the form of an upper-air trough over the southern parts will enhance precipitation potential further. With the Atlantic Ocean Anticyclone ridging around the country between the 13th and 15th and with the tropical system to the north, easterly to northerly winds will dominate, introducing a deep layer of moisture over the country.

The ridging high in the south will cause strong south easterlies over the southwestern Cape during the early to middle part of the week. With the tropical low moving out eastwards by Saturday (16th) and the upper-air trough in the south also moving out east, rainfall will clear from Saturday, when significant rain is only indicated over parts of Limpopo. During the weekend and early next week, westerly winds with high temperatures may return to the western parts of the interior.

Conditions in main agricultural production regions (12 - 18 February)
Maize production region: Cloudy conditions with above-normal rainfall and near-normal to below-normal temperatures will continue for the remainder of the week. Widespread rain and thundershowers will dominate the entire region except for the south western parts until Friday. Rainfall totals until Friday (15th) may exceed 50 mm over most of central to eastern North West, northern Free State and southern Mpumalanga. Scattered thundershowers are possible over the southwestern parts from Wednesday (13th) to Friday (15th). Little to no rain is expected during the weekend according to current projections.

Cape Wine Lands and Ruens: Light showers are possible initially over the Garden Route on Tuesday (12th). It should be warm and dry for the most part, with temperatures increasing until Friday (15th). It will be very hot along the West Coast and Swartland by Thursday (14th) and Friday (15th). Southeasterlies will dominate, becoming strong in the southwest and reaching maximum strength on Tuesday (12th) and Wednesday (13th). Hot and windy conditions over large parts will be conducive for the development and spread of wild fires. The weekend should be somewhat cooler, with a small chance of showers on Saturday. Early projections indicate hotter conditions again towards Monday (18th). 
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:
• Significant daily rainfall totals, following wetter conditions earlier, may be conducive to flash floods:
Over the Lowveld and Escarpment of Mpumalanga (13th). 
 Over central to eastern North West, northern free State, Gauteng, southern Mpumalanga on Thursday and Friday (14th and 15th). 
 Over the northern parts of Limpopo on Friday (15th) and Saturday (16th). 
• Moderate to strong south easterlies over the southwestern parts of the Cape during most of the period (reaching a maximum on Wednesday (13th)) may be conducive to the development and spread of wild fires where vegetation is dry. 
• Warm to hot and windy conditions (westerly winds) are possible over the Northern Cape interior from Saturday (16th) towards early next week. This may be conducive to the development and spread of wild fires where (if) vegetation is dry.

Seasonal overview
El Niño and seasonal forecasts
The El Niño-Southern Oscillation (ENSO) is currently neutral. Tropical Pacific sea surface temperatures have cooled slightly in the past fortnight but remain warmer than average. Importantly, in the central to western Pacific, warmer than average sub-surface waters persist. This is often a precursor to El Niño events. While most atmospheric indicators of ENSO are neutral, trade winds have been weaker than average, or even reversed, across the central and western Pacific in recent weeks, leading to sub-surface warming. Westerly wind anomalies over the western tropical Pacific are forecast to persist a little longer, which is likely to assist further warming of the tropical Pacific.

The majority of climate models suggest ENSO-neutral sea surface temperatures will continue through autumn. However, the current ocean warmth, and the forecast for warmer than average sea surface temperatures later in the year, means the possibility of El Niño remains. Three of eight models suggest that El Niño may establish by mid-2019, with another three models on the warm side of neutral. It should be noted that model outlooks that span the southern autumn period tend to have lower accuracy than outlooks issued at other times of the year. This means outlooks beyond May should be used with some caution...Australian Bureau of Meteorology - http://www.bom.gov.au

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. However, recent values are in the neutral to positive range - signalling weak coupling as mentioned above.

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.

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.

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 are 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.


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