Cumulus-El Niño and seasonal forecasts- March 2019

Mostly dry conditions expected over the interior -Isolated to scattered thundershowers occurred over most of the interior during the last few days, with somewhat more widespread falls Saturday and Sunday over the Free State and southern North West.


The next few days will generally be dry over most of the country. Precipitation over the interior will be in the form of isolated thundershowers that may become severe in certain instances. It will start out with cloudy and cool conditions over the eastern to northeastern parts of the country early in the period, but it will be warm and dry for the most part during the rest of the period over the interior. The southeastern to eastern coastal areas may experience relatively cool and wet conditions regularly. Towards Sunday (1st) and early next week, there may be widespread rainfall over the interior while a cold front may result in cold and wet conditions over the winter rainfall region and southern parts.
The following is a summary of weather conditions during the next few days:
• General:
The Eastern Highveld as well as the eastern to southern coastal areas and adjacent interior will receive normal rainfall.
Most of the interior will receive below-normal rainfall during the next few days.
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.
The eastern parts of the country will be cloudy and cool on Wednesday (27th) and Thursday (28th).
The eastern to southeastern coastal areas and adjacent interior will be cloudy, cool and wet for most of the period, fussing on Tuesday to Wednesday (26th and 27th), Saturday (30th) and again by Monday (1st).
Little to no rain is expected over the country from Thursday (28th) to Saturday (30th).
Fresh to strong southeasterlies will dominate the southwestern parts on Tuesday (26th) and Monday (1st).
A cold front will influence the winter rainfall region and southern parts of the country from Sunday (31st) to Monday (1st).
There are indications (still fairly uncertain so far ahead of time) of widespread rain over the eastern and possibly central interior by early next week.
• Rainfall:
Isolated thundershowers are possible over the eastern to northeastern parts of the country on Tuesday (26th) and Wednesday (27th).
Cloudy and cool conditions with rain or showers are possible along the southeastern to eastern coastal areas on Tuesday and Wednesday (26th and 27th), Friday (30th) and Monday (1st).
Thundershowers over the eastern Free State, and Mpumalanga may become severe on Tuesday (26th) and Wednesday (27th).
Rain and showers are possible over the winter rainfall region and the Garden Route on Sunday (31st) and Monday (1st).
• Temperatures:
Maximum temperatures over the western maize-production region will range between 26 and 34 °C while minimums will be in the order of 14 – 22°C.
Maximum temperatures over the eastern maize-production region will range between 21 and 31°C while minimums will be in the order of 11 – 17°C.
It will be cool over the eastern to southeastern parts of the country on Tuesday (26th) and Wednesday (27th).
It will be cool over the southern to southeastern parts of the country by Tuesday (26th) and Monday (2nd).
It will be warm to hot over most of the interior during the weekend.

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:

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 - http://www.bom.gov.au
SSTs in the tropical Pacific increased within the weak El Niño category during February and early March, while subsurface waters became more strongly warmer than average. Patterns in the atmosphere now clearly suggest El Niño conditions. Collective model forecasts show a continuation of at least weak El Niño-level SSTs through spring and summer, likely even lasting through 2019. 


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.

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.

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
Top left: Observed rainfall (% of long-term mean, based on satellite rainfall estimates). Top right: Rainfall forecast for OND – SAWS – issued in September 2018. Bottom left: Forecast based on decadal climate variability – issued September 2018. Bottom right: Rainfall forecast for OND – issued in September 2018 by the IRI.
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. Issued by the IRI in September 2018.

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): 1 – 20 March 2019
The first 20 days saw widespread rainfall especially over the Free State and southern North West (mostly during the period 8 – 12 March). Less rain occurred over Mpumalanga.

Vegetation Condition Index: 11 – 20 March 2019
Vegetation activity remains near normal to above normal over most of the maize-production region.

Overview of expected conditions over South Africa during the next few days
Ridging high-pressure systems will result in cool conditions over the southern to eastern parts of the country by Tuesday (26th) and again Monday (1st). Circulation towards the east will generally be cyclonic – resulting in relatively dry conditions over the interior and below-normal rainfall. The upper-air over the interior will also be fairly unfavorable, except for perturbations over the northern to northeastern parts which may support isolated thundershowers from time to time. Isolated thundershowers are possible over the eastern parts on Tuesday (26th) and Wednesday (27th). Due to interaction with dry air to the west and fairly strong upper-air winds, some thundershowers may become severe. The ridging Atlantic Ocean Anticyclone by Monday (1st) may result in widespread thundershowers, depending on the development of a trough in the upper-air. Outlooks this far ahead of time is still fairly uncertain.
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 (26 March – 1 April)
Maize production region: Isolated thundershowers are possible early in the period, with dry conditions from about mid-week, continuing into the weekend. Cool, cloudy conditions will occur over the eastern parts on Tuesday (26th) and Wednesday (27th). 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 14 – 22°C. Maximum temperatures over the eastern maize-production region will range between 21 and 31°C while minimums will be in the order of 11 – 17°C. Chances for widespread thundershowers improve from Sunday / Monday (31st / 1st).
Cape Wine Lands and Ruens: This region will be warm and dry for the most part, becoming cloudy and cool to cold with rain on Sunday / Monday. Temperatures over the interior will reach the lower 30s around mid-week this week. Except for Tuesday (26th) and Monday (1st), the wind will be weak with a westerly component. Strong southeasterlies will again dominate on Tuesday (26th) and Monday (1st) in the southwest. Where vegetation is dry, the windy conditions may be conducive to the development and spread of wild fires.


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