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2016-03-30 By: J Malherbe, R Kuschke

Contents

Summary .................................................................................................................................2

Rainfall: 1 – 28 March 2016 ....................................................................................................3

Lowest minimum temperatures: 22 – 28 March 2016 ..............................................................4

Overview of expected conditions over South Africa during the next few days .........................5

Significant weather events (30 March – 5 April 2016) .........................................................5

Conditions in main agricultural production regions (30 March – 5 April 2016) ....................5

Conditions across South Africa during the next 4 days ........................................................7

Possible extreme conditions - relevant to agriculture ......................................................... 10

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Summary Two widespread rainfall events (7 to 11 and 15 to 19 March) have resulted in large areas in the northeast receiving

above-normal rainfall in March. Heavy rain was mostly confined to the eastern and northern parts of the summer

rainfall region. Following the wet conditions in the northeast, the main activity shifted towards the central and western parts. A significant cold front reached the Cape by the 25th, producing cold weather and widespread rain

over the winter rainfall area (with snow reported from the Matroosberg). The associated upper-air low resulted in

widespread showers over the central interior, including much of the western Free State and Northern Cape interior.

The upper-air low flattened and surface westerlies advected cold air from the west across the interior, resulting in

dry and cool conditions over much of the country since the 27th. Isolated areas over the eastern high-lying interior

experienced the first frost for the winter.

The cold front moving across the interior happened as the Southern Hemisphere experienced large variations in the

pressure distributions towards the south, as indicated by the Southern Annular Mode (SAM). The index has

decreased considerably after reaching very high values, indicating a tendency for the westerlies to shift northwards.

Over the southern African area, this was reflected firstly by the development of the deep upper-air low responsible

for rain around the 25th over the central parts followed by the inland penetration of a cold front from the southwest.

The SAM is expected decrease somewhat more before increasing later in the week.

The Annular Mode Website - http://www.atmos.colostate.edu/ao/index.html

Circulation patterns are projected to result in a continuation of cold conditions in the southwest, with some showers

indicated over the (southern parts of the) winter rainfall region during the next few days. Night-time temperatures

may still remain low at first over the high-lying interior, but are indicated to increase steadily during the next few

days, especially by the weekend when a more summer-like pattern will result in moisture being advected from the

east over the interior.

During similar years in the decadal rainfall cycle, rainfall during March and early April tended to remain near

normal to above normal, following similar conditions during January and February. During the current summer,

observations over the northeastern parts confirmed earlier tendencies, but the central parts experienced a resurgence

of dry weather during February.

The 2015/16 El Niño is weakening: ”The decline of the 2015-16 El Niño continues in the tropical Pacific.

Temperatures below the ocean surface have cooled steadily, with only the top 50 meters more than +1 °C warmer

than normal. It is likely this is the coolest this top layer of ocean has been since January 2015. Atmospheric

indicators reflect such changes in the ocean. For instance, the Southern Oscillation Index (SOI) has returned to

weak El Niño levels……….. International climate models suggest El Niño will continue to weaken during the

southern autumn, returning to neutral levels by mid-2016. For winter and spring, climate models suggest neutral

and La Niña are equally likely……....” - Australian Bureau of Meteorology - http://www.bom.gov.au

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Rainfall: 1 – 28 March 2016

While rainfall totals during the first 28 days of March exceeded 100 mm over large parts of the

northern and eastern provinces, totals over the Free State remained on average close to 50 mm.

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Lowest minimum temperatures: 22 – 28 March 2016

The southern and eastern high-lying areas experienced lowest minimum temperatures below 2°C

during the period.

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Overview of expected conditions over South Africa during the next few days

Significant weather events (30 March – 5 April 2016)

The coming week will see a gradual increase in temperatures as we enter another build-up

period, with anticyclones generally increasing in strength along the mid-latitudes and subtropics,

as reflected in an increasing SAM. Frontal systems will continue to brush the southwestern parts

with more showers expected over the winter rainfall region during the week. Westerlies will

advect cold air over the southwestern interior, but minimum temperatures should increase over

the interior where the wind will have a northerly component. Colder air from the southwest may

once again reach the central parts, before large amounts of warmer moist air will be advected

from the east as the Indian Ocean Anticyclone intensifies during the weekend.

Large amounts of surface moisture from the east and northeast, together with an upper-air trough

over the southwestern parts, will result in a band of isolated to scattered showers and

thundershowers from Sunday, currently projected to focus over the southern and western Free

State and surrounding areas. Conditions during the early part of April may continue to be more

favorable for precipitation over the interior.

On average, rainfall expected over the central interior by next week may result in above-normal

rainfall over those areas during the period while frontal systems during the remainder of the

current week may result in normal rainfall over the southwestern winter rainfall region also. The

rest of the country is expected to remain largely dry. Temperatures will on average be above

normal over most of the interior, and below normal over the southwestern parts.

Conditions in main agricultural production regions (30 March – 5 April 2016)

Maize production region: Sunny and mild conditions will dominate during the early part of the period,

with cloudy to partly cloudy conditions by later the weekend. There is no indication currently of a widespread rainfall event influencing the area during the coming week. Minimum temperatures are

expected to gradually increase during the period.

Swartland, Cape Wine Lands and Ruens: Cool conditions with westerlies and some showers are

expected to last until the weekend. Clearance is indicated towards next week, with drier conditions in the southwest associated with strong southerlies to southeasterlies.

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Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society

(IGES) – http://Wxmaps.org

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Conditions across South Africa during the next 4 days

Rainfall

While the interior is expected to remain dry during the next three days, some showers are

expected over the winter rainfall region as frontal systems move across the area.

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Minimum temperatures

Minimum temperatures are expected to remain low over the southwestern parts due to frontal

activity, but will steadily increase over the northeastern parts.

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Maximum temperatures

Daytime temperatures will remain low in the southwest, including much of the Northern Cape

and southern Free State whilst increasing slightly in the northeast.

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Possible extreme conditions - relevant to agriculture

The South African Weather Service issues warnings for any severe weather that may develop, based on

much more information (and in near-real time) than the output of one single weather model (GFS

atmospheric model - Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) – http://Wxmaps.org)

considered here in the beginning of a week-long (starting 30 March) period. It is therefore advised to keep

track of warnings that may be issued by the SAWS (www.weathersa.co.za) as the week progresses.

According to current model projections (GFS atmospheric model) of weather conditions during the

coming week, the following may be deduced:

Low minimum temperatures are still indicated especially over the southern high-lying interior,

with a slight possibility of frost also over the high-lying eastern interior early in the period.