By: J Malherbe, R Kuschke Contents

2014-10-28
By: J Malherbe, R Kuschke
Contents
Summary ..................................................................................................................................... 2 Overview of conditions during the previous week over South Africa: ...................................... 3 Rainfall.................................................................................................................................... 3 Wind ........................................................................................................................................ 4 Temperatures........................................................................................................................... 5 Percentage of Average Seasonal Greenness ........................................................................... 6 Latest status of indicators of global climate ............................................................................... 7 SSTs ........................................................................................................................................ 7 El Niño-Southern Oscillation (SSTs and SOI) ....................................................................... 8 El Niño-Southern Oscillation (Probabilistic forecast)) ........................................................... 9 Southern Annular Mode (SAM) ............................................................................................. 9 Overview of significant weather conditions during coming week globally ............................. 10 (Sourced from the output of Global Coupled Climate Models, published online) ................... 10 Conditions over the USA, Europe and Australia – 28 October – 3 November ........................ 11 USA....................................................................................................................................... 11 Europe ................................................................................................................................... 13 Australia ................................................................................................................................ 14 Overview of expected conditions over South Africa over the next few days ........................... 15 Significant weather events (28 October – 3 November) ....................................................... 15 Conditions in main agricultural production regions (28 October – 3 November) ................ 15 Daily rainfall and temperatures across South Africa ............................................................ 17 Possible extreme conditions - relevant to agriculture ........................................................... 20 1
Summary
An upper air low moving across the country from the 22nd to 25th was responsible for isolated to scattered
thundershowers over much of the central to northeastern parts of the country. The heaviest falls occurred
mainly on the 25th as the low moved to a position over southern North West. Atmospheric conditions such
as the southwesterly flow in the upper air and large moisture gradients resulted in some storms becoming
severe, resulting mostly in strong winds and sometimes also hail over the eastern parts during the period 22
to 25 October. Systems causing significant rainfall over the northeastern parts quite often develops when
the Southern Annular Mode becomes negative from relatively high positive values, as was the case during
the weekend.
Unlike the previous week, the main area of precipitation during the coming week will be located towards
the central, southern and western parts, including possibly much of the winter rainfall region later in the
period.
At the medium time scale, previous years such as the current were associated with relatively wet conditions
during November over the northeastern parts. There are indications that these periods may be concentrated
around the 9th and 24th.
At seasonal time scale, Coupled Global Climate Models are still dominated by an expectation of the
development of ENSO Warm conditions. Therefore, current seasonal projections favor relatively dry
conditions towards mid summer and late summer. Current SST anomalies towards the Mid and High
Southern Latitudes have in the past been associated with wetter conditions during mid-to late summer,
while decadal variability may have a positive impact on rainfall towards late summer over the summer
rainfall region despite the negative impact of ENSO.
2
Overview of conditions during the previous week over South Africa:
Rainfall
Precipitation was confined mostly to the northeastern parts and also into southern KwaZulu-Natal
and northeastern Eastern Cape. Large parts of Gauteng and eastern Mpumalanga received between
25 and 50 mm. The western parts and northeastern Limpopo remained dry or received very little
rain.
3
Wind
Several stations across the northeastern provinces recorded maximum wind gusts in the high wind to gale
category, associated with several storms that became severe over these areas from the 22nd to 25th. Strong
southeasterlies also occurred over the southwestern extremes, associated with large scale circulation
patterns. (see table below for the colour code and Beaufort Wind Scale)
6
Strong breeze
39–49 km/h
Large branches in motion. Whistling
heard in overhead wires. Umbrella use
becomes difficult. Empty plastic bins tip
over.
7
High wind,
50–61 km/h
Whole trees in motion. Effort needed to
walk against the wind.
Gale
62–74 km/h
Some twigs broken from trees. Cars veer
on road. Progress on foot is seriously
impeded.
9
Strong gale
75–88 km/h
Some branches break off trees, and some
small
trees
blow
over.
Construction/temporary
signs
and
barricades blow over.
10
Storm
89–102 km/h
Trees are broken off or uprooted,
structural damage likely.
moderate gale
8
4
Temperatures
Temperatures exceeded 40°Cover the northwestern interior (mainly by the 22nd). The eastern and
southeastern parts as well as the southern coastal areas remained relatively cool, due to the flow from the
east and south that dominated during the period.
5
Percentage of Average Seasonal Greenness
After a dry and cold winter, cumulative vegetation activity since July is below normal over much of the
eastern parts. Some rainfall events during August and September resulted in positive anomalies over the
central to northwestern parts. Above-normal cumulative vegetation activity also dominates over the winter
rainfall region where the rainfall during the past few months was near normal.
6
Latest status of indicators of global climate



SSTs (Sea Surface Temperatures) in the Central and Eastern Equatorial Pacific remain near normal to slightly above normal, with the development of an El Niño still a possibility within the next 2 months SSTs in the Southern Hemisphere however still seem to indicate a positive influence on rainfall during the coming summer, while The SAM has become negative, indicating drier conditions possibly developing on the short to medium term over the northeastern parts. SSTs
SSTs across the equatorial Pacific are slightly above average, showing a possible progression towards the
development of an El Niño. “…The tropical Pacific Ocean has remained warmer than average for more
than six months, while the Southern Oscillation Index has remained negative since early June. However
neither has reached typical El Niño levels for any sustained period, and only weak atmosphere-ocean
coupling appears to have taken place so far….” - Australian Bureau of Meteorologyhttp://www.bom.gov.au
Lower SSTs towards the high southern Latitudes and relatively high SSTs over the Mid Latitudes are
however linked to positive summer rainfall anomalies over South Africa.
SST map: NOAA Climate Prediction Centre - http://www.cpc.ncep.noaa.gov
7
El Niño-Southern Oscillation (SSTs and SOI)
Australian
Bureau
of
Meteorologyhttp://www.bom.gov.au
SST anomalies over the
central-to-eastern Pacific
remain very small (+0.62)
as average for the last week,
showing a slow increase
towards El Niño thresholds.
Australian
Bureau
of
Meteorologyhttp://www.bom.gov.au
The SOI remains negative at a
30-day average of -5.1. The
relatively low value is possibly
indicative of atmospheric
conditions
leaning
more
towards ENSO Warm Event
behaviour
with
possible
responses in the Walker
circulation.
(The
Walker
circulation refers to the
location of upward and
downward large scale air flow
along the equator. When the
SOI becomes negative, it is an
indication that one of the
ascending limbs of this
circulation pattern, usually
located over Africa, may be
shifted off the continent,
usually leading to lower
rainfall over southern Africa).
8
Australian Bureau of Meteorology- http://www.bom.gov.au
El Niño-Southern Oscillation (Probabilistic forecast))
The consensus probabilistic forecast by the Climate Prediction Center (CPC) and International Research Institute for
Climate and Society (IRI) indicates a 65% chance for the development of warm ENSO conditions during Austral midsummer.
“Most models predict El Niño to develop during October-December 2014 and to continue into early 2015. This
El
Niño
will
likely
remain
weak.”
CPC
ENSO
outlook
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/
IRI - http://iri.columbia.edu/
Southern Annular Mode (SAM)
The Annular Mode Website - http://www.atmos.colostate.edu/ao/index.html
The SAM (Southern Annular Mode / Antarctic Oscillation) is an indication of the position and strength of pressure
anomalies over the Southern Hemisphere. When this index is positive (negative), it is usually an indication of highpressure (low-pressure) anomalies over the mid-latitudes. As positive anomalies over the mid-latitudes may result in
strong subtropical high pressure regions over the oceanic regions surrounding South Africa, the SAM is positively
(negatively) related to South African summer (winter) rainfall.
The SAM has fallen sharply since the 22nd and will remain negative over the next few days. There are indications of
a recovery towards early November.
9
Overview of significant weather conditions during coming week
globally
(Sourced from the output of Global Coupled Climate Models, published online)
Tropical Cyclones
Currently, 4 active tropical systems are being tracked:
 Tropical Cyclone Nilofar in the Arabian Sea, expected to move northeastwards towards northwestern India and
southern Pakistan,
 X-tropical Storm Hana, moving westwards across Honduras and Guatemala,
 Tropical system 96L in the N Atlantic, projected to move northwards without affecting major land areas directly,
 Tropical system 93L Pacific, projected to move westwards and possibly recurve back to the northeast and affect
western Mexico later.
Weather Underground - http://www.wunderground.com
Tropical Cyclone Centre La Reunion http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/index.html
(CIMMS) - Tropical Cyclone Group -http://tropic.ssec.wisc.edu/
In any event of a tropical system posing a threat to southern Africa, the South African Weather Service
(www.weathersa.co.za) will issue the relevant warnings.
10
Conditions over the USA, Europe and Australia – 28 October – 3
November
USA
Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society
(IGES) - http://Wxmaps.org
Over the next few days, normal to above-normal temperatures are expected over the western two thirds of
the country, with below-normal temperatures towards the east. Above-normal precipitation is likely from
the Pacific Northwest into the northern Great Plains as well as in a band from Texas into the central to
northeastern Great Lakes region. Below-normal precipitation is expected over the Mid-Atlantic and New
England.
11
http://droughtmonitor.unl.edu/data/jpg/current/current_usdm.jpg
The major weather system that affected much of the nation’s midsection last week left abundant
precipitation this week from the mid-Atlantic up into New England. Hurricane Ana lost strength as it
approached Hawaii and Tropical Storm Ana passed south of the Hawaiian Island dumping up to 10 inches
of rain in its path.- United States Drought Monitor- http://droughtmonitor.unl.edu
This information in relevant for the week ending 21 October.
12
Europe
Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society
(IGES) – http://Wxmaps.org
Conditions will remain wet over the western and northern parts of Europe as well as the Mediterranean and
surrounding land area into Turkey. Temperatures should be near normal to above normal except over
southeastern Europe, including the eastern Mediterranean and the Black Sea, where average temperatures
should be near normal to below normal.
An important feature of the atmospheric circulation will be a low pressure area that will remain quasi
stationary over the eastern Mediterranean, resulting in precipitation over much of that area during the week.
Several frontal systems will also move across the northwestern parts, including Scandinavia, resulting in
above-normal precipitation there. In between, much of mainland Central and Western Europe will be under
the influence of a ridge and remain relatively dry.
13
Australia
Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society
(IGES) – http://Wxmaps.org
Both eastern Western Australia and parts of Queensland should receive above-normal precipitation during
the week while the rest of the country should remain relatively dry. Precipitation should be associated with
upper air perturbations while circulation over much of the country should be unfavourable. Above-normal
temperatures should dominate except for the southeastern parts. New Zealand should also be anomalously
cool and wet during the next few days, with frequent frontal activity in association with a negative SAM
currently experienced.
14
Overview of expected conditions over South Africa over the next few
days
Significant weather events (28 October – 3 November)
During the first part of the week, the presence of some upper air perturbations over the western and central
parts together with some tropical moisture from the north and surface moisture from the east will result in
isolated thundershowers over the western and central interior as well as the eastern escarpment and
Highveld.
Over the weekend, a large upper air low will develop towards the west, and may result in widespread rain
firstly over the southeastern parts some distance ahead of the low and later directly in the vicinity of the
system in the southwest. There are indications that heavy falls are possible over some parts in the southwest.
Total rainfall during the period is expected to be above normal over the central to southwestern parts and
below normal towards the northeast. The rainfall over the southwestern parts will be associated to a large
extent to the projected development of the low to the southwest by the weekend and into the early part of
next week.
Average temperatures should be normal to slightly above normal across the interior while normal to belownormal over the coastal areas.
Conditions in main agricultural production regions (28 October – 3
November)
Maize production region: Isolated to scattered thundershowers will occur during especially by Thursday
and Friday. Currently, total rainfall during the coming week should be concentrated more towards the
southern parts of the region, with the more significant contributions only by the weekend into early next
week.
Swartland, Cape Wine lands and Ruens: A cold front moving across the southern parts and the
development of the low towards the west of the country may result in wet conditions over this part
in the coming week. Early indications are that heavy falls may occur over the eastern production
region towards early next week, but forecasts at this time scale are not seen as reliable to determine
the exact location of rainfall events.
Strong southeasterlies may also develop over parts of this region by the weekend.
15
Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society
(IGES) – http://Wxmaps.org
16
Daily rainfall and temperatures across South Africa
Daily rainfall (mm) for 28 – 31 October 2014
Isolated to scattered thundershowers may occur over the central to western interior today, slowly
moving eastwards to cover parts of Northwest and towards the eastern Seaboard by Friday. Frontal
activity and an onshore flow will also result in some rain along the coast and adjacent interior in
the southwest from Wednesday, spreading northeastwards along the coast during the following
days.
17
Daily minimum temperature (°C)
Minimum temperatures will be relatively high over the northwestern interior over the next few days and
will become a few degrees lower than current over the southern parts by Friday.
18
Daily maximum temperatures (°C)
Maximum temperatures should be relatively high over the northern parts. By Friday, temperatures over the
northeastern parts may become very high due to a northwesterly flow in association with the front moving
over the southern parts where temperatures should decrease. Temperatures over the northeast will however
decrease again on Saturday.
19
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 28 October) 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:

Hot conditions may occur over the northeastern parts, especially the Limpopo River Valley and the
Lowveld, on Friday

There are early indications that the developing upper air low towards the west may result in heavy
rainfall over the mountainous areas in the southwest (at this stage projected for mainly over the
southern parts of the Western Cape)

Strong southeasterlies may develop over the southwestern coastal region towards Friday and
Saturday
20
Sources of information
Rainfall, temperature and wind maps over South Africa for the past week:
Agricultural Research Council - Institute for Soil, Climate and Water (ISCW) – Climate Data Bank. Data recorded by the automatic
weather station network of the ARC-ISCW.
Vegetation condition maps: Coarse Resolution Imagery Database (CRID), ARC-ISCW.
Information related to: ENSO, IOD and SOI:
Australian Bureau of Meteorology - http://www.bom.gov.au
Climate Prediction Center - http://www.cpc.ncep.noaa.gov
International Research Institute for Climate and Society- http://iri.columbia.edu/
Information related to the SAM:
The Annular Mode Website - http://www.atmos.colostate.edu/ao/index.html
SST map:
NOAA Climate Prediction Center - http://www.cpc.ncep.noaa.gov
Daily conditions over South Africa:
CSIR NRE (National Resources and theEnvironment)
“CSIR NRE produces forecasts on an experimental basis, doesn’t guarantee the accuracy of the daily forecasts and cannot be held
accountable for the results of decisions taken based on the forecasts”
Tropical cyclone/hurricane/typhoon information:
Weather Underground - http://www.wunderground.com
Cooperative Institute for Meteorological Satellite Studies (CIMMS) - Tropical Cyclone Group -http://tropic.ssec.wisc.edu/
Tropical Cyclone Centre La Reunion -http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/index.html
Information on drought conditions over the USA:
United States Drought Monitor - http://droughtmonitor.unl.edu
Precipitation and temperature outlooks for the coming week:
Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) – http://Wxmaps.org
“COLA and IGES make no guarantees about and bear no responsibility or liability concerning the accuracy or timeliness of the
images being published on these web pages. All images are generated by COLA and do not represent the actual forecasts issued by
the National Weather Service. These products are not a substitute for official forecasts and are not guaranteed to be complete or
timely. The underlying data are the direct product of the various operational forecast models run by the National Centers for
Environmental Prediction, National Weather Service, NOAA and are supplied without interpretation or correction.”
21