Climate
Seasonal Climate Watch
September 2019 to January 2020
Overview
The El Niño-Southern Oscillation
(ENSO) is currently in a neutral
state and the forecast indicates that
it will most likely remain in a neutral
state for the coming seasons. ENSO
forecasts are currently extremely
uncertain with a wide variety of
outcomes predicted by different
forecasting centres. Usually when
this is the case seasonal forecasts
for the summer rainfall areas tend
to be very uncertain as well.
The uncertainty in the ENSO
forecast seems to be reflected in
the rainfall forecast for the summer
rainfall areas as well. The late spring
(Oct-Nov-Dec) period indicates
confident forecasts that below-
normal rainfall is more likely over
the central and north-eastern parts
of the country. Early-summer (Nov-
Dec-Jan), however, indicates that
above-normal rainfall is more likely
for the same areas. This sudden
change can be attributed to the
uncertainty in the ENSO forecasts
mentioned above. The threshold
forecasts mainly indicate a higher
number of rainfall days during
the late-spring and early-summer
seasons, further adding to the
uncertainty of rainfall forecasts for
the start of the summer season.
With regards to temperatures,
mostly
higher
than
normal
temperatures are expected for
the northern most parts of the
country from spring through
to early-summer.
The South African Weather Service
will continue to monitor and provide
updates of any future assessments
that may provide more clarity on
the current expectations for the
coming seasons.
Seasonal Totals and Averages Rainfall Frequency Predictions
In an effort to improve the predictions made by the SCM, which
struggles to produce reliable rainfall and temperature forecasts at a
local scale, the Multi-Model System (MMS) has been implemented to
statistically downscale various global forecasts, including the SCM and
the Climate Forecasting System version 2 (CFSv2) administered by the
National Oceanographic and Atmospheric Administration (NOAA).
Below are the current three-season forecasts issued in August 2019.
Three maps are shown for each season which include the raw MMS
probabilistic prediction (left), the probabilistic prediction with skill
masked out (middle) and the climatological average (right) for the
specific season. The user is advised to consider the skill masked map
(middle) as the official SAWS forecast, however, the two additional
maps may be used as tools in such a case where skill for a specific area
is deemed insufficient This product is a result of the SAWS operational multi-model system
(MMS) where the 850-hPa geopotential heights hindcast outputs are
first statistically recalibrated and downscaled to observed number of
rainfall days exceeding desired thresholds (derived from high resolution
0.1 X 0.1 degree (ARCv2) African Rainfall Climatology version 2 rainfall
dataset) within seasons of interest over South Africa by using model
output statistics (MOS). The 850-hPa geopotential heights are used
here because they are found to be the best predictor of rainfall over
southern Africa.
These forecasts can be used together with the traditional seasonal
rainfall total forecasts in that it can indicate the frequency of rainfall
days where seasonal rainfall forecast areas expect below- or above-
normal conditions.
September-October-November (SON) 2019 seasonal precipitation
prediction without skill taken into account (left), as well as skill masked out
(middle). Also included is the climatological average for SON
(right, in mm) calculated over the period 1979-2009.
October-November-December (OND) 2019 seasonal precipitation
prediction without skill taken into account (left), as well as skill masked out
(middle). Also included is the climatological average for OND (right, in
mm) calculated over the period 1979-2009.
September-October-November 2019 rainfall-days forecast. Forecast for
high and low number of rainfall days exceeding 5 and 15mm without skill
taken into account (left) and with skill taken into account (middle). Also
included is the climatology for rainfall days (right) exceeding 5 and 15mm
calculated over the period 1983-2009.
For more information please contact:
Cobus Olivier - Scientist: Prediction Research:
[email protected]
Website: www.weathersa.co.za -
Weatherlines: 012 367 6000 and USSD - *120*7297#
November-December-January (NDJ) 2019 seasonal precipitation
prediction without skill taken into account (left), as well as skill masked
out (middle). Also included is the climatological average for NDJ (right, in
mm) calculated over the period 1979-2009.
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SABI | OCTOBER/NOVEMBER 2019
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