IRI NET ASSESSMENT FORECAST
DISCUSSION
In early April 1999, the IRI
Experimental Forecast Division prepared a Climate Outlook for April
to June 1999. The Precipitation and Temperature Probabilities presented
in this Outlook are based on predictions of the continuation of cooler
than average conditions in the eastern and central equatorial Pacific Ocean
(La Niña), and the persistence of warmer
than average conditions in the western equatorial Pacific. The sea
surface temperatures of the central and western tropical Indian Ocean have
been cooling from their record high temperatures over most of the basin
and this forecast was made under the assumption that this trend would continue.
It has been assumed that the northern and tropical Atlantic Ocean will
remain warmer than average, and that sea surface temperatures in the South
Atlantic will increase during the forecast period.
PRECIPITATION OUTLOOK
The Outlook for April to June 1999 can also be found at http://iri.ucsd.edu/forecast/net_asmt.
Maps are given showing expected precipitation probabilities in tercile
classes. The maps indicate probabilities that seasonal precipitation will
fall into the wettest third of the years (top number), the middle third
of years (middle number) or the driest third of the years (bottom). An
outlook of climatology "C" indicates equal probabilities in each class
i.e.; there is no basis for favoring the forecast of any particular category.
Areas marked "D" experience a pronounced dry season during the forecast
period, typically receiving less than 15% of their annual precipitation
total during this three-month period. Boundaries between sub-regions should
be considered transition zones, and their location considered to be only
qualitatively correct.
METHODS The
following procedures and information were used to prepare this Climate
Outlook: 1) Coupled ocean-atmosphere model predictions of tropical Pacific
SST Particularly heavy weighting has been given to the NOAA /NCEP, Climate
Modeling Branch coupled model which suggests continued slow decrease from
moderate La Niña conditions, with the persistence of strong positive
sea-surface temperature anomalies near Indonesia, 2) statistical forecasts
of Indian Ocean and Atlantic Ocean sea surface temperature, 3) Atmospheric
global circulation model (GCM) predictions response to the present and
predicted SST patterns, 4) Statistical analyses, 5) Appropriate Regional
Climate Outlook Forum consensus guidance.
April to June 1999 Precipitation Probabilities |
The procedures, models, and data used to derive this Climate
Outlook may be somewhat different from those used by National Meteorological
Services in particular regions and may differ from the official forecasts
issued in those areas. This Outlook has been prepared in the time available,
using all information that was reasonably accessible. Inclusion of other
climate information and guidance requires further arrangements. The IRI
is engaged in establishing collaborative arrangements with the goal of
improving its capability to provide the best and most complete global climate
guidance.
The Climate Outlook for April to June 1999 is dependent
on the quality of the SST predictions. For the tropical Pacific, these
predictions can be expected to provide useful information, but there is
considerable spread in coupled model predictions concerning the evolution
of SSTs. This spread in predictions is a primary source of uncertainty
in the Outlook provided here, which assumes that tropical Pacific SST's
will continue to decline at a modest rate, resulting in moderate La Niña
conditions by the end of the year. Also, it is known that Indian and Atlantic
Ocean SSTs may play some role in modulating precipitation and temperature
patterns. Thus, the uncertainties in Indian and Atlantic Ocean SST values
during the forecast period may lead to additional uncertainty over some
parts of the world.
It is stressed that the current status of seasonal-to-interannual
climate forecasting allows prediction of spatial and temporal averages,
and does not fully account for all factors that influence regional and
national climate variability. This Outlook is relevant only to seasonal
time scales and relatively large areas; local variations should be expected.
For further information concerning this and other guidance products, users
are strongly advised to contact their National Meteorological Services.
TEMPERATURE OUTLOOK
The Outlook for April to June 1999 can also be found at http://iri.ldeo.columbia.edu/climate/forecast/net_asmt.
The temperature map show expected probabilities that the seasonal temperatures
will fall into the warmest third of the years
(top number), the middle third of the years,
or the coldest third of the years (bottom
number). A qualitative outlook of climatology "C" indicates
equal probabilities in each class i.e.; there is no basis for favoring
the forecast of any particular category. Boundaries between sub-regions
should be considered transition zones, and their location considered to
be only qualitatively correct.
April to June 1999 Temperature Probabilities |
Sources of information
include ACMAD, Caribbean Meteorological Institute, CPTEC, CPC/NOAA, Department
of Natural Resources (Queensland, Australia), ECMWF, Indian Meteorological
Department and the South African Weather Bureau. |
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