IRI Home



Jan-Mar 1999 Precipitation Probabilities 
DISCUSSION In October 1998 the IRI Experimental Forecast Division prepared a Climate Outlook for January through March 1999. The Precipitation and Temperature Probabilities presented in this Outlook are based on predictions of the continued evolution 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, and this trend is expected to continue. It has been assumed that the northern and tropical Atlantic Ocean will remain warmer than normal, and that sea surface temperatures in the South Atlantic will increase during the forecast period.

PRECIPITATION OUTLOOK The Outlook for January to March 1999 can also be found at 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 development of 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.

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 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 January to March 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 Temperature Outlook for January - March 1999 will appear in the next (November) issue of the IRI Climate Information Digest that will be available on 14 December 1998 and can also be found at

Sources of information include ACMAD, CPTEC, CPC/NOAA, Department of Natural Resources (Australia), ECMWF, Indian Meteorological Department and the  South African Weather Bureau.