IRI NET ASSESSMENT FORECAST

DISCUSSION The IRI Experimental Forecast Division has prepared a Climate Outlook for October through December 1998. An update will be the October to December 1998 season and a forecast for January to March 1999 will be issued in mid October.  The Precipitation and Temperature Probabilities presented in this Outlook are based on the forecast for continued evolution towards cooler than average sea surface temperatures (SSTs) in the central equatorial Pacific Ocean including the west coast of South America. The tropical Atlantic and Indian Oceans are currently warmer than average, but the Indian Ocean and South Atlantic are expected to cool also. The maps presented here are regional enlargements of the Net Assessment Forecasts which appeared in the previous Digest.
 
PRECIPITATION OUTLOOK The Outlook for October to December 1998 can also be found at http://iri.columbia.edu/climate/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).  Areas in darker gray 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 weak to moderate La Niña conditions, with the persistence of strong positive sea surface temperature anomalies near Indonesia, 2) Atmospheric global circulation model (GCM) predictions response to the present and predicted SST patterns, 3) Statistical analyses, 4) 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 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 October - December 1998 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 changes over parts of Africa. 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.